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Measurements of liquid scintillator properties for the Borexino detector
INSTNUMENTS
&rk4mmDs
IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A 400 (1997) 53-68
Measurements of liquid scintillator properties for the Borexino detector F. Elisei”, F. Gattib, A. GorettiC, T. Hagnerd, F. Masetti”, U. Mazzucato”, G. Ranucci”, S. Schoenertd, G. Testerab**, P. Ullucci”, S. Vitaleb a Dipartimento di Chimica dell’liniversita’ and Sezione INFN di Perugia, 06123 Perugia, Ita1.y b Dipartimento di Fisica dell’Uniuersita’ and Sezione INFN di Geneva, 16146 Genoua, fta(v c Dipartimento di Fisica dell’~nivers~ta~ and Sezione INFN di ~i~ana, 20133 ~ila~a, Italy d Technische Universitaet Muenchen, 85747 Garching, Germany
Received 23 April 1997; received in revised form 21 May 1997
Abstract The optical properties of various scintillators (both single components and their mixtures) have been measured in order to choose the scintillator composition offering the best compromise among the requirements of the large volume detector of the Borexino experiment on solar neutrinos. A pre-selection of the most interesting binary or ternary mixtures has been carried out by using a relatively simple and fast optical method. This is based on the W excitation of the solvent and the observation of the emission properties in the spectral region of interest. The results obtained allowed the light output, response time and attenuation lengths to be evaluated for a broad range of mixtures. The mixtures offering the best properties for Borexino have been investigated in greater depth by using ionizing radiation as excitation source. The results were found to be in good agreement with those obtained by the optical method. In addition, the cx quenching and the time response of the scintillators to 01and p particles have been tested to evaluate the a/p discrimination capability. The comparison of the results obtained in four laboratories using different techniques and methodologies, offered an exhaustive picture of the scintillator properties and provided the criteria for a good choice of the mixture for Borexino. PACS:
42.20.E; 29.40.M; 96.60.K
1. Introduction Various detectors have shown that the Sun is a source of neutrinos [l]. This fact confirms the main hypothesis of our model of the Sun [Z] and shows that energy is really produced in stars by fusion processes. On the other hand, the experimental data indicate that there are still phenomena not completely
*Corresponding author. Tel.: +39 103536433: fax: +39 10 313358; e-mail: [email protected]. 0168-9002/97/$17.00
PII SO I68-9002(
0 1997 Elsevier Science B.V. All rights reserved 97)00933-9
understood either in the nuclear reactions occurring in the core of the Sun or in the neutrino physics 131. In fact, the measured neutrino fluxes turn out to be less than expected and their different energy components seem to be suppressed in a different way. Other detectors will be running during the next years with the aim of sampling the neutrino energy spectrum in various regions and of detecting neutrinos of any flavour f4, 51. Among these, Borexino [5] will be a liquid scintillator based detector designed to detect low energy neutrinos of every flavour and particularly
54
F. Elisei et al. / Nucl. Instr. and Meth. in Phys. Res. A 400 (1997) 5348
the mon~omatic neu~nos (0.86 MeV) o~ginated in the reaction 7Be + e- = 7Li + v in the sun ( 7Be neutrinos). The aim of this paper is to discuss how the characteristics of the liquid scintillator influence the Borexino performances and to show the results of the optical properties measmements of different candidate mixtures. The paper describes the results of a research carried out in collaboration in four laboratories, using different techniques and methodologies, to study the performances of different scintillator mixtures chosen after a pre-selection based on the optical properties measured under UV excitation. The comparison of the results obtained by the four groups gives important information on the absorption and emission properties of the scintillators and offers the criteria for a good choice of an optimized scintillator mixture to be used in the Borexino experiment.
2. General requirements of the Borexiuo scintillator Neutrinos will be detected by Borexino mainly through their elastic scattering on electrons. The monocromatic 7Be neutrinos will originate a recoil electron energy spectrum extending from 0 to about 0.7MeV. Despite the high neutrinos flux, the low value of the cross section imposes the use of a large mass detector. The scintillator volume of Borexino will be 300 tons and the number of neutrinos interactions predicted by the Standard Solar Model is 50 events/day in the fiducial volume. The scintillator will be contained by a thin spherical (8.5 m diameter) nylon vessel viewed by about 2000 photomultipliers. Borexino will measure the electron energy spectrum by recording the photomultiplier charge signals. The selection of the mentioned low rate and low energy signals requires a very careful control of the background. The contribution from cosmic rays will be suppressed by placing the detector underground (Laboratori Nazionali Gran Sasso, Italy) and here the scintillator volume will be shielded in each direction from the residual background coming from the rocks by about 4 m of ultrapure water. At the end, the main con~ibutions to the background signals come from the radioactivity of all the materials used to construct and to shield the detector (external background) and
from the activity of the s~~ntillatoritself (internal background). A software reconstruction of the scintillation event position will be used to define a fiducial volume contained inside the overall scintillator volume and to eliminate residual external background signals. The spatial re~ons~ction wil be provided by measuring the arrival time of the signals on the photomultipliers. The internal background will be tagged by looking for correlated events: clol and pcl cascades linking intermediate nuclei of the natural radioactive decay series can be selected if the lifetime is convenient. A further background subtraction will be performed by recognizing c1signals of not correlated events by means of the different time response of a scintillator to electrons and heavy particles. The amount of others impurities in equilibrium with a taggable nuclide will then be inferred. A prototype of the Borexino detector (called Counting Test Facility, CTF) has been mounted in the Gran Sasso Laboratory and has been running for about two years 161. It is clear that the optical properties of the scintillator largely influence the Borexino performances. The main requirements for the Borexino scintillator can be summarized as follows: (a) it must be as free as possible from radioactive contaminants. Chemical and radiochemical purification methods will be used to enhance the ra~op~~ before filling the detector vessel and during the run of the experiment. To facilitate the use of an online purification system a mixture having a chemical compositions as simple as possible should be selected. For this reason our research has been concentrated on mixtures made by only one solvent, a primary fluor and a wavelength shifter, if needed. (b) The detection of the mentioned low energy signals requires to maximize the amount of light reaching the photomultipliers. This means that the scintillator must have a light yield as high as possible and it must be as transparent as possible to its light. The number of detected photoelectrons &h determines the energy resolution AE/E of the apparatus (AE/E is proportional to l/m) and this is particularly important in the low energy region of the spectrum where the p signals of the 14Cdecay (end point equal to 156 keV) should be identified.
I? Elisei et al. / Nucl. Ins@. and Meth. in Phys. Rex A 400 (1997) 5.668
(c) The decay time of the s~intil~~torlight must be as fast as possible to allow the spatial reconstruction necessary to define the fiducial volume. Neglecting for the moment the possible contribution of phenomena of light absorption and reemission, the difference of the time of arrival of the signals on the photomultipliers are due to difference in the light pathlengths (the maximum difference - 8.5 m - originates a time difference of 42.5 ns), to the transit time spread of the photomultipliers (about 1 ns for each of them in our conditions) and to the fluctuations in the decay time of the scintillator light. Scintillators having light decay time longer than a few ns strongly spoil the space resolution. For a given scintillator time response, the spatial resolution depends on the number of detected photoelectrons: the requirement of a high yield and transparent mixtures is then linked to the problem of the event position reconstm~tion. (d) Pulse shape discrimination will be one of the more powerful tools for background tagging. Therefore another parameter largely affecting the choice of the scintillator is its response to CIparticles: the light quenching by charged particles must be known as a function of their energy and the amount of light emitted in the delayed component typical of a liquid scintillator response to heavy particles should be as high as possible. (e) Finally the scintillator must be available in large quantities with a reasonable cost. The choice of the scintillator will clearly be a compromise among the discussed requirements. Many measurements on different mixtures have been performed. The results concerning the optical properties (light yield, a~enuation properties, time response and 01 quenching) are reported here without discussing the radiopurity data.
3. Choice of the solvents and fluors The optical properties of the candidate mixtures have been experimentally investigated by two different methods: ( 1) the first one [7] is based on the optical excitation of the solvent in the UV region and on the measurement of the optical properties in the spectral region corresponding to the sensitivity curve of the EM1 935 1 photomultipliers that have been selected for Borexino.
55
(2) in the second method the scintillator excitation is obtained by using ionizing particles [8, 91. The first method allows the measurement of intrinsic properties of the solvents and fluors (emission intensity and extinction coefficients as a function of the wavelength, fluorescence lifetimes and energy transfer efficiencies) that are of fundamental importance to predict the behaviour of mixtures with different fluor concentrations. The second method gives only an integrated response but leads to results which are obtained in conditions more similar to the Borexino ones. Being the results obtained by the two methods in good agreement, and being the first one simpler and faster, we used it to investigate the properties of most of the scintillators. Before studying the behaviour of the scintillation mixtures a careful characterization of each separate component (solvent, primary fluor and secondary fluor) has been performed. The fluorescence spectra I&A) and quantum yields @ro have been measured in a front-face geometry (where the optical path x is equal to zero) by a Spex Fluorolog-2 mod.FL 112 spectrofluorimeter and the fluorescence lifetimes rr have been measured by an Edingurg Instruments 199s spectro~uo~meter, based on the single photon counting method, with a time resolution of 0.3 ns. For the solvents and fluors, the attenuation lengths have been measured by a Perkin Elmer Lambda 16 spectrophotometer using cells of optical path varying in a range of four orders of ma~i~de, from 0,001 to 10 cm, thus being able to measure the entire absorption spectrum with the same solution. The intensity Z(J.,x) of a monochromatic light beam travelling inside the liquid under investigation can be described by the Lambert-Beer law 1(&X) = 1()(A)x 10-ee(~)X,
(1)
where ~(1~) is the molar extinction coefficient, c the molar concentration and x the optical path. The attenuation length A(,?) at each wavelength ,Ais given by the usual relationship
(2) The A values are markedly dependent on the wavelength. It has to be noted that the values measured at i longer than 410-430 nm (depending on the solvent), using the longest cells of 10 cm path, correspond to absorbances less than 0.01; therefore, their uncertainty
56
F. Elisei et al. / Nucl. Instr. and Meth. in Phys. Rex A 400 (1997) 53-68
Table 1 Solvent properties: fluorescence quantum yield @so (relative to that of the mixture PC/BIBUQ 4.34gdn-3), lifetime rr (ns), and attenuation length A(A) (m) are listed values for some distilled deaerted solvent? (the values in parenthesis refer to aerated solvents). The wavelength values are in nm. Solventa
@FO
TF
A(320)
A(365)
A(424)
A(460)
C
0.18
25.0
0.96 (0.34)
PC PX p-PX/MCHb o-PX ml-PX EB
0.40 0.32 0.38 0.19 0.20 0.15
28.0 28.8 28.7 23.6 27.9 18.4
0.84 (0.04) 1.3 (0.06) 1.9 (0.15) 1.8 (0.09) 1.7 (0.09) 2.5 (0.3)
4.8 3.0 3.7 5.0 4.9 5.0 6.6
10.6 6.8 8.7 10.3 9.2 9.0 9.4
16.1 11.1 15.5 16.7 14.5 14.0 14.5
a Fluka samples b 50/50 (v/v).
