A large-area position-sensitive detector for fast neutrons

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

Nuclear Instruments and Methods in Physics Research A 334 (1993) 495-503 North-Holland

Section A

A large-area position-sensitive detector for fast neutrons L. Lüdemann, K. Knoche, W. Scobel and K. Woller I. Institut für Experimentalphysik

Unioerstdt Hamburg, Hamburg, Germany

Received 8 April 1993

3

Six plastic-scintillator (BC408) neutron detectors of dimensions 10 x 10 x 100 cm with position sensitivity along their length have been developed. Test measurements of the time, position and pulse-height resolution as a function of position and amount of the deposited energy have been performed using a 3 GeV electron beam . For pulse-heights exceeding those of 9 MeV (40 MeV) protons, the resolution of the time difference Ot12 was better than 280 (150) ps, yielding a position resolution <<-4 cm (2 cm) FWHM . The effective pulse-height response varied less than 7% over the whole length of the detector . The detection efficiency was measured using a 2s2Cf source ; comparisons with the Monte Carlo code NEFF4 showed good agreement.

1. Introduction Nuclear reaction studies via neutron spectroscopy in the energy range from 5 McVP to more than 100 McVP, that is characteristic e.g . for the preequilibrium emission in light and heavy ion induced reactions [1], require not only the measurement of time-of-flight (TOF) and pulse-height, but also position information. For this purpose, large solid angles may either be covered with many small neutron detectors or with large-area detector arrays . If the neutron multiplicities allow, position-sensitive large-area detectors, mostly in form of plastic or liquid organic scintillators, are preferred. Their advantages over systems of many conventional neutron detectors rest in the continuous position information and in the considerable reduction of electronics equipment. For pulse-heights above 10 McVP, discrimination between y's and neutrons is usually dispensable, because the yield of y's drops by orders of magnitude. For low pulse-heights the discrimination properties of large volume liquid scintillators deteriorate anyway [2] and must be replaced, if necessary, by TOF discrimination . A frequently used detector geometry is that of a long, rectangular bar with photomultiplier (PM) readout on both sides, aligned perpendicularly to the incoming neutrons [3-6] . Such scintillator bars may be stacked to larger assemblies that are superior to quadratic units of same front area . The bars offer two possible ways [7] to derive a position information : (i) The reconstruction of the position x of the incoming particle can be performed by comparing the

measured propagation times TI , T2 of the scintillation light to the ends of the detector

ef +x o ,

x = (T2 - Tl) c

where ceff is the effective velocity of light in the scintillator and xo fixes the absolute position scale. At the same time, a position independent mean time Td for TOF purposes can be generated from the sum T I + T2 as Td

L =z(TI+T2) -- .

(2)

Ceff

(ii) Alternatively, position reconstruction may be derived from a comparison of the (normalized) pulseheights PHI, PH2 read out on either side ; in case of an exponential dependence PH(x) a exp(-x/.l) one obtains x=

2(

L- .lln

(PH2)) .

In this expresssion, a denotes the effective attenuation length and L the total length of the scintillator bar. A position independent pulse height PH d is given by the geometrical average, viz. PH d =

(PHI

x PH2)

a exp(-L/A) .

(4)

Method (i) is expected to describe the position more accurate, especially at the ends, where frequent reflections take place [8], and generally for designs with high attenuation lengths. In contrast, eq . (3) can be used to monitor a detector in its pulse height and timing stability [6].

0168-9002/93/$06 .00 C 1993 - Elsevier Science Publishers B.V . All rights reserved

49 6

L. Lüdemann et al. / Large-area PSD for fast neutrons

For high energy neutrons, an experimentally acceptable efficiency requires an appropriate detector thickness . In view of the then considerable scintillator volume, it is mandatory to inspect the detector performance with respect to light output, pulse height, time and spatial resolution . Radioactive sources placed on the detector surface are not appropriate, because they do not allow one to control the exact position of the deposited energy relative to both PMs and the trajectory length . Triggered cosmics are excluded because of count rate estimates. A collimated 3 GeV electron energy beam, however, is an excellent tool to test scintillators under conditions corresponding to those of applications involving recoil proton ranges of different lengths and depths in the scintillator material . An exact alignment of the detector relative to the electron beam is possible and the minimum ionizing electrons depose their energy uniformly along their track. In addition will a spatial dependence of the pulseheight have influence on the detector efficiency 77, that is a function of both neutron energy En and electronic pulse height threshold Ethr . Therefore an exact knowledge of the pulse-height response along the detector is a prerequisite for the elimination of the position dependence in q(E, Ethr) . Our paper is organized as follows: in section 2 we describe the mechanical design of the detector . Section 3 represents the main body of the paper and is devoted to studies with the 3 GeV electron test beam . In