(4.7) (0.64) (1.3) (2.8) (2.3) (2.3) (6.5)
(10.3) (6.9) (8.0) (10.1) (9.4) (9.4) (10.0)
(17.4) (11.1) (14.0) (16.7) (14.5) (14.5) (15.5)
> 99%.
can be very high, considering the sensitivity and scattered light of the normal spectrophotometers. Therefore, the attenuation lengths measured at 1 longer than 400nm should be considered as lower limits; however, we believe that their relative values are reliable. 3.1. Solvents Several aromatic solvents have been investigated to find the best compromise among the various requirements mentioned in Section 2. Fluorescence quantum yield @~a and lifetime rr, transparency, thermal and photochemical stability are the properties mainly investigated for MN (1-methylnaphthalene), A (methoxybenzene or anisole), M (1,3,5-trymethylbenzene or mesitylene), p-X ( 1,4_dimethylbenzene or para-xylene), o-X ( 1,2-dimethylbenzene or orthoxylene), m-X ( 1,3-dimethylbenzene or meta-xylene), PC (1,2,4,-trimethylbenzene or pseudocumene), C (isopropylbenzene or Cumene), EB (ethylbenzene). The solvents have been examined as received by the producers and also after purification by distillation. In a few cases, other purification methods, such as water extraction and chemical purification (treatment with LiCl in methanol) have been tested. Some of these aromatics showed formation of long-lived poor emitting excimers. The latter, however, negatively affects the light output and decay time of the pure solvents, but should not reduce the energy transfer efficiency from the solvent to the primary fluor in the presence of rela-
tively high fluor concentration (> 1OW2mol dmh3 ). On the basis of this preliminary investigation, Xs, PC, C, EB have been found to have the best transmittance in the region of interest and have been then investigated in deeper detail. Since oxygen forms contact complexes with some of these aromatics with a worsening of the transmittance at the red edge of their first electronic absorption band (So - $), the study has been carried out in de-aerated solvents by bubbling nitrogen or argon through them. Table 1 reports the fluorescence properties and the attenuation lengths at four 1 values for some of the solvent investigated. Their comparison led to the following considerations. EB and Xs have good optical properties and low prices, but too low flash points, moreover, p-X has a too high melting point (1213°C). C shows very good optical properties but an higher instability in the presence of some fluors. PC displays long enough attenuations lengths and high chemical stability: for these reasons, it was chosen for the preliminary tests in the Borexino prototype (CTF [lo]) and is a good candidate for the final detector. 3.2. Fluors Several organic compounds have been characterized in view of their possible use as primary solutes: PBD (2-Phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole), Buthyl-PBD (2-(4-biphenylyl)-5-(4-tert-butyl-phenyl) -1,3,4-oxadiazole, PPO (2,5 diphe-nyloxazole),
F. Elisei et al. / Nucl. Ins&. and Merh. in Phys. Rex A 400 (1997)
PMP (1-phenyl-3-mesityl-2-pyrazoline), TP ( 1,4diphenylbenzene or para terphenyl) or A-shifters, bis-MSB (~-his-(o-methylsty~l)-be~ene), BBOT (2, 5-bis-[5-te~-bu~lbe~oxazolyl](2~thio-phene, BiBuQ (4,4”‘-his-(2-butyloctyloxy)-p-quater-phenyl, POPOP (p-bis[2-(5phenyloxazolyl)l benzene. The requirements of these compunds are high @FO and short rr, large Stokes shift, high stability and sufficient solubility. The fluorescence efficiency of these fluors has been measured in different experimental conditions: (1) in dilute solutions, by direct excitation in their absorption band; (2) in concentrated solutions, again by direct excitation monitoring the emission by reflectance (front-face geometry); (3) in concentrated solutions, again by reflectance, under direct excitation of the solvent at 267nm and monitoring their emission after the energy transfer from the solvent to the fluor. The photophysical parameters obtained by the second procedure in concentrated solutions of de-aerated PC were already reported [7] for most of the fluors. They generally showed high emission yield (>O.S) and lifetimes of the order of few nanoseconds.
270
57
53-68
340
410
480
550
35
45
X/rim
lo4 1
3.3. Scintillator mixtures The mixtures have been studied under excitation at /z = 267 nm (corresponding to the absorption region of the solvents investigated) and with different solute concentrations. The energy of the excited solvent is transferred to the high concentrated primary fluor mostly by a nonradiative mechanism. In a few cases a A-shifter at a much smaller concen~ation was added to these binary solutions. The excitation energy in this case is transferred to the secondary solute by both non radiative and radiative mechanisms. On the basis of the results obtained with the twocomponent mixtures, we concluded that the best compromise between high light output and short lifetime was offered by PC/PPO. Mixtures containing PMP, characte~zed by a larger Stokes shift, showed a smaller self-absorption but a longer lifetime and a spectrum which does not match well the sensitivity curve of the photomultipliers. It has to be noted that PCjPPO solutions, checked after a period of about two years, have been found to show unchanged optical properties (spectra, emission yields, lifetimes and attenuation lengths).
II 5
ib)
25
15
time/l
O-‘5
Fig. I. (a) Emission spectrum of PC (1) and of PCjPPO mixtures: [PPO] =0.37(2), 0.75(3), 1.5(4), 3(5) and 6(6) gdm-“; the excitation is obtained with i, = 267 nm. The numbers in parenthesis are the labels of the curves in the plot. (b) Emission decay of the PCjPPO mixtures: [PPO] = 6( I), 3(2), 1S(3). 0.75(4), 0.37(5) gdne3; the mixtures are excited at A = 267 nm.
The solute concentration largely influences the scintillator response and the performances of the Borexino detector. In fact, an increase in the fluor concen~ation leads to an increase in the solvent to solute energy transfer efficiency and a decrease in the fluor lifetime but also, as it will be described in Section 4), because of the self absorption process influences the light propagation properties. The first two effects are illustrated in Fig. l(a) and l(b) for the system PC/PPO. For this scintillator a practically complete energy transfer
F. Elisei et al. i Nucl. Ins&. and Meth. in Phys. Rex A 400 (19973 5368
58
efficiency (>95%) has been reached at the highest fluor concentmt~on (6 gdmF3) while the decay time shortens to the intrinsic lifetime measured by direct excitation of the fluor.
0.8 F
4. Xight propagation inside the scintillator mixture To evaluate the self absorption effects, the fluorescence of the mixtures has been studied as a function of the optical path. Following the first method described in Section 3), the fluorescence intensity &o(A) of the solution was recorded in front face geometry (in this condition the excitation light is absorbed within a path of few pm and the system detects the light emitted at 22’ with respect to the direction of the primary beam) and the extinction coefficients of the solvents and solutes have been measured. Then the quantum yield at various pathlengths in the direction of the primary beam can be calculated by
300
400
500
600
Xlnm Pig. 3. Emission spectra, corrected for the sensitivity profile of the EM1 9351 photomultiplier, of PC/PPO (6 g dmm3) measured in front face geometry (absorption path equal to zero - curve 1) and calculated at various pathlengths d in the direction of the primary beam.(d=1cm(2),d=5cm(3),d=10cm(4),d=20cm(5), d =50cm (6), d = IOOcm (7) and d =200cm (8).
@F(x) @FoJ lFO@) exp-
la ~~(E(~%)C+EI(~)CI+EZ(~)CZ)X d;l .j- ho(n)
dJ.
’
1.0
c
1
(3) where @FOis the quantum yield at x = 0, E,~1,~2 and c, cl, c2 refer to the solvent, the primary fluor and the secondary fluor if present. Fig. 2 shows the attenuation lengths of PC and PPO as a function of the waveI
0.1
1 !
0
,
,
50
,
,
,
100
150
,
,
J
200
optical path/cm Fig. 4. Light output of PC/TP (3 gdme3) (triangles) and PCjPPO (1.5 gdmw3) (circles) as a function of the optical path.
400
300
500
X/rim Fig. 2. Attenuation lengths obtained by method 1 for PC (curve 1) and PPO (1.5) gdme3 (curve 2).
lengths and Fig. 3 shows how the self-absorption effect modifies the light spectrum at various pathlengths: the emission yield is reduced and the red part of the spectrum is enhanced by increasing the optical path. The amount of light reaching the distance x from the emission point depends both on the fluors and solvent as Fig. 2 shows: PPO (and generally the primary fluor) is
F Elisei et al. / Nucl. Instr. and h4eth. in Phys. Res. A 400 (1997)
5348
59
Table 2 Scintillator properties obtained by method 1: The table lists the I and L values (expressed in m) of some scintillator mixtures together with the emission lifetime 5~ (ns) and the fluorescence quantum yield @a (under solvent excitation with 1 exe = 267 nm relative to that of PC/BIBUQ 4.34 g dnm3 ). At the concentrations used, the fluorescence output (measured in front-face geometry) is practically that of the primary fluor, the energy transfer to the I-shifter being negligible in these conditions. However, in the bulk solution, this light is completely absorbed by the d-shifter within a volume of a few cm3 (radiative transfer). The observed yields were then corrected for both the quantum efficiency of the i-shifter (0.96 for bis-MSB and 0.90 for BBOT) and the sensitivity curve of the photomultipliers, thus obtaining the @gX values in the last column. Solvent/solute P(JTP(3.0) PrJTP(3.0) PC/PPO(6.0) PC/PPO(6.0) PC/PPO(6.0) PC/PPO(l.S) PC/TP( 1.5) PC/TP(l.S)
(g dme3)
i-shifter
(g dme3 )
bis-MSB(O.02) bis-MSB(0.02) BBOT(0.0276) bis-MSB(0.02) BBOT(0.0276)
1
L
0.19f2 0.17f2 0.17f2 0.16f2 0.23 f 2 0.17f2 0.17f2 0.23 f 2
2.35 5.06 2.62 4.38 5.00 3.25 4.98 5.58
responsible for the light absorption at short distances x while the long distance light behaviour is mainly influenced by PC (and generally by the solvent). As a consequence of that, the @r(x) curves obtained by relation (3) show an initial fast light decrease followed by a slower light attenuation (see Fig. 4). At least for x longer than a few cm the mentioned curves can be described by a two exponential law IF(X) = Al exp( -x/Z) + A2 exp( -x/L),
(4)
where 1 and L can be considered “effective attenuation lengths” independent of ;1: 1 is due to the self absorption of short wavelength photons by the fluor while the L values is mainly (but not only) influenced by the solvent. Table 2 reports the I and L values obtained using relation (4) and the fluorescence parameters measured by method 1) described in Section 3), for some of the mixtures investigated. Among the binary mixtures, PC/PPO( 1.5 g dm-3 ) offered acceptable values for both L and rr and therefore has been chosen for the CTF experiment. Further investigations (Table 2), have been performed using as fluor an hydrocarbon without heteroatoms, such as TP, which may present the advantage of a better inertness and an easier purification. Then the effect of the addition of a I-shifter to both the PC/PPO and PC/TP mixtures has been examined. Being the absorption spectrum of TP blue-shifted compared with
zt f * f It zt It &
0.06 0.07 0.06 0.07 0.09 0.06 0.07 0.10
TF
@FO
@cm FO
2.1 2.7 2.0 2.6 2.7 3.6 3.9 4.0
0.87 0.87 0.79 0.79 0.79 0.73 0.73 0.73
0.83 0.71 0.79 0.65 0.55 0.73 0.60 0.50
the PPO one, its emission ligth can be absorbed by the solvent at a larger extent compared with PPO. Therefore the addition of the i-shifter becomes necessary using TP. For this reason, we started to study the behaviour of a three component mixture containing PC/TP/bis-MSB. Fig. 5 shows the absorption and fluorescence spectrum of TP and the corresponding spectra of bis-MSB. The good overlap of the emission spectrum of TP and the absorption of MSB ensures that practically all the light emitted by TP is absorbed by the shifter within about 10 cm. Fig. 6 shows similar spectra for the system PC/PPO/bis-MSB. In this case, the emission spectrum of PPO is red-shifted compared with TP: this implies that a fraction of its emission is directly seen from the photomultiplier without being absorbed by the shifter. Fig. 7 shows similar spectra for PC/PPO using a different shifter, BBOT, with a situation similar to PC/TP/bis-MSB. Fig. 8 shows the light output versus optical path in the direction of the primary beam for the system PC/TP with and without addition of the shifter bisMSB. The curves with the shifter has been calculated by assuming that all the light from TP is absorbed by MSB and converted into MSB fluorescence. The improvement in L in the presence of the shifter is evident. It has to be noted that, as expected, such improvement is accompanied by an increase in the fluorescence lifetime. Similar parameters for the system PC/PPO using
60
F. Elisei et al. / hkl.
Instr. and Meth. in Phys. Res. A 400 (1997)
53-68
0.75 iti ii +j 0.50 0 4 0.25
0.25
0.00
300
400
500
600
400
300
X/rim
500
600
Xlnm
Fig. 5. Absorption (fub lines) and emission (dashed lines) spectra of TP (1) and bis-MSB (2) in PC.