section 4 we determine the neutron efficiency by means of a ZSZCf source and compare the results with a Monte Carlo simulation . 2. Detector design and test set-up Position sensitive scintillator bars for neutron TOF spectroscopy have been studied before . Most constructions made use of plastic scintillators [3-6,9]; in some applications, e.g . for evaporation neutron spectroscopy, liquid scintillators have been preferred [10,11]. General design criteria aim at high light output and attenuation length for good time and position resolution, high neutron detection efficiency i7(E , E t ,,) and modularity and have been amply discussed [3,10]. We have chosen the plastic scintillator material BC408 (bulk attenuation length d0 = 380 cm, decay constant r = 2.1 ns, H/C ratio 1.10, and 64% of the anthracene light output [12]) with dimensions 10 cm X 10 cm X 100 cm . The thickness is a compromise between good overall time resolution and high detector efficiency, the front area one between high solid angle coverage and good light collection [4]. The whole detector surface was polished, loosely wrapped with diffusely reflecting TYVEK [l3] and a light tight foil . The light output on both ends was transferred with light guides to photomultipliers (PMs) of 52 mm cathode diameter (Hamamatsu 329) that were equipped with 6

Paddle Cross 41

Veto Cross42 aperture

7 (m) -~ Distance ix

Ani

Dynl An2 Dyn2

Fig. 1 . Schematic setup of scintillator bar at the 3 GeV electron test beam, with beam defining scintillators, electronics for time and pulse height read out on both ends, and data acquisition.

49 7

L. Lüdemann et al. / Large-area PSD for fast neutrons

conventional voltage dividers [14] . The optical contacts consisted of cement (BC600) and grease (Dow Corning 607), respectively. The light guide was given a length of 10.3 cm, that minimizes the time resolution [3,5,15,16]. Two surface variants were tested : type I was covered with TiO, paint (NE561), type 11 was polished and wrapped with TYVEK, too. Six identical detectors were built. The mechanical design and support allowed us to stack them in units of 2, 3, and 6 bars on top of each other. Scintillator paddles in front consisting of 4 mm thick plastic scintillator material permitted us to veto charged particles against neutrons . Tests of the individual scintillator bars have been

performed by exposing them to the 3 GeV electron test beam, that was produced at the DESY II electron synchrotron facility . Fig. 1 shows the setup. The beam intensity was surveyed with a paddle (P) behind the beam exit. Beam alignment was obtained by means of two (C12, C34) pairs of crossed scintillator fingers, that define the beam spot on the test object to 1 x 1 cm z, and a vetoing aperture (VA) with a central hole of 1.5 mm diameter . The trigger section (lower left part in fig. 1) allowed us to distinguish between two types of events . A fivefold coincidence P n C12 n C34 defined the raw trigger yielding rates of up to 100 Hz ; for the central trigger P n C12 n C34 n VA count rates were typically

deposited energy

E, (MeV)

20

w

11,

17 .5

á 500 Z6

15

400

12 .5

300

10 7.5

200

5 100

2 .5 0

0

10

20

deposited energy

30

E,

(MeV)

deposited energy E, (MeV)

Fig. 2. (a) Energy Ee deposited by 3 GeV electrons in BC408 of thickness D. The symbols give the experimental pulse height distributions, the solid lines a simultaneous fit. (b) Pulse height resolution 8(OPH) per PM readout vs . E.. Solid (open) symbols are experimental results for the type 1 (11) light guide, the solid (dashed) line is the best fit of eq . (5). (c) Time resolution b(AT) vs E e and the resulting position resolution 8(x).