Fig. 7. Absorption (full lines) and emission (&shed lines) spectra of PPO (1) and BBOT (2) in PC.
1.00
0.75 p: ii -ff 0.50
5: 54 0.25
0.00 300
400
500
600
X/nm Fig. 6. Absorption (fall lines) and emission (dashed lines) spectra of PPO (1) and bis-MSB (2) in PC.
different shifters, MSB and BBOT, are reported in Table 2. In the latter case we obtained the best L value among those derived from the measurements in our laboratories. The relationship described by Eq. (4) has been experimen~lly found using the second method mentioned in Section (3). Fig. 9 schematically shows the experimental apparatus described in detail elsewhere [8]. Briefly, it consists of a cylindrical Pyrex tube (2 m long and 8 cm in diameter) viewed by two EM1 935 1 photomultipliers and filled by the scintillator. A CS’~~ source, emitting 667 keV y, can be moved parallel to the tube axis and excites the scintillator in different positions. An external NaI counter allows the selectwo
O.lt_,
,
0
,
,
50
(
,
100
optical
I
150
/i
,I 200
path/cm
Fig. 8. Light output versus optical path in the direction of the primary beam for PCjPT (3 gdme3) (circles) and PC/TP (3 g dm-3)/bis-MSB(0.02 g dnp3) (squares).
tion of events with the Compton scattered photons at an angle of 150” f 15”. The time difference between the signals reaching the two photomultipliers and the charge collected by each of them can be measured. The geometry of the apparatus ensures that the geometrical efficiency is almost position independent [7] and so the light a~nuation properties can be easily measured. A two exponential attenuation law has been experimentally found for all the mixtures investigated. See Fig. 10 as an example. Due to the possible presence
61
F. Elisei et al. ,!Nucl. Instr. and Meth. in Phys. Res. A 400 (1997) 53-68
Fig. 9. Experimental apparatus used for the light propagation measurements following method 2.
0
Source position (cm) Fig. 10. Light yield as a function of the source position measured by method 2 for a sample of the PC/PPO mixtures used in CTF.
of impurities, large fluctuations of the L values have been found for mixtures having the same composition but a good agreement with the values obtained by method 1) has been reached after a proper purification of the samples. Table 3 reports some results (see next section for the explanation of the data reported in the last column). The presence of dust has
been found to negatively influence the mixture transparency especially for the system PCjPMP and, for this reason, all the scintillator samples were filtered with two nylon filters (having 0.2 and 0.5 pm holes) before the insertion into the glass tube. Due to the superposition of the absorption and emission spectra, in a large scintillator sample the
62
F. Elisei et al. / Nucl. Instr. and Meth. in Phys. Rex A 400 (1997) 5368
Table 3 I(m) and L(m) values obtained by method 2 and number of photoelectrons close to PMTl Solvent/solute
(g dm-3)
i-shifter
(g dme3)
PC/PMP(2.5) PrJPMP(2.5) PC/PPO(5.5) PC/PPO( 1.5) PC/PPO( 1.5) PC/PPOa( 1.5)
bis-MSB(0.02) -
(Ns) measured
when the source is 5 cm far from the tube end
1
L
Filter
NS
0.43 f 0.1 0.21 Et 0.02
5.0 f 0.5 1.9 f 0.2
yes no
5.14 f 0.08 -
0.10 0.11 0.22 0.12
1.8 2.0 3.6 3.3
yes yes yes no
4.14 4.07 4.42 4.33
f f f f
0.03 0.04 0.05 0.03
f It zt f
0.2 0.3 0.3 0.3
f 0.07 f 0.08 f 0.05 l 0.03
a Sample of the mixture used in CTF
photons absorbed by the fluor can be reemitted more times before leaving the scintillator volume. The cascade process stops when the energy of the reemitted photons falls out of the fluor absorption band. This is a well known phenomenon [ 10, 141 whose importance depends on different factors: among these there are the Stokes shift, the fluor concentration and its quantum efficiency as well as the geometry of the system. The reemission phenomenon influences the scintillator time response, the spectrum shape and the attenuation properties in a way which depends on the detection geometry and on the volume of the sample. The contribution of the reemission phenomenon to the results of the measurements obtained by the optical excitation method is expected to be negligible due to the particular geometry of the system. In fact, no evidence for this effect has been found by this method. Some indications for the presence of such process come from the time response measured by method 2). The difference of the arrival time of the photomultiplier pulses has been measured using the apparatus of Fig. 9 as a function of the source position. Fig. 11 is a typical plot obtained when the arrival time is defined as the time when the pulses reach 40% of their maximum amplitude. In absence of reemission, a linear plot is expected. Deviations from the linear law have been observed when the source is close to the end of the tube. A careful analysis excluded a geometrical effect origin while the presence of some reemission process can explain these data. In fact, the reemission mechanism transforms a pointlike scintillation source in an extended source (a cloud) having dimensions related to the mean free path for the absorption by PPO. What
is expected to be linear is the relationship between the mentioned photomultipliers time difference and the position corresponding to the center of mass of the cloud. When the cloud is axially contained inside the tube, the latter coincides with the source position but this is not true when the source is close to the tube end. Now the center of mass of the cloud is located more inside the tube than the source position. No deviations from linearity have been found when the time difference has been measured by a low threshold discriminator, that is when the time has been defined as the arrival time of the first photoelectron. This fact indicates that, as expected, the first photoelectron detected in each event comes from the primary scintillation. The propagation of the light inside the cylindrical detector has been studied by a Montecarlo code using the emission spectra and extinction coefficients measured by method 1 ), including the photomultiplier quantum efficiency versus wavelength and the geometry of the detector. The PPO reemission process has been included assuming that the emission spectrum is independent of the absorbed wavelength and that the PPO quantum efficiency is equal to the value obtained by the optical method (0.82). The calculation reproduces the behaviour of the experimental data as a function of the excitation point and shows that 20% of the detected light reaches the photomultipliers after at least one reemission process. As regard to the solvent behaviour, it has to be noted that the E(A) values measured by method 1) include contributions from inelastic light scattering processes (leading to photon disappearence) and elastic light scattering leading only to a change in the direction of
63
F: Elisei et al. 1 Nucl. Instr. and Meth. in Phys. Res. A 400 (1997) 5348 T
-7
_:_
.I.
.:.
60 ‘.............:
._...................... _;_
..
\
‘:. _;.
.;_
.;.
.i .
_;. i+
0
2
4
i 6
+
L
10
6 time
difference
(ns)
Fig. Ii. Plot of the source position versus time difference measured by method 2 for PC/PPO (3 gdm-‘). The time is measured when the photomultiplier pulses reach 40% of their maximum amplitudes. The source position is measured from the end tube close to PMTI A linear fit of the data with the sonrce position ranging from 100 to 40 cm (solid line) and its extrapolation (dotted line) for lower values of the source position is shown.
propagation of the photons. The first process dominates for short wavelengths while the second one is important for the longest A of the spectrum. Because the elastic scatte~ng is ch~acterized by mean free path of the order of several tenths of cm or meters, in the used apparatus both the elastic and inelastic scattering practically lead to a loss of photons in the x direction and so the relative contribution of these two kinds of processes cannot be distin~ished, The important point is that a spherical detector, like Borexino or CTF, will collect the light in every direction and so the amount of light reaching the surface of a sphere of radius R, due to a scintillation in its center, should be higher than that obtained using relation (4) with x = R. In others words, when the elastic scattering processes are the dominant ones, the L values measured by the described apparatus cannot be interpreted as “long distance global
attenuation length” in Borexino. Anyway, L is a parameter useful for comparing the performances of different mixtures.
5. Light yield The last column of Table 3 reports the number Ns of photoelectrons measured by method 2) when the source is 5cm away from the end of the tube close to PMTl . The comparison of the Montecarlo and the experimental data, taking into account the photom~tiplier response, indicates that the absolute yield of the sample of the mixture used in CTF should be 11500 f 1000 y/MeV. As already discussed in Ref. [S], only relative yields are reliable because no attempt has been made to have an absolute calibration of the light yield measured by the apparatus.
64
F Elisei et al. 1 Nucl. Instr. and Meth. in Phys. Rex A 400 (1997)
6. Scintillator time response and pulse shape discrimination The determination of the time evolution of the scintillator response under a and p irradiation, has been performed using the single photon sampling technique described in Refs. [ 11,121. The apparatus which accomplishes this method can be realized observing a small scintillator volume (we used a cell having linear dimensions of about 0.5 cm) by means of two photomultipliers. One tube, closely coupled to the scintillator, detects all the occurring events, thus providing the trigger signal, while the other is weakly coupled to the sample and observes practically only one photoelectron. The determination of the light waveshape is obtained by measuring the time difference between the two photomultiplier signals for a huge number of events. Actually the measured curve is the convolution between the decay curve of the scintillator and the resolution function of the apparatus, which is mainly constituted by the transit time jitter of the photomultiplier [13]. Ref. [9] gives a detailed description. Additional sources of loss in resolution are the residual time walk of the constant fraction discriminator and the uncertainty in the triggering time of the high level tube. The resolution function of our apparatus has been evaluated as described in Ref. [9] and it is characterized by a FWHM of 1.6 ns. 6.1.
Time measurement results
In addition to the measurements performed using the mixtures interesting for Borexino, some tests have been carried using the standard scintillator NE213, i.e. the commercial standard for a/p discrimination. Small traces of oxygen in the liquid could influence the time decay of the light [14]. Therefore, as usual, oxygen was removed from the liquids degassing the samples by bubbling nitrogen trough them. Electron excitation was achieved by using a 137Cs y source. The y interacts in the scintillator through Compton scattering and the Compton scattered electrons are the origin of the scintillation light. A 210Po source emitting 5.3 MeV 01 has been also adopted. Fig. 12 shows as an example, the average light profiles experimentally determined for the mixture PC/PPO (1.5 g dmp3) both for cxand fl irradiation. In
53-68
order to extract from the data the true scintillator time decay function, a suitable deconvolution procedure taking into account the apparatus resolution, has been performed. Tables 4 and 5 report the results of the measurements. The time signals s(t) have been fitted using a multiexponential model expressed by ni
1”
s(t) =
z zI exp -t/Zi.
(5)
In practice we found that three components are enough to describe the curves obtained through electron irradiation, while the a excited curves are better reproduced with four term fits. The first component is related to the actual lifetime of the fluorescent state and, because of the energy transfer process, to the solute concentration. The other terms have been introduced to describe the long tail due to the slow portion of the pulse, which is essentially connected with the delayed fluorescence mechanism. It is important to note that, for all the mixtures investigated, zt is very similar for c1 and p irradiation and it is also essentially equal to the value obtained by luminescence spectrometry, as can be noted by comparing the results reported in Table 2 and Table 5 for the PC/PPO (6 g dmp3) and PC/PPO (1.5 g dn-3 ) scintillators. Another important point is the difference in the tails for p and c1 excitation: while for the former the tail accounts only for a negligible fraction of light, in the latter it comprises an amount of it which can be almost half of the total. 6.2. Evaluation of the pulse shape discrimination properties Given the shapes measured with the previous setup, it is possible to carry out a quantitative prediction of the pulse shape discrimination (PSD) capability which can be obtained with a specific scintillator. The calculation to be performed consists in the determination of the spread of the probability density function of the amount of signal in the tail for ct and p, or, in other words, the probability of the number of photoelectrons in the tail in both cases of excitation of the scintillator. In the calculation performed, the tail has been assumed to start 30 ns after the beginning of the pulse.