498

L . Lüdemann et al. / Large-area PSD for fast neutrons

a factor 100 lower. The electronic and data acquisition set-up was conventional ; additional details may be found in ref. [17] . The detector itself could be moved remotely controlled to both directions (x, y) perpendicular to the beam . In addition it was possible to rotate the detector by 45° with respect to its longitudinal (x) axis . In this position the deposited energy was varied with the effective thickness D by moving the scintillator in the vertical direction (see insert in fig. 2a).

The time and the relative pulse height resolution were obtained by calculating AT= (TI - T2) and A PH = (PH1 - PH2)/ v/2- x PH d, respectively. Their distributions were of Gaussian shapes whose widths 8(OT) and 8(APH) were taken as the respective resolutions and plotted as FWHM in figs . 2b and 2c . Both data sets showed a monotonic and comparable dependence on E e . Because of the linear light response to electrons, 6(APH) was interpreted as energy resolution 8(Ee)/Ee . It is customary to fit this quantity with the expression

3. Detector resolutions

6

3 .1 . General procedure and energy dependence

Detector tests were performed by scanning a scintillator bar with the electron beam either in the horizontal (x) or in the vertical (y) direction with several points of incidence. In each position, the number of events for the central trigger reached at least 2 x 10 3 . For most scans, the raw trigger could be used without loss in resolution . As an example, fig. 2a shows the pulse height spectra resulting from a scan in the y direction across the middle of a rotated scintillator bar. The 3 GeV electrons deposit energies Ee with Landau distributions and average energies of 1.9 MeV per cm BC408 traversed. The curves represent calculated Landau distributions with the parameters derived from ref. [18] . The onset of discrepancies to the experimental result observed for the high energy part and D >- 4 cm was attributed to electromagnetic showering that depletes the Landau maximum and gives rise to increased energy deposition . Therefore in the subsequent data analysis gates of FWHM size were set to select the maxima . For these events, pulse heights and time resolutions as well as the quantities x, Td and PH d defined in eqs . (1)-(4) were calculated off-line as a function of Ee .

(Ee) Ee

a2

ß2 Y2 +-+-2 .

Ee

Ee

Here, the first term represents a contribution scaling linearly with the light output, the /3 term includes the statistics of photons and photoelectrons, and the third one, not depending on the light output, the noise contributions from PM and electronics [6]. The resulting fit in a, /3, y shown in fig. 2b describes the data well . It approaches a resolution of 5% for high energies, whereas at low energies the photon statistics dominates. The polished variant II of the light guide improves the photon statistics and thus reduces /3 ; the dashed curve has therefore been calculated with a and y being kept-constant . The corresponding [6] fit of the right hand side of eq . (5) to the time resolution 8(AT) in fig. 2c yielded similar results : a statistics term (,ß=560 psMeV 1 / 2 ) that dominates at low energies where the PM term (a = 86 ps) is negligible . According to eq . (1) the time resolution can be converted to a position resolution 8(x) to be read from the ordinate scale on the right hand side in fig. 2c . For this purpose the effective velocity of light ceff was determined from scans OT(x) along the scintillator axis (see section 3.2). It can be seen that 8(x) for deposited energies above EP = 9 McVP (or, electron equivalent, 5 McVee), is better than

Table 1 Comparison of time resolutions b(AT) obtained with scintillator bars . A PM /A çe denotes the PM area to the scintillator cross section, AF the front area per PM readout Scintillator material

Size H x T xL [cm']

Energy deposited [MeVee ]

8(AT) FWHM [Ps]

ApM Açe

Ref.

AF [cm 2]

BC408 NE213 SCSN-38 SCSN-23 NE110 NE102A BC408 NE 1l0

lox lox 100 11 .5 x6 x 100 20 x3 x 150 3.5 x 4x 100 20x10x180 10 x 5 x 200 3.8 x 3.8 x 51 20 x 5 x 300

1 .6-28 0.6-3 5 7 17 14 7,22 8.4

450-140 >_ 500 330 190, 120 250 470 250, 160 380

0.20 >1 0.33 >1 0.66 0.30 0.42 0.46

this work [10] [7] [19] [4] [3] [6] [5]

500 580 1500 180 1800 1000 100 3000

a

Including time walk corrections.