65
F. Elisei et iai./ Nucl. Instr. and Met& im Phys. Res. A 400 (1997) 53-68
Scintillation pulse shape
10
1 200
0
600
400
1000
800
1200
1400
1600
Time (ns) Fig. 12. Experimental light profiles measured for PCjPPO (1.5 gdtn3)
for CLand electron excitation.
Table 4 Fit results for time curves obtained under e- excitation. The time values are expressed in ns. The errors are discussed in Ref. ]8] Solvents/solute (g dmw3 )
rl
rz
B
41
42
43
PC/PMP(G.O) pX/PMP(6.4) PC/PPO(6.0) pX/PPO(6.6) PC/PPO(3.0) PC/PPO( 1.5) NE213
2.99 2.90
8.65 8.37 8.12 6.87 11.35 17.61 24.28
54.34 72.89 71.25 71.47 108.7 59.50 73.62
0.915 0.907 0.856 0.852 0.941 0.895 0.912
0.080 0.089 0.131 0.138 0.05 0.063 0.045
0.005
1.75 1.54 2.18 3.57 3.86
0.004 0.013 0.01 0.009 0.042 0.043
Table 5 Fit results for decay curves obtained under a excitation. The time values are expressed in ns. The errors are discussed in Ref. [8] Solvents/solute PC,‘F’MP(&O) pX/PMP(6.6) PC/PPO(c;.O) pX/PPO(6.6) PC/PPo( 3 .O) PC/PPO( I .5) NE213
(g dmm3 )
“1
72
t3
z4
41
42
43
94
3.02 2.71 2.03 1.44 2.19 3.25 3.89
16.64 14.55 13.1 10.52 12.02 13.49 20.60
64.39 58.21 56.19 49.09 56.13 59.95 92.36
374.2 340.1 399.6 335.1 433.6 279.1. 440.0
0.55 0.5 I 0.625 0.616 0.636 0.630 0.470
0.124 0.14 0.162 0.165 0.153 0.178 0.223
0.144 0.161 0.108 0.119 0.104 0.119
0.182 0.189 0.105 0.100 0.107 0.073
0.191
0.116
66
F. Eltiei et al. /Nucl. Instr. and Meth. in Phys. Rex A 400 (1997) 53-68
The degree of overlap of the two distributions represents the inherent discrimination capability of the scintillator. The details of such an approach are described for example in Ref. [ 1511.The kind of dis~bution obtained by this method is illustrated in Fig. 13 in the case of the mixture PC/PPO (1.5 g dm-3), which is the one adopted for CTF, and for a total number of 150 photoelectrons making up the signal. The choice of the pulse height for this example is due to the fact that it represents the amplitude range of the a background signal expected in Borexino; therefore Fig. 13 represents what kind of separation could be achieved in Borexino on the basis of the intrinsic feature of the scintillator. The actual determination of the probability of a identification and of p misidentification depends upon a choice of the discrimination threshold. If we assume, for example, that the threshold is equal to 12 photoelectrons (e.g., a signal is assumed to be due to an electron if in the tail it has less than 12 photoelectrons, or to an a if its tail comprises more than 12 photoelectrons), than from the two distributions displayed in the figure it results that the cc signals are identified with the extreme high efficiency of 99.7%, while the loss of electrons pulses due to an erroneous identification is about 0.4%. Actually, this extremely good separation is limited by some factors as the lack of perfect homogenei~ in the scintillator response over its entire volume, the pathlength spread introduced by the elastic scattering and the effects at the boundaries of the various media in detector which cause an additional broadening of the two dis~butions thus increasing the amount of overlap. For the purpose of comparing the different scintillators, however, it is enough to evaluate in a quantitative way how much the two ideal dis~butions are overlapped. This is usually performed by introducing a factor of merit parameter D defined as
D= d&T
(6)
where AS represents the distance between the peaks of the two distributions, and oe and [T, their standard deviations. A scintillator exhibits good enough discrimination capabili~ if the parameter D is at least equal to 1. D depends upon the intrinsic pulse shape of the scintillator and on the pulse height: indeed, the separation between the two curves is enhanced as the num-
Table 6 PSD factor of merit D for various scintillators tested for cl/p separation Soiven~/so~ute (g dx~-~ ) 50 photoel. IO0photoel. I 50 photoef PC/PMP(6.0) pX/PMP(6.6) PC/PPO(6.0) pX/PPO(6.6) PC/PPO(3.0) PC/PPO( 1S) NE213
3.89 3.86 2.37 2.50 2.76 2.41 3.64
5.71 5.69 3.35 3.47 3.34 3.16 5.35
6.92 6.88 4.33 4.25 4.32 3.77 6.63
ber of photoelectrons increases. The various factors of merit of the mixtures investigated are listed in Table 6 for the three different pulse heights of 50, 100 and 150 photoelectrons. From these data it appears that all the evaluated scintillators show good disc~mination capability, which, as expected, increases with the number of photoelectrons. However, it should be noted that the two cocktails containing PMP are characterized by better pulse shape discrimination pe~o~ances than those containing PPO: indeed, they are in the same range of performances of the optimized PSD scintillator NE213. On the other hand, there is practically no difference between the two solvents, i.e. PC and p-X, and there is also no essential dependence upon the solute concentration, as desumed from the comparison of the data reported in Tables 4-6. In conclusion, the results obtained point out that even the less powerti PSD scintillators among those tested assure a degree of discrimination more than adequate to fulfill the experimental requirements of Borexino.
7. a Quenching measurements It is well known that light emission of scintillators in response to CLparticle irradiation is strongly suppressed in comparison with electron irradiation of the same energy [14]. The number of created photons is typically one order of magnitude lower for c1than for j3 excitation at equal energy deposition. One usually defines an electron equivalent energy of the 01particle and refers to the ratio between c1energy and electron equivalent energy as the c1 quenching factor Q. The value of Q depends on the ionization density and hence
l? E&i
et al. 1 Nucl. Instr. and Meth.
in Phys. Res. A 400 (1997)
53-68
0.06
3’,
. _ 4. ._
50
60
Number of photoelectrons in the tail Fig. 13. Distribution of the number of photoelectrons of 150 photoelectrons.
in the tail for ~1 and p excitation
on the 01energy. F~he~ore, the quenching depends on the specific scintillator solution and is unknown a priori. The c1 quenching, (see Table 7), has been determined from the comparison of the pulse heights of 222Rn (5.49 MeV) and its short lived daughters 218Po (6.0 MeV) and 214Po (768MeV) with the pulse heights of the Compton energies of 137Cs(4’78keV) and 54Mn (640 keV). For this purpose the scintillator has been loaded with the noble gas 222Rn.A 226Raemanation source has been immersed in about 100cm3 of scintillator. After the insertion of the source the residual oxygen, dissolved in the scintillator, has been removed by flushing with nitrogen. The typical time of exposure of the emanation source to the scintillator was 24 h. Thereafter, the source has been removed and the scintillator transferred into a quartz glass flask for measuring. In order to avoid contact with ambient air all operations have been carried out in nitrogen atmosphere. The flask has been subsequently optically coupled to a photomultiplier of type EMI 9351. In order to increase the homogeneity of the
in the scintillator
PC/PPO
(1.5 gdmm3 ), for pulses
Table 7 Quenching factors. The pulse height response of PC/PPO (1.5 gdmm3) to cL particles of different pressed in terms of the equivalent electron energy. factor Q represents the ratio between the c1 energy alent electron energy
the scintillator energies is exThe quenching and the equiv-
CLenergy (MeV)
Equiv. b energy (keV)
&
5.49 6.00 7.68
477 545 826
11.5f0.2 ll.Of0.1 9.3 f 0.2
light collection, the walls of the measuring flask have been coated with titanium oxide reflection paint. The detector response with respect to electron interaction has been determined by Compton scattering, The energy deposition has been defined by selecting Compton events with the photon being scattered in backward directions. This has been realized by a coincidence setup with a NaI detector and the liquid scintillator. The obtained pulse height
68
F. E&i
et al. / Nucl. Instr. and h4eth. in Phys. Rex A 400 (1997) 5368
distributions, measured with the liquid scintillator, were gaussian shaped with centroid values at the corresponding Compton edge energies. With this technique one can measure simultaneously the detector response to c1particle interaction as well as to electron interaction. Furthermore, both a and y interactions are homogeneously distributed in space within the scintillator. This method minimizes systematic uncertainties related to inhomogeneities in light collection, to pulse height drifts with time, or to quenching related to the introduction of chemically dissolved sources into the scintillator.
8. Conclusions The results obtained in the four laboratories allowed a complete characterization of the chemical stability, optical properties, light output, a- fl discrimination and c1 quenching factors of a number of scintillator mixtures. This information has been useful to evaluate the criteria for a good choice of the composition which is being used in CTF and will be used in the Borexino experiment. The optical measurements, based on the direct excitation of the solvent, led to a pre-selection of the best candidates among a large number of solvents and fluors and to the choice of suitable concentrations. This reduced the number of mixtures to be investigated in more detail in the time-consuming experiments with radioactive sources using much larger volumes of scintillator. Parallel experiments with 01and p particles, carried out on some optimized mixtures, gave results in good agreement with those obtained by optical excitation. This study showed that the best solvent is probably PC and the simplest two-component mixture is PC/PPO. Addition of a third component, e.g. the L-shifter bis-MSB, could be useful to increase the light output. In such case, it would be convenient to use TP instead of PPO as primary fluor since its emission spectrum offers a better overlap with the absorption spectrum of bisMSB, thus assuring a complete energy transfer to the secondary fluor. The results of this study evidenced the importance of specific phenomena (such as the absorption-
reemission of light emitted by the fluors and the possible contribution of the elastic scattering of the light in the system) which will be better investigated by using the three-dimensional detector of CTF. The comparison of the present data with those obtained in CTF will allow us to reach a complete understanding of the behaviour of large-scale scintillation detectors. The results of such comparison will be described in a subsequent paper.
References [l] B.T. Cleveland et al., Nucl. Phys. B 38 (1995) 47; P. Anselmann et al., Phys. Lett. B 327 (1994) 377; J. Abdurashitov et al., Phys. Lett. B 328 (1994) 234; Y. Suzuki et al., Nucl. Phys. B 38 (1995) 54; Y. Fukuda et al., Phys. Rev. Lett. 77 (1996) 1683. [2] J.N. Bahcall, Neutrino Astrophysics, Cambridge University Press, Cambridge, 1989; S. Turck-Chieze et al., Astrophys. J. 408 (1993) 347; J.N. Bahcall et al., Rev. Mod. Phys. 67 (1995) 1. [3] PI. Krastev et al., Phys. Rev. D 53 (1996) 1665; J.N. Bahcall et al., Phys. Lett. B 29 (1969) 623; S. Mikheyev et al., Sov. J. Nucl. Phys. 42 (1985) 913. [4] M. Takita, Frontiers of Neutrino Astrophysics, Y. Suzuki, K. Nakamura (Eds.), Universal Academy Press, Tokyo, 1993, p. 147; H.H Chen, Phys. Rev. Lett. 55 (1985); B.T. Cleveland, Proc. 23rd Int. Cosmic Ray Conf., Calgary, 1993. [5] C. Arpesella et al., Borexino at Gran Sasso: proposal for a real time detector for low energy solar neutrinos, Milano, 1991. [6] G. Alimonti et al., Nucl. Instr. and Meth., submitted. [7] F. Masetti, F. Elisei, U. Mazzucato, J. Lumin. 68 (1996) 15. [8] F. Gatti, G. Morelli, G. Testera, S. Vitale, Nucl. Instr. and Meth. A 370 (1996) 609. [9] G. Ranucci et al., Nucl. Instr. and Meth. A 350 (1994) 338. [lo] H.K. Holt, Phys. Rev. A 13 (4) (1976) 1142; J.M.G. Martinho et al., J. Chem. Phys. 90 (1) (1989) 53; P. Wiorkowski et al., Opt. Comm. 53 (4) (1985) 217; M.N. Barberas-Santos et al., J. Chem. Phys. 103 (8) (1995) 3022. [ll] L.M. Bollinger, G.E. Thomas, Rev. Sci. Instrum. 32 (1961) 1044. [12] Y. Koechlin, Thesis, University of Paris, 1961. [13] J. Kirkbride et al., Nucl. Instr. and Meth. 52 (1967) 293. 1141 J.B. Birks, The Theory and Practice of Scintillation Counting, Pergamon Press, Oxford, 1964. [15] G. Ranucci, Nucl. Instr. and Meth. A 354, 1995, p. 389.
&rk4mmDs
IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A 400 (1997) 53-68
Measurements of liquid scintillator properties for the Borexino detector F. Elisei”, F. Gattib, A. GorettiC, T. Hagnerd, F. Masetti”, U. Mazzucato”, G. Ranucci”, S. Schoenertd, G. Testerab**, P. Ullucci”, S. Vitaleb a Dipartimento di Chimica dell’liniversita’ and Sezione INFN di Perugia, 06123 Perugia, Ita1.y b Dipartimento di Fisica dell’Uniuersita’ and Sezione INFN di Geneva, 16146 Genoua, fta(v c Dipartimento di Fisica dell’~nivers~ta~ and Sezione INFN di ~i~ana, 20133 ~ila~a, Italy d Technische Universitaet Muenchen, 85747 Garching, Germany
Received 23 April 1997; received in revised form 21 May 1997
Abstract The optical properties of various scintillators (both single components and their mixtures) have been measured in order to choose the scintillator composition offering the best compromise among the requirements of the large volume detector of the Borexino experiment on solar neutrinos. A pre-selection of the most interesting binary or ternary mixtures has been carried out by using a relatively simple and fast optical method. This is based on the W excitation of the solvent and the observation of the emission properties in the spectral region of interest. The results obtained allowed the light output, response time and attenuation lengths to be evaluated for a broad range of mixtures. The mixtures offering the best properties for Borexino have been investigated in greater depth by using ionizing radiation as excitation source. The results were found to be in good agreement with those obtained by the optical method. In addition, the cx quenching and the time response of the scintillators to 01and p particles have been tested to evaluate the a/p discrimination capability. The comparison of the results obtained in four laboratories using different techniques and methodologies, offered an exhaustive picture of the scintillator properties and provided the criteria for a good choice of the mixture for Borexino. PACS:
42.20.E; 29.40.M; 96.60.K
1. Introduction Various detectors have shown that the Sun is a source of neutrinos [l]. This fact confirms the main hypothesis of our model of the Sun [Z] and shows that energy is really produced in stars by fusion processes. On the other hand, the experimental data indicate that there are still phenomena not completely
*Corresponding author. Tel.: +39 103536433: fax: +39 10 313358; e-mail: [email protected]. 0168-9002/97/$17.00
PII SO I68-9002(
0 1997 Elsevier Science B.V. All rights reserved 97)00933-9
understood either in the nuclear reactions occurring in the core of the Sun or in the neutrino physics 131. In fact, the measured neutrino fluxes turn out to be less than expected and their different energy components seem to be suppressed in a different way. Other detectors will be running during the next years with the aim of sampling the neutrino energy spectrum in various regions and of detecting neutrinos of any flavour f4, 51. Among these, Borexino [5] will be a liquid scintillator based detector designed to detect low energy neutrinos of every flavour and particularly
54
F. Elisei et al. / Nucl. Instr. and Meth. in Phys. Res. A 400 (1997) 5348
the mon~omatic neu~nos (0.86 MeV) o~ginated in the reaction 7Be + e- = 7Li + v in the sun ( 7Be neutrinos). The aim of this paper is to discuss how the characteristics of the liquid scintillator influence the Borexino performances and to show the results of the optical properties measmements of different candidate mixtures. The paper describes the results of a research carried out in collaboration in four laboratories, using different techniques and methodologies, to study the performances of different scintillator mixtures chosen after a pre-selection based on the optical properties measured under UV excitation. The comparison of the results obtained by the four groups gives important information on the absorption and emission properties of the scintillators and offers the criteria for a good choice of an optimized scintillator mixture to be used in the Borexino experiment.
2. General requirements of the Borexiuo scintillator Neutrinos will be detected by Borexino mainly through their elastic scattering on electrons. The monocromatic 7Be neutrinos will originate a recoil electron energy spectrum extending from 0 to about 0.7MeV. Despite the high neutrinos flux, the low value of the cross section imposes the use of a large mass detector. The scintillator volume of Borexino will be 300 tons and the number of neutrinos interactions predicted by the Standard Solar Model is 50 events/day in the fiducial volume. The scintillator will be contained by a thin spherical (8.5 m diameter) nylon vessel viewed by about 2000 photomultipliers. Borexino will measure the electron energy spectrum by recording the photomultiplier charge signals. The selection of the mentioned low rate and low energy signals requires a very careful control of the background. The contribution from cosmic rays will be suppressed by placing the detector underground (Laboratori Nazionali Gran Sasso, Italy) and here the scintillator volume will be shielded in each direction from the residual background coming from the rocks by about 4 m of ultrapure water. At the end, the main con~ibutions to the background signals come from the radioactivity of all the materials used to construct and to shield the detector (external background) and
from the activity of the s~~ntillatoritself (internal background). A software reconstruction of the scintillation event position will be used to define a fiducial volume contained inside the overall scintillator volume and to eliminate residual external background signals. The spatial re~ons~ction wil be provided by measuring the arrival time of the signals on the photomultipliers. The internal background will be tagged by looking for correlated events: clol and pcl cascades linking intermediate nuclei of the natural radioactive decay series can be selected if the lifetime is convenient. A further background subtraction will be performed by recognizing c1signals of not correlated events by means of the different time response of a scintillator to electrons and heavy particles. The amount of others impurities in equilibrium with a taggable nuclide will then be inferred. A prototype of the Borexino detector (called Counting Test Facility, CTF) has been mounted in the Gran Sasso Laboratory and has been running for about two years 161. It is clear that the optical properties of the scintillator largely influence the Borexino performances. The main requirements for the Borexino scintillator can be summarized as follows: (a) it must be as free as possible from radioactive contaminants. Chemical and radiochemical purification methods will be used to enhance the ra~op~~ before filling the detector vessel and during the run of the experiment. To facilitate the use of an online purification system a mixture having a chemical compositions as simple as possible should be selected. For this reason our research has been concentrated on mixtures made by only one solvent, a primary fluor and a wavelength shifter, if needed. (b) The detection of the mentioned low energy signals requires to maximize the amount of light reaching the photomultipliers. This means that the scintillator must have a light yield as high as possible and it must be as transparent as possible to its light. The number of detected photoelectrons &h determines the energy resolution AE/E of the apparatus (AE/E is proportional to l/m) and this is particularly important in the low energy region of the spectrum where the p signals of the 14Cdecay (end point equal to 156 keV) should be identified.
I? Elisei et al. / Nucl. Ins@. and Meth. in Phys. Rex A 400 (1997) 5.668
(c) The decay time of the s~intil~~torlight must be as fast as possible to allow the spatial reconstruction necessary to define the fiducial volume. Neglecting for the moment the possible contribution of phenomena of light absorption and reemission, the difference of the time of arrival of the signals on the photomultipliers are due to difference in the light pathlengths (the maximum difference - 8.5 m - originates a time difference of 42.5 ns), to the transit time spread of the photomultipliers (about 1 ns for each of them in our conditions) and to the fluctuations in the decay time of the scintillator light. Scintillators having light decay time longer than a few ns strongly spoil the space resolution. For a given scintillator time response, the spatial resolution depends on the number of detected photoelectrons: the requirement of a high yield and transparent mixtures is then linked to the problem of the event position reconstm~tion. (d) Pulse shape discrimination will be one of the more powerful tools for background tagging. Therefore another parameter largely affecting the choice of the scintillator is its response to CIparticles: the light quenching by charged particles must be known as a function of their energy and the amount of light emitted in the delayed component typical of a liquid scintillator response to heavy particles should be as high as possible. (e) Finally the scintillator must be available in large quantities with a reasonable cost. The choice of the scintillator will clearly be a compromise among the discussed requirements. Many measurements on different mixtures have been performed. The results concerning the optical properties (light yield, a~enuation properties, time response and 01 quenching) are reported here without discussing the radiopurity data.
3. Choice of the solvents and fluors The optical properties of the candidate mixtures have been experimentally investigated by two different methods: ( 1) the first one [7] is based on the optical excitation of the solvent in the UV region and on the measurement of the optical properties in the spectral region corresponding to the sensitivity curve of the EM1 935 1 photomultipliers that have been selected for Borexino.
55
(2) in the second method the scintillator excitation is obtained by using ionizing particles [8, 91. The first method allows the measurement of intrinsic properties of the solvents and fluors (emission intensity and extinction coefficients as a function of the wavelength, fluorescence lifetimes and energy transfer efficiencies) that are of fundamental importance to predict the behaviour of mixtures with different fluor concentrations. The second method gives only an integrated response but leads to results which are obtained in conditions more similar to the Borexino ones. Being the results obtained by the two methods in good agreement, and being the first one simpler and faster, we used it to investigate the properties of most of the scintillators. Before studying the behaviour of the scintillation mixtures a careful characterization of each separate component (solvent, primary fluor and secondary fluor) has been performed. The fluorescence spectra I&A) and quantum yields @ro have been measured in a front-face geometry (where the optical path x is equal to zero) by a Spex Fluorolog-2 mod.FL 112 spectrofluorimeter and the fluorescence lifetimes rr have been measured by an Edingurg Instruments 199s spectro~uo~meter, based on the single photon counting method, with a time resolution of 0.3 ns. For the solvents and fluors, the attenuation lengths have been measured by a Perkin Elmer Lambda 16 spectrophotometer using cells of optical path varying in a range of four orders of ma~i~de, from 0,001 to 10 cm, thus being able to measure the entire absorption spectrum with the same solution. The intensity Z(J.,x) of a monochromatic light beam travelling inside the liquid under investigation can be described by the Lambert-Beer law 1(&X) = 1()(A)x 10-ee(~)X,
(1)
where ~(1~) is the molar extinction coefficient, c the molar concentration and x the optical path. The attenuation length A(,?) at each wavelength ,Ais given by the usual relationship
(2) The A values are markedly dependent on the wavelength. It has to be noted that the values measured at i longer than 410-430 nm (depending on the solvent), using the longest cells of 10 cm path, correspond to absorbances less than 0.01; therefore, their uncertainty
56
F. Elisei et al. / Nucl. Instr. and Meth. in Phys. Rex A 400 (1997) 53-68
Table 1 Solvent properties: fluorescence quantum yield @so (relative to that of the mixture PC/BIBUQ 4.34gdn-3), lifetime rr (ns), and attenuation length A(A) (m) are listed values for some distilled deaerted solvent? (the values in parenthesis refer to aerated solvents). The wavelength values are in nm. Solventa
@FO
TF
A(320)
A(365)
A(424)
A(460)
C
0.18
25.0
0.96 (0.34)
PC PX p-PX/MCHb o-PX ml-PX EB
0.40 0.32 0.38 0.19 0.20 0.15
28.0 28.8 28.7 23.6 27.9 18.4
0.84 (0.04) 1.3 (0.06) 1.9 (0.15) 1.8 (0.09) 1.7 (0.09) 2.5 (0.3)
4.8 3.0 3.7 5.0 4.9 5.0 6.6
10.6 6.8 8.7 10.3 9.2 9.0 9.4
16.1 11.1 15.5 16.7 14.5 14.0 14.5
a Fluka samples b 50/50 (v/v).