L. Lüdemann et al. / Large-area PSD for fast neutrons

5 cm (FWHM) and approaches 2 cm for high energies. Again, variant II of the light guides improved the resolution and was chosen for all subsequent applications. Table 1 allows one to compare time resolutions obtained with different types of scintillator bars. Of particular interest is the comparison with the results of Armand et al. [4] for a detector of similar size because NE110 has practically the same (bulk) attenuation length as BC408. These authors improved their photoelectron statistics by using large PMs that made the ratio A,,/Ase a factor of 3 .3 larger than that of the present system . At the same time, however, the increased intrinsic transit time spread of a 13 cm vs 5 cm diameter phototube more than counterbalances this advantage, because for high deposited energies the overall time resolution is mainly determined by the intrinsic term [3,6]. The detectors developed in refs. [6,19] are superior to ours in time resolution. This comes about through the smaller scintillator cross section that allows one either to reach the maximum possible light yield (A pm >>A se) with the same type of PM as in the present work [19], or to apply smaller PMs with accordingly reduced intrinsic time spread [6] . A small cross section is, however, incompatible with our design goal of a detector with good efficiency for high energy neutrons and large solid angle coverage. 3.2. Position dependence

For the investigation of the resolutions on the position (x, y) of electron incidence and of the effective light attenuation length d, each detector was scanned in increments Ax = 4 or 15 cm on the center line (y = 5 cm) . For these measurements, the bars were aligned with their front being perpendicular to the beam such that 10.00 ± 0.02 cm of scintillator material was traversed in each case and on the average Ee = 18.9 MeV deposited. The linear response OT(x) of eq. (1) is shown in fig . 3. The best fitting lines correspond to effective velocities varying between 14.76 and 14.88 cm/ns with an average value ceff = 14.82 ± 0.03 cm/ns; their deviations from the positions x of electron incidence are within the uncertainty of the beam position . The energy resolution (eq. (5)) turned out to be 5.6 ± 0.3% and independent, within these uncertainties, of x for positions being 1 cm or more away from either end of the bar . A deviation was only observed for the extreme corner positions (x = 1 or 99 cm, y = 1 or 9 cm); here a deterioration by up to 0.7% was found, that was attributed to the varying number of reflections and their intensity and, to a lesser extent, to inhomogencities of the photocathode . The time (position) resolution, however, remained constant within

499

Fig . 3 . Time difference Tz -T, vs position x of the 3 GeV electron beam for all six detectors, arbitrarily displaced by 1 ns each. the uncertainties stated (2.3 ± 0.25 cm) over the whole front area scanned, as was also reported in ref. [7]. The normalized average light output PH(x) could not be described by a pure exponential decrease . Fig. 4a shows that the pulse height for energies deposited at the far and near end of the bar increased beyond that expectation, which was attributed to the reflectivity of and the transmission through the nearby optical joints between bar, light pipe and PM, respectively . It adds to the direct light and experiences an attenuation along the path of length 2L -x such that PH(x) a exp(-x/a) +A exp(+x/a) . (6) The best fit of this expression to the experimental light attenuation (fig. 4a) resulted in A = 0.303 and a = 131 cm. However, A and a are highly correlated and therefore a cannot be compared to attenuation lengths of BC408 obtained in bulk material (380 cm) or extended objects (230 cm [9]). Despite this deviation of PH(x) from the standard attenuation law, the ratio PHI/PH2 closely follows an approximate (see eq. (3) for PH1= PH2) linear dependence on x as is shown in fig . 4b. The accuracy, though, is not sufficient for quantitative position resolution . It is interesting to note that the light attenuation features of all six bars produced are very similar such that the general fit of eq. (6) comes very close to a (representative) individual fit (see fig. 4b). The parameterization eq. (6) can therefore be used to calculate the average pulse height PHd (eq . (4)) . It is predicted with eq. (6) to stay constant within ±3 .5% over the whole length of the bar (cf. fig. 4a) and can be applied

500

L. Lüdemann et al. / Large-area PSDfor fast neutrons 102 a:D

chamber filled with isobutane. The ionization chamber covered almost the whole solid angle of 2, r sr. The

100

pulse-height allowed the separation between a-particles and fission fragments. To ensure long term

98

pulse-height stability, the isobutane pressure of 2 mbar was stabilized by a feedback system controlling the gas flow through the chamber.