(4.7) (0.64) (1.3) (2.8) (2.3) (2.3) (6.5)
(10.3) (6.9) (8.0) (10.1) (9.4) (9.4) (10.0)
(17.4) (11.1) (14.0) (16.7) (14.5) (14.5) (15.5)
> 99%.
can be very high, considering the sensitivity and scattered light of the normal spectrophotometers. Therefore, the attenuation lengths measured at 1 longer than 400nm should be considered as lower limits; however, we believe that their relative values are reliable. 3.1. Solvents Several aromatic solvents have been investigated to find the best compromise among the various requirements mentioned in Section 2. Fluorescence quantum yield @~a and lifetime rr, transparency, thermal and photochemical stability are the properties mainly investigated for MN (1-methylnaphthalene), A (methoxybenzene or anisole), M (1,3,5-trymethylbenzene or mesitylene), p-X ( 1,4_dimethylbenzene or para-xylene), o-X ( 1,2-dimethylbenzene or orthoxylene), m-X ( 1,3-dimethylbenzene or meta-xylene), PC (1,2,4,-trimethylbenzene or pseudocumene), C (isopropylbenzene or Cumene), EB (ethylbenzene). The solvents have been examined as received by the producers and also after purification by distillation. In a few cases, other purification methods, such as water extraction and chemical purification (treatment with LiCl in methanol) have been tested. Some of these aromatics showed formation of long-lived poor emitting excimers. The latter, however, negatively affects the light output and decay time of the pure solvents, but should not reduce the energy transfer efficiency from the solvent to the primary fluor in the presence of rela-
tively high fluor concentration (> 1OW2mol dmh3 ). On the basis of this preliminary investigation, Xs, PC, C, EB have been found to have the best transmittance in the region of interest and have been then investigated in deeper detail. Since oxygen forms contact complexes with some of these aromatics with a worsening of the transmittance at the red edge of their first electronic absorption band (So - $), the study has been carried out in de-aerated solvents by bubbling nitrogen or argon through them. Table 1 reports the fluorescence properties and the attenuation lengths at four 1 values for some of the solvent investigated. Their comparison led to the following considerations. EB and Xs have good optical properties and low prices, but too low flash points, moreover, p-X has a too high melting point (1213°C). C shows very good optical properties but an higher instability in the presence of some fluors. PC displays long enough attenuations lengths and high chemical stability: for these reasons, it was chosen for the preliminary tests in the Borexino prototype (CTF [lo]) and is a good candidate for the final detector. 3.2. Fluors Several organic compounds have been characterized in view of their possible use as primary solutes: PBD (2-Phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole), Buthyl-PBD (2-(4-biphenylyl)-5-(4-tert-butyl-phenyl) -1,3,4-oxadiazole, PPO (2,5 diphe-nyloxazole),
F. Elisei et al. / Nucl. Ins&. and Merh. in Phys. Rex A 400 (1997)
PMP (1-phenyl-3-mesityl-2-pyrazoline), TP ( 1,4diphenylbenzene or para terphenyl) or A-shifters, bis-MSB (~-his-(o-methylsty~l)-be~ene), BBOT (2, 5-bis-[5-te~-bu~lbe~oxazolyl](2~thio-phene, BiBuQ (4,4”‘-his-(2-butyloctyloxy)-p-quater-phenyl, POPOP (p-bis[2-(5phenyloxazolyl)l benzene. The requirements of these compunds are high @FO and short rr, large Stokes shift, high stability and sufficient solubility. The fluorescence efficiency of these fluors has been measured in different experimental conditions: (1) in dilute solutions, by direct excitation in their absorption band; (2) in concentrated solutions, again by direct excitation monitoring the emission by reflectance (front-face geometry); (3) in concentrated solutions, again by reflectance, under direct excitation of the solvent at 267nm and monitoring their emission after the energy transfer from the solvent to the fluor. The photophysical parameters obtained by the second procedure in concentrated solutions of de-aerated PC were already reported [7] for most of the fluors. They generally showed high emission yield (>O.S) and lifetimes of the order of few nanoseconds.
270
57
53-68
340
410
480
550
35
45
X/rim
lo4 1
3.3. Scintillator mixtures The mixtures have been studied under excitation at /z = 267 nm (corresponding to the absorption region of the solvents investigated) and with different solute concentrations. The energy of the excited solvent is transferred to the high concentrated primary fluor mostly by a nonradiative mechanism. In a few cases a A-shifter at a much smaller concen~ation was added to these binary solutions. The excitation energy in this case is transferred to the secondary solute by both non radiative and radiative mechanisms. On the basis of the results obtained with the twocomponent mixtures, we concluded that the best compromise between high light output and short lifetime was offered by PC/PPO. Mixtures containing PMP, characte~zed by a larger Stokes shift, showed a smaller self-absorption but a longer lifetime and a spectrum which does not match well the sensitivity curve of the photomultipliers. It has to be noted that PCjPPO solutions, checked after a period of about two years, have been found to show unchanged optical properties (spectra, emission yields, lifetimes and attenuation lengths).
II 5
ib)
25
15
time/l
O-‘5
Fig. I. (a) Emission spectrum of PC (1) and of PCjPPO mixtures: [PPO] =0.37(2), 0.75(3), 1.5(4), 3(5) and 6(6) gdm-“; the excitation is obtained with i, = 267 nm. The numbers in parenthesis are the labels of the curves in the plot. (b) Emission decay of the PCjPPO mixtures: [PPO] = 6( I), 3(2), 1S(3). 0.75(4), 0.37(5) gdne3; the mixtures are excited at A = 267 nm.
The solute concentration largely influences the scintillator response and the performances of the Borexino detector. In fact, an increase in the fluor concen~ation leads to an increase in the solvent to solute energy transfer efficiency and a decrease in the fluor lifetime but also, as it will be described in Section 4), because of the self absorption process influences the light propagation properties. The first two effects are illustrated in Fig. l(a) and l(b) for the system PC/PPO. For this scintillator a practically complete energy transfer
F. Elisei et al. i Nucl. Ins&. and Meth. in Phys. Rex A 400 (19973 5368
58
efficiency (>95%) has been reached at the highest fluor concentmt~on (6 gdmF3) while the decay time shortens to the intrinsic lifetime measured by direct excitation of the fluor.
0.8 F
4. Xight propagation inside the scintillator mixture To evaluate the self absorption effects, the fluorescence of the mixtures has been studied as a function of the optical path. Following the first method described in Section 3), the fluorescence intensity &o(A) of the solution was recorded in front face geometry (in this condition the excitation light is absorbed within a path of few pm and the system detects the light emitted at 22’ with respect to the direction of the primary beam) and the extinction coefficients of the solvents and solutes have been measured. Then the quantum yield at various pathlengths in the direction of the primary beam can be calculated by
300
400
500
600
Xlnm Pig. 3. Emission spectra, corrected for the sensitivity profile of the EM1 9351 photomultiplier, of PC/PPO (6 g dmm3) measured in front face geometry (absorption path equal to zero - curve 1) and calculated at various pathlengths d in the direction of the primary beam.(d=1cm(2),d=5cm(3),d=10cm(4),d=20cm(5), d =50cm (6), d = IOOcm (7) and d =200cm (8).
@F(x) @FoJ lFO@) exp-
la ~~(E(~%)C+EI(~)CI+EZ(~)CZ)X d;l .j- ho(n)
dJ.
’
1.0
c
1
(3) where @FOis the quantum yield at x = 0, E,~1,~2 and c, cl, c2 refer to the solvent, the primary fluor and the secondary fluor if present. Fig. 2 shows the attenuation lengths of PC and PPO as a function of the waveI
0.1
1 !
0
,
,
50
,
,
,
100
150
,
,
J
200
optical path/cm Fig. 4. Light output of PC/TP (3 gdme3) (triangles) and PCjPPO (1.5 gdmw3) (circles) as a function of the optical path.
400
300
500
X/rim Fig. 2. Attenuation lengths obtained by method 1 for PC (curve 1) and PPO (1.5) gdme3 (curve 2).
lengths and Fig. 3 shows how the self-absorption effect modifies the light spectrum at various pathlengths: the emission yield is reduced and the red part of the spectrum is enhanced by increasing the optical path. The amount of light reaching the distance x from the emission point depends both on the fluors and solvent as Fig. 2 shows: PPO (and generally the primary fluor) is
F Elisei et al. / Nucl. Instr. and h4eth. in Phys. Res. A 400 (1997)
5348
59
Table 2 Scintillator properties obtained by method 1: The table lists the I and L values (expressed in m) of some scintillator mixtures together with the emission lifetime 5~ (ns) and the fluorescence quantum yield @a (under solvent excitation with 1 exe = 267 nm relative to that of PC/BIBUQ 4.34 g dnm3 ). At the concentrations used, the fluorescence output (measured in front-face geometry) is practically that of the primary fluor, the energy transfer to the I-shifter being negligible in these conditions. However, in the bulk solution, this light is completely absorbed by the d-shifter within a volume of a few cm3 (radiative transfer). The observed yields were then corrected for both the quantum efficiency of the i-shifter (0.96 for bis-MSB and 0.90 for BBOT) and the sensitivity curve of the photomultipliers, thus obtaining the @gX values in the last column. Solvent/solute P(JTP(3.0) PrJTP(3.0) PC/PPO(6.0) PC/PPO(6.0) PC/PPO(6.0) PC/PPO(l.S) PC/TP( 1.5) PC/TP(l.S)
(g dme3)
i-shifter
(g dme3 )
bis-MSB(O.02) bis-MSB(0.02) BBOT(0.0276) bis-MSB(0.02) BBOT(0.0276)
1
L
0.19f2 0.17f2 0.17f2 0.16f2 0.23 f 2 0.17f2 0.17f2 0.23 f 2
2.35 5.06 2.62 4.38 5.00 3.25 4.98 5.58
responsible for the light absorption at short distances x while the long distance light behaviour is mainly influenced by PC (and generally by the solvent). As a consequence of that, the @r(x) curves obtained by relation (3) show an initial fast light decrease followed by a slower light attenuation (see Fig. 4). At least for x longer than a few cm the mentioned curves can be described by a two exponential law IF(X) = Al exp( -x/Z) + A2 exp( -x/L),
(4)
where 1 and L can be considered “effective attenuation lengths” independent of ;1: 1 is due to the self absorption of short wavelength photons by the fluor while the L values is mainly (but not only) influenced by the solvent. Table 2 reports the I and L values obtained using relation (4) and the fluorescence parameters measured by method 1) described in Section 3), for some of the mixtures investigated. Among the binary mixtures, PC/PPO( 1.5 g dm-3 ) offered acceptable values for both L and rr and therefore has been chosen for the CTF experiment. Further investigations (Table 2), have been performed using as fluor an hydrocarbon without heteroatoms, such as TP, which may present the advantage of a better inertness and an easier purification. Then the effect of the addition of a I-shifter to both the PC/PPO and PC/TP mixtures has been examined. Being the absorption spectrum of TP blue-shifted compared with
zt f * f It zt It &
0.06 0.07 0.06 0.07 0.09 0.06 0.07 0.10
TF
@FO
@cm FO
2.1 2.7 2.0 2.6 2.7 3.6 3.9 4.0
0.87 0.87 0.79 0.79 0.79 0.73 0.73 0.73
0.83 0.71 0.79 0.65 0.55 0.73 0.60 0.50
the PPO one, its emission ligth can be absorbed by the solvent at a larger extent compared with PPO. Therefore the addition of the i-shifter becomes necessary using TP. For this reason, we started to study the behaviour of a three component mixture containing PC/TP/bis-MSB. Fig. 5 shows the absorption and fluorescence spectrum of TP and the corresponding spectra of bis-MSB. The good overlap of the emission spectrum of TP and the absorption of MSB ensures that practically all the light emitted by TP is absorbed by the shifter within about 10 cm. Fig. 6 shows similar spectra for the system PC/PPO/bis-MSB. In this case, the emission spectrum of PPO is red-shifted compared with TP: this implies that a fraction of its emission is directly seen from the photomultiplier without being absorbed by the shifter. Fig. 7 shows similar spectra for PC/PPO using a different shifter, BBOT, with a situation similar to PC/TP/bis-MSB. Fig. 8 shows the light output versus optical path in the direction of the primary beam for the system PC/TP with and without addition of the shifter bisMSB. The curves with the shifter has been calculated by assuming that all the light from TP is absorbed by MSB and converted into MSB fluorescence. The improvement in L in the presence of the shifter is evident. It has to be noted that, as expected, such improvement is accompanied by an increase in the fluorescence lifetime. Similar parameters for the system PC/PPO using
60
F. Elisei et al. / hkl.
Instr. and Meth. in Phys. Res. A 400 (1997)
53-68
0.75 iti ii +j 0.50 0 4 0.25
0.25
0.00
300
400
500
600
400
300
X/rim
500
600
Xlnm
Fig. 5. Absorption (fub lines) and emission (dashed lines) spectra of TP (1) and bis-MSB (2) in PC.
Fig. 7. Absorption (full lines) and emission (&shed lines) spectra of PPO (1) and BBOT (2) in PC.
1.00
0.75 p: ii -ff 0.50
5: 54 0.25
0.00 300
400
500
600
X/nm Fig. 6. Absorption (fall lines) and emission (dashed lines) spectra of PPO (1) and bis-MSB (2) in PC.
different shifters, MSB and BBOT, are reported in Table 2. In the latter case we obtained the best L value among those derived from the measurements in our laboratories. The relationship described by Eq. (4) has been experimen~lly found using the second method mentioned in Section (3). Fig. 9 schematically shows the experimental apparatus described in detail elsewhere [8]. Briefly, it consists of a cylindrical Pyrex tube (2 m long and 8 cm in diameter) viewed by two EM1 935 1 photomultipliers and filled by the scintillator. A CS’~~ source, emitting 667 keV y, can be moved parallel to the tube axis and excites the scintillator in different positions. An external NaI counter allows the selectwo
O.lt_,
,
0
,
,
50
(
,
100
optical
I
150
/i
,I 200
path/cm
Fig. 8. Light output versus optical path in the direction of the primary beam for PCjPT (3 gdme3) (circles) and PC/TP (3 g dm-3)/bis-MSB(0.02 g dnp3) (squares).
tion of events with the Compton scattered photons at an angle of 150” f 15”. The time difference between the signals reaching the two photomultipliers and the charge collected by each of them can be measured. The geometry of the apparatus ensures that the geometrical efficiency is almost position independent [7] and so the light a~nuation properties can be easily measured. A two exponential attenuation law has been experimentally found for all the mixtures investigated. See Fig. 10 as an example. Due to the possible presence
61
F. Elisei et al. ,!Nucl. Instr. and Meth. in Phys. Res. A 400 (1997) 53-68
Fig. 9. Experimental apparatus used for the light propagation measurements following method 2.
0
Source position (cm) Fig. 10. Light yield as a function of the source position measured by method 2 for a sample of the PC/PPO mixtures used in CTF.
of impurities, large fluctuations of the L values have been found for mixtures having the same composition but a good agreement with the values obtained by method 1) has been reached after a proper purification of the samples. Table 3 reports some results (see next section for the explanation of the data reported in the last column). The presence of dust has
been found to negatively influence the mixture transparency especially for the system PCjPMP and, for this reason, all the scintillator samples were filtered with two nylon filters (having 0.2 and 0.5 pm holes) before the insertion into the glass tube. Due to the superposition of the absorption and emission spectra, in a large scintillator sample the
62
F. Elisei et al. / Nucl. Instr. and Meth. in Phys. Rex A 400 (1997) 5368
Table 3 I(m) and L(m) values obtained by method 2 and number of photoelectrons close to PMTl Solvent/solute
(g dm-3)
i-shifter
(g dme3)
PC/PMP(2.5) PrJPMP(2.5) PC/PPO(5.5) PC/PPO( 1.5) PC/PPO( 1.5) PC/PPOa( 1.5)
bis-MSB(0.02) -
(Ns) measured
when the source is 5 cm far from the tube end
1
L
Filter
NS
0.43 f 0.1 0.21 Et 0.02
5.0 f 0.5 1.9 f 0.2
yes no
5.14 f 0.08 -
0.10 0.11 0.22 0.12
1.8 2.0 3.6 3.3
yes yes yes no
4.14 4.07 4.42 4.33
f f f f
0.03 0.04 0.05 0.03
f It zt f
0.2 0.3 0.3 0.3
f 0.07 f 0.08 f 0.05 l 0.03
a Sample of the mixture used in CTF
photons absorbed by the fluor can be reemitted more times before leaving the scintillator volume. The cascade process stops when the energy of the reemitted photons falls out of the fluor absorption band. This is a well known phenomenon [ 10, 141 whose importance depends on different factors: among these there are the Stokes shift, the fluor concentration and its quantum efficiency as well as the geometry of the system. The reemission phenomenon influences the scintillator time response, the spectrum shape and the attenuation properties in a way which depends on the detection geometry and on the volume of the sample. The contribution of the reemission phenomenon to the results of the measurements obtained by the optical excitation method is expected to be negligible due to the particular geometry of the system. In fact, no evidence for this effect has been found by this method. Some indications for the presence of such process come from the time response measured by method 2). The difference of the arrival time of the photomultiplier pulses has been measured using the apparatus of Fig. 9 as a function of the source position. Fig. 11 is a typical plot obtained when the arrival time is defined as the time when the pulses reach 40% of their maximum amplitude. In absence of reemission, a linear plot is expected. Deviations from the linear law have been observed when the source is close to the end of the tube. A careful analysis excluded a geometrical effect origin while the presence of some reemission process can explain these data. In fact, the reemission mechanism transforms a pointlike scintillation source in an extended source (a cloud) having dimensions related to the mean free path for the absorption by PPO. What
is expected to be linear is the relationship between the mentioned photomultipliers time difference and the position corresponding to the center of mass of the cloud. When the cloud is axially contained inside the tube, the latter coincides with the source position but this is not true when the source is close to the tube end. Now the center of mass of the cloud is located more inside the tube than the source position. No deviations from linearity have been found when the time difference has been measured by a low threshold discriminator, that is when the time has been defined as the arrival time of the first photoelectron. This fact indicates that, as expected, the first photoelectron detected in each event comes from the primary scintillation. The propagation of the light inside the cylindrical detector has been studied by a Montecarlo code using the emission spectra and extinction coefficients measured by method 1 ), including the photomultiplier quantum efficiency versus wavelength and the geometry of the detector. The PPO reemission process has been included assuming that the emission spectrum is independent of the absorbed wavelength and that the PPO quantum efficiency is equal to the value obtained by the optical method (0.82). The calculation reproduces the behaviour of the experimental data as a function of the excitation point and shows that 20% of the detected light reaches the photomultipliers after at least one reemission process. As regard to the solvent behaviour, it has to be noted that the E(A) values measured by method 1) include contributions from inelastic light scattering processes (leading to photon disappearence) and elastic light scattering leading only to a change in the direction of
63
F: Elisei et al. 1 Nucl. Instr. and Meth. in Phys. Res. A 400 (1997) 5348 T
-7
_:_
.I.
.:.
60 ‘.............:
._...................... _;_
..
\
‘:. _;.
.;_
.;.
.i .
_;. i+
0
2
4
i 6
+
L
10
6 time
difference
(ns)
Fig. Ii. Plot of the source position versus time difference measured by method 2 for PC/PPO (3 gdm-‘). The time is measured when the photomultiplier pulses reach 40% of their maximum amplitudes. The source position is measured from the end tube close to PMTI A linear fit of the data with the sonrce position ranging from 100 to 40 cm (solid line) and its extrapolation (dotted line) for lower values of the source position is shown.
propagation of the photons. The first process dominates for short wavelengths while the second one is important for the longest A of the spectrum. Because the elastic scatte~ng is ch~acterized by mean free path of the order of several tenths of cm or meters, in the used apparatus both the elastic and inelastic scattering practically lead to a loss of photons in the x direction and so the relative contribution of these two kinds of processes cannot be distin~ished, The important point is that a spherical detector, like Borexino or CTF, will collect the light in every direction and so the amount of light reaching the surface of a sphere of radius R, due to a scintillation in its center, should be higher than that obtained using relation (4) with x = R. In others words, when the elastic scattering processes are the dominant ones, the L values measured by the described apparatus cannot be interpreted as “long distance global
attenuation length” in Borexino. Anyway, L is a parameter useful for comparing the performances of different mixtures.
5. Light yield The last column of Table 3 reports the number Ns of photoelectrons measured by method 2) when the source is 5cm away from the end of the tube close to PMTl . The comparison of the Montecarlo and the experimental data, taking into account the photom~tiplier response, indicates that the absolute yield of the sample of the mixture used in CTF should be 11500 f 1000 y/MeV. As already discussed in Ref. [S], only relative yields are reliable because no attempt has been made to have an absolute calibration of the light yield measured by the apparatus.
64
F Elisei et al. 1 Nucl. Instr. and Meth. in Phys. Rex A 400 (1997)
6. Scintillator time response and pulse shape discrimination The determination of the time evolution of the scintillator response under a and p irradiation, has been performed using the single photon sampling technique described in Refs. [ 11,121. The apparatus which accomplishes this method can be realized observing a small scintillator volume (we used a cell having linear dimensions of about 0.5 cm) by means of two photomultipliers. One tube, closely coupled to the scintillator, detects all the occurring events, thus providing the trigger signal, while the other is weakly coupled to the sample and observes practically only one photoelectron. The determination of the light waveshape is obtained by measuring the time difference between the two photomultiplier signals for a huge number of events. Actually the measured curve is the convolution between the decay curve of the scintillator and the resolution function of the apparatus, which is mainly constituted by the transit time jitter of the photomultiplier [13]. Ref. [9] gives a detailed description. Additional sources of loss in resolution are the residual time walk of the constant fraction discriminator and the uncertainty in the triggering time of the high level tube. The resolution function of our apparatus has been evaluated as described in Ref. [9] and it is characterized by a FWHM of 1.6 ns. 6.1.
Time measurement results
In addition to the measurements performed using the mixtures interesting for Borexino, some tests have been carried using the standard scintillator NE213, i.e. the commercial standard for a/p discrimination. Small traces of oxygen in the liquid could influence the time decay of the light [14]. Therefore, as usual, oxygen was removed from the liquids degassing the samples by bubbling nitrogen trough them. Electron excitation was achieved by using a 137Cs y source. The y interacts in the scintillator through Compton scattering and the Compton scattered electrons are the origin of the scintillation light. A 210Po source emitting 5.3 MeV 01 has been also adopted. Fig. 12 shows as an example, the average light profiles experimentally determined for the mixture PC/PPO (1.5 g dmp3) both for cxand fl irradiation. In
53-68
order to extract from the data the true scintillator time decay function, a suitable deconvolution procedure taking into account the apparatus resolution, has been performed. Tables 4 and 5 report the results of the measurements. The time signals s(t) have been fitted using a multiexponential model expressed by ni
1”
s(t) =
z zI exp -t/Zi.