96 94

The six neutron detectors were horizontically stacked to a wall of 60 cm x 100 cm and the 252Cf source was placed at a distance of s = 190 cm from the

92 90

center of the wall. The neutron positions were reconstructed eventwise from the time differences T2 - T, with eq . (1) (see fig. 5a); from that information the

88 86 84

exact lengths of the TOF paths were determined . The absolute time-scale was derived from the prompt ypeaks and then the TOF spectra of eq. (2) converted into energy spectra. Data were combined to 6 bins per detector covering the sections < 10 cm, 10-30 cm,

a-

30-50 cm, 50-70 cm, 70-90 cm and >_ 90 cm as indicated in fig. 5a. Fission correlated background was

taken care of by inserting for one run a shadow bar of 1 % transmission midway in the TOF path . 0 .9

0 .8

5000

v

k 0

I

I

I

I

20

1

1

1

1

1

1

40

1

1

1

1

1

1

1

1

1

1

0 4000

1111

60 80 100 position x (cm)

E 3000

Fig. 4. (a) Measured and calculated light outputs for energies deposited in position x . Experimental data are normalized at x = 5 cm and averaged over all twelve read out PMs. The lines give the best fit of eq. (6). (b) Pulse height ratio for readout on both sides of one individual scintillator bar . The solid (dashed) line gives the average (individual) best fit from eq. (6).

C

2000 1000 0

2 0

200

as offline threshold for the definition of the neutron detection efficiency .

wa

4. Neutron detection efficiency The characteristics of the emission of fission coinci252Cf dent neutrons from a spontanous fissioning source are well known. The standard spectrum can be described by d Nstd _ dE

M T(3/2)T3/2

E exp(-E P/T)

To trigger the data acquisition, fission events were detected by means of a small and low mass ionization

400 T2-T1 (channel)

0 0

Z

0

10

(7)

with the multiplicity M= 3.76 and the temperature T= 1.42 MeV [20,211. These neutrons are used to determine the detection efficiency of the scintillator .

45

10 z

I

0

2 .5

I 5 7 .5 10 neutron energy (MeV)

Fig. 5. (a) Spectrum of time differences T2 - Tl (or positions x, cf . eq. (1)) from irradiation with 252Cf neutrons . (b) Experimental neutron energy spectrum observed in bin 3 of the bar.

L. Lüdemann et al. / Large-area PSD for fast neutrons

For higher neutron energies and/or count rates the bin widths can be reduced in accordance with the position resolution shown in fig . 2b . The two bins overlapping with the readout sections, however, are not suited for quantitative measurements, because the detection efficiency drops as a consequence of edge effects . Among them are (i) the loss of charged reaction products whose contributions do not reach the (electronic) pulse height threshold ; (ii) the loss of events due to the finite position resolution that is not balanced by the corresponding gain from the adjacent bin ; (iii) systematic uncertainties in T2 - Tl originating from the enhanced importance of light reaching the PMs directly, i .e . with a velocity exceeding ceff in eq . (1) . In addition the neutron flux through the side faces of the detector traverses less detector material . The extreme bins are usually [3,8,10] discarded and the bar here was used on an effective length of 80 cm (bins #2-5) . For each detector and bin, the measured spectral distribution dNeXP /dE  was also corrected for the finite effective solid angle f2 e, referring to vertical incidence and for the energy dependent neutron atten-

U C N U c

50 1

uation (Att(E )) by construction material within the flight path . The neutron multiplicity spectrum was then obtained by normalization to the number Nff of fission events as dN dE

dNeXP

4Tr

(8)

dE  Att(E )f 2 effNff '

One example is shown in fig . 5b . These multiplicities were compared with the standard spectrum, eq . (7) ; the efficiency q(E n, Ethr) is then defined as the ratio dN/dE  . 7l(En , Emr) dNstd/dE  The results are shown in fig . 6 with their statistical uncertainties Ail . For comparison with Monte Carlo calculations the pulse-height thresholds Ethr must be known . For this purpose, the pulse heights have been calibrated with electron sources (3 GeV test beam, "Sr) and the light response for protons from ref . [22] . The energy was deposited through the center point of each bar and the actual pulse height for each section

0 .7 0 .6

0

0.5

v c

0.3

" U 0.4

0 cC

0.2 0 .1 0 0 .7 0 .6 0 .5 0 .4 0 .3 0 .2 0 .1 0

0

4

8

0

4

8 neutron energy E, (MeV)

Fig . 6. Neutron detection efficiencies determined with eq . (9) for the inner bins #2-5 of a bar ; also shown the result of the Monte Carlo calculation with the code NEFF4 (solid lines) .

502

L . Lüdemann et al. / Large-area PSD for fast neutrons

calculated with eq . (6). Thresholds Eth, had typical values of 1 .3 MeV with an uncertainty of ±50 keV and a variation along the bar of 130 keV. The Monte Carlo code NEFF4 by Dietze et al . [23] computes the efficiency for neutron energies up to 20 MeV with an uncertainty of 8% for E >_ 2Ethr . In fig. 6, the results are compared with the experimental efficiencies from eq . (9) for the inner bins #2-5 of a bar. In general, the agreement is very good ; a slight discrepancy close to threshold (E < 1.5 MeV) may be due to the uncertainties entering into the determination of Ethr (light response function, light attenuation, energy calibration) . 5. Summary

to threshold. The efficiency then was in good agreement with the Monte Carlo code NEFF4, if light attenuation along the bar was taken care of for the determination of Ethr, Meanwhile [24] the detectors have been successfully applied to the spectroscopy of fission coincident preequilibrium neutrons from the reaction of 494 MeV projectiles t9F with 2°9Bi . For E >_ 8 MeV the detection efficiency was calculated with Monte Carlo codes and cross checked by comparison with a simultaneously operated 10 .1 cm diameter X 10 .1 cm liquid scintillator with excellent n-y discrimination and well known efficiency that was positioned equivalent to one bin of a scintillator bar. By this technique it is expected to establish the detector efficiency up to E = 100 MeV with uncertainties < 20%.

A plastic-scintillator detector of dimensions 10 cm

X 10 cm X 100 cm and two-sided photomultiplier read-

out for position sensitivity has been studied with 3 GeV electrons. It was intended for time-of-flight spectroscopy of energetic (E . >_ 5 MeV), rare reaction neutrons. Its cross section Ase in combination with a large bulk attenuation length, totally reflecting surfaces and optimized light guides improved the number Ne of photoelectrons . The readout with compact photomultipliers (AP,/As, = 0.2) reduced Ne , but contributed with a small intrinsic transit time spread to a good time and position resolution for large pulse heights. Tests of the resolution properties as a function of both amount and position of deposited electron energy Ee, yielded the following results: (1) The pulse height resolution 8(Ee)IEe varies essentially like Ne t/2 approaching 5% for Ee >_ 30 MeV. It is constant over the whole detector front area, with the exception of the corner positions 1 cm from the detector surfaces . (2) The overall variation of pulse height along the detector is less than 14%. It can be described by a sum of two exponentials which reflect the impact of both ends . The geometrical mean PH d, however, remains constant within 7% (4 .5%) over the whole length L (Leff = 0.8L). (3) Position and time resolution benefit at higher energies from the small photomultipliers used ; they reach values of 280 ps and 4 cm (150 ps and 2 cm) for pulse heights corresponding to proton energies EP = 9 MeV (40 MeV) with negligible variation across the detector front area . These features were found with all six detectors built. Neutron detection efficiency ?1(Ethr, E) was determined experimentally with 252Cf neutrons for E < 8 MeV. Due to the edge effects the useful detector length Leff excludes the end sections of 10 cm each . This value corresponds to the position resolution close

Acknowledgements We would like to thank the staff of the DESY II test beam operation for their help . The continuous support of the mechanical workshop led by B. Leicht is appreciated . This work was financially supported by the HMI Berlin GmbH (contract 120/531/012435) and by the Bundesministerium fidr Forschung and Technologie (contract 06HH142) .

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[21] H.H . Knitter et al ., Nucl . Phys . A 490 (1988) 307 . [22] V .V . Verbinski et al., Nucl . Instr . and Meth. 65 (1986) 8 . [23] G . Dietze et al ., Report PTB-ND-22, PTB Braunschweig (1982). [24] K. Knoche, PhD Thesis, Universität Hamburg (1993).