(5)
In practice we found that three components are enough to describe the curves obtained through electron irradiation, while the a excited curves are better reproduced with four term fits. The first component is related to the actual lifetime of the fluorescent state and, because of the energy transfer process, to the solute concentration. The other terms have been introduced to describe the long tail due to the slow portion of the pulse, which is essentially connected with the delayed fluorescence mechanism. It is important to note that, for all the mixtures investigated, zt is very similar for c1 and p irradiation and it is also essentially equal to the value obtained by luminescence spectrometry, as can be noted by comparing the results reported in Table 2 and Table 5 for the PC/PPO (6 g dmp3) and PC/PPO (1.5 g dn-3 ) scintillators. Another important point is the difference in the tails for p and c1 excitation: while for the former the tail accounts only for a negligible fraction of light, in the latter it comprises an amount of it which can be almost half of the total. 6.2. Evaluation of the pulse shape discrimination properties Given the shapes measured with the previous setup, it is possible to carry out a quantitative prediction of the pulse shape discrimination (PSD) capability which can be obtained with a specific scintillator. The calculation to be performed consists in the determination of the spread of the probability density function of the amount of signal in the tail for ct and p, or, in other words, the probability of the number of photoelectrons in the tail in both cases of excitation of the scintillator. In the calculation performed, the tail has been assumed to start 30 ns after the beginning of the pulse.
65
F. Elisei et iai./ Nucl. Instr. and Met& im Phys. Res. A 400 (1997) 53-68
Scintillation pulse shape
10
1 200
0
600
400
1000
800
1200
1400
1600
Time (ns) Fig. 12. Experimental light profiles measured for PCjPPO (1.5 gdtn3)
for CLand electron excitation.
Table 4 Fit results for time curves obtained under e- excitation. The time values are expressed in ns. The errors are discussed in Ref. ]8] Solvents/solute (g dmw3 )
rl
rz
B
41
42
43
PC/PMP(G.O) pX/PMP(6.4) PC/PPO(6.0) pX/PPO(6.6) PC/PPO(3.0) PC/PPO( 1.5) NE213
2.99 2.90
8.65 8.37 8.12 6.87 11.35 17.61 24.28
54.34 72.89 71.25 71.47 108.7 59.50 73.62
0.915 0.907 0.856 0.852 0.941 0.895 0.912
0.080 0.089 0.131 0.138 0.05 0.063 0.045
0.005
1.75 1.54 2.18 3.57 3.86
0.004 0.013 0.01 0.009 0.042 0.043
Table 5 Fit results for decay curves obtained under a excitation. The time values are expressed in ns. The errors are discussed in Ref. [8] Solvents/solute PC,‘F’MP(&O) pX/PMP(6.6) PC/PPO(c;.O) pX/PPO(6.6) PC/PPo( 3 .O) PC/PPO( I .5) NE213
(g dmm3 )
“1
72
t3
z4
41
42
43
94
3.02 2.71 2.03 1.44 2.19 3.25 3.89
16.64 14.55 13.1 10.52 12.02 13.49 20.60
64.39 58.21 56.19 49.09 56.13 59.95 92.36
374.2 340.1 399.6 335.1 433.6 279.1. 440.0
0.55 0.5 I 0.625 0.616 0.636 0.630 0.470
0.124 0.14 0.162 0.165 0.153 0.178 0.223
0.144 0.161 0.108 0.119 0.104 0.119
0.182 0.189 0.105 0.100 0.107 0.073
0.191
0.116
66
F. Eltiei et al. /Nucl. Instr. and Meth. in Phys. Rex A 400 (1997) 53-68
The degree of overlap of the two distributions represents the inherent discrimination capability of the scintillator. The details of such an approach are described for example in Ref. [ 1511.The kind of dis~bution obtained by this method is illustrated in Fig. 13 in the case of the mixture PC/PPO (1.5 g dm-3), which is the one adopted for CTF, and for a total number of 150 photoelectrons making up the signal. The choice of the pulse height for this example is due to the fact that it represents the amplitude range of the a background signal expected in Borexino; therefore Fig. 13 represents what kind of separation could be achieved in Borexino on the basis of the intrinsic feature of the scintillator. The actual determination of the probability of a identification and of p misidentification depends upon a choice of the discrimination threshold. If we assume, for example, that the threshold is equal to 12 photoelectrons (e.g., a signal is assumed to be due to an electron if in the tail it has less than 12 photoelectrons, or to an a if its tail comprises more than 12 photoelectrons), than from the two distributions displayed in the figure it results that the cc signals are identified with the extreme high efficiency of 99.7%, while the loss of electrons pulses due to an erroneous identification is about 0.4%. Actually, this extremely good separation is limited by some factors as the lack of perfect homogenei~ in the scintillator response over its entire volume, the pathlength spread introduced by the elastic scattering and the effects at the boundaries of the various media in detector which cause an additional broadening of the two dis~butions thus increasing the amount of overlap. For the purpose of comparing the different scintillators, however, it is enough to evaluate in a quantitative way how much the two ideal dis~butions are overlapped. This is usually performed by introducing a factor of merit parameter D defined as
D= d&T
(6)
where AS represents the distance between the peaks of the two distributions, and oe and [T, their standard deviations. A scintillator exhibits good enough discrimination capabili~ if the parameter D is at least equal to 1. D depends upon the intrinsic pulse shape of the scintillator and on the pulse height: indeed, the separation between the two curves is enhanced as the num-
Table 6 PSD factor of merit D for various scintillators tested for cl/p separation Soiven~/so~ute (g dx~-~ ) 50 photoel. IO0photoel. I 50 photoef PC/PMP(6.0) pX/PMP(6.6) PC/PPO(6.0) pX/PPO(6.6) PC/PPO(3.0) PC/PPO( 1S) NE213
3.89 3.86 2.37 2.50 2.76 2.41 3.64
5.71 5.69 3.35 3.47 3.34 3.16 5.35
6.92 6.88 4.33 4.25 4.32 3.77 6.63
ber of photoelectrons increases. The various factors of merit of the mixtures investigated are listed in Table 6 for the three different pulse heights of 50, 100 and 150 photoelectrons. From these data it appears that all the evaluated scintillators show good disc~mination capability, which, as expected, increases with the number of photoelectrons. However, it should be noted that the two cocktails containing PMP are characterized by better pulse shape discrimination pe~o~ances than those containing PPO: indeed, they are in the same range of performances of the optimized PSD scintillator NE213. On the other hand, there is practically no difference between the two solvents, i.e. PC and p-X, and there is also no essential dependence upon the solute concentration, as desumed from the comparison of the data reported in Tables 4-6. In conclusion, the results obtained point out that even the less powerti PSD scintillators among those tested assure a degree of discrimination more than adequate to fulfill the experimental requirements of Borexino.
7. a Quenching measurements It is well known that light emission of scintillators in response to CLparticle irradiation is strongly suppressed in comparison with electron irradiation of the same energy [14]. The number of created photons is typically one order of magnitude lower for c1than for j3 excitation at equal energy deposition. One usually defines an electron equivalent energy of the 01particle and refers to the ratio between c1energy and electron equivalent energy as the c1 quenching factor Q. The value of Q depends on the ionization density and hence
l? E&i
et al. 1 Nucl. Instr. and Meth.
in Phys. Res. A 400 (1997)
53-68
0.06
3’,
. _ 4. ._
50
60
Number of photoelectrons in the tail Fig. 13. Distribution of the number of photoelectrons of 150 photoelectrons.
in the tail for ~1 and p excitation
on the 01energy. F~he~ore, the quenching depends on the specific scintillator solution and is unknown a priori. The c1 quenching, (see Table 7), has been determined from the comparison of the pulse heights of 222Rn (5.49 MeV) and its short lived daughters 218Po (6.0 MeV) and 214Po (768MeV) with the pulse heights of the Compton energies of 137Cs(4’78keV) and 54Mn (640 keV). For this purpose the scintillator has been loaded with the noble gas 222Rn.A 226Raemanation source has been immersed in about 100cm3 of scintillator. After the insertion of the source the residual oxygen, dissolved in the scintillator, has been removed by flushing with nitrogen. The typical time of exposure of the emanation source to the scintillator was 24 h. Thereafter, the source has been removed and the scintillator transferred into a quartz glass flask for measuring. In order to avoid contact with ambient air all operations have been carried out in nitrogen atmosphere. The flask has been subsequently optically coupled to a photomultiplier of type EMI 9351. In order to increase the homogeneity of the
in the scintillator
PC/PPO
(1.5 gdmm3 ), for pulses
Table 7 Quenching factors. The pulse height response of PC/PPO (1.5 gdmm3) to cL particles of different pressed in terms of the equivalent electron energy. factor Q represents the ratio between the c1 energy alent electron energy
the scintillator energies is exThe quenching and the equiv-
CLenergy (MeV)
Equiv. b energy (keV)
&
5.49 6.00 7.68
477 545 826
11.5f0.2 ll.Of0.1 9.3 f 0.2
light collection, the walls of the measuring flask have been coated with titanium oxide reflection paint. The detector response with respect to electron interaction has been determined by Compton scattering, The energy deposition has been defined by selecting Compton events with the photon being scattered in backward directions. This has been realized by a coincidence setup with a NaI detector and the liquid scintillator. The obtained pulse height
68
F. E&i
et al. / Nucl. Instr. and h4eth. in Phys. Rex A 400 (1997) 5368
distributions, measured with the liquid scintillator, were gaussian shaped with centroid values at the corresponding Compton edge energies. With this technique one can measure simultaneously the detector response to c1particle interaction as well as to electron interaction. Furthermore, both a and y interactions are homogeneously distributed in space within the scintillator. This method minimizes systematic uncertainties related to inhomogeneities in light collection, to pulse height drifts with time, or to quenching related to the introduction of chemically dissolved sources into the scintillator.
8. Conclusions The results obtained in the four laboratories allowed a complete characterization of the chemical stability, optical properties, light output, a- fl discrimination and c1 quenching factors of a number of scintillator mixtures. This information has been useful to evaluate the criteria for a good choice of the composition which is being used in CTF and will be used in the Borexino experiment. The optical measurements, based on the direct excitation of the solvent, led to a pre-selection of the best candidates among a large number of solvents and fluors and to the choice of suitable concentrations. This reduced the number of mixtures to be investigated in more detail in the time-consuming experiments with radioactive sources using much larger volumes of scintillator. Parallel experiments with 01and p particles, carried out on some optimized mixtures, gave results in good agreement with those obtained by optical excitation. This study showed that the best solvent is probably PC and the simplest two-component mixture is PC/PPO. Addition of a third component, e.g. the L-shifter bis-MSB, could be useful to increase the light output. In such case, it would be convenient to use TP instead of PPO as primary fluor since its emission spectrum offers a better overlap with the absorption spectrum of bisMSB, thus assuring a complete energy transfer to the secondary fluor. The results of this study evidenced the importance of specific phenomena (such as the absorption-
reemission of light emitted by the fluors and the possible contribution of the elastic scattering of the light in the system) which will be better investigated by using the three-dimensional detector of CTF. The comparison of the present data with those obtained in CTF will allow us to reach a complete understanding of the behaviour of large-scale scintillation detectors. The results of such comparison will be described in a subsequent paper.
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