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On the performance of helium scintillation counters
NUCLEAR INSTRUMENTS AND METHODS 72 (I969) t95-200;
© NORTH-HOLLAND PUBLISHING CO.
O N T H E P E R F O R M A N C E OF H E L I U M S C I N T I L L A T I O N C O U N T E R S P. GUAZZONI and M. PIGNANELLI lstituto di Fisica dell" Universit?t, Milano, Italy
and lstituto Nazionale Fisica Nucleare, Milano, Italy
Received 3 March 1969 A systematic study of the mechanism of helium scintillation counters is reported. The decay time and light yield in scintillations produced by ~ particles in pure helium or helium containing other gases, at low concentrations, has been measured.
Some mixtures, as (He-CO2) and (He-N~-NO) can have practical interest. The factors determining the energy resolution are discussed.
I. Introduction
2. Experimental apparatus
In recent applications the gas scintillators are often used as "living targets", in which the gaseous medium serves not only as scintillator but also as target for nuclear physics experiment. Well known is the use of 3He or 4He targets in neutron detectors or polarization analysersl). In view of the interest of helium targets in very high energy nuclear interactions2'3), a systematic study of helium scintillation counters has been undertaken. In a previous paper 4) we pointed out that there are two main sources for radiative processes in helium: dlestruction of metastable atomic states and subsequent vacuum ultraviolet emission, or charge exchange processes between helium ions and molecules of other gases contained in helium as impurities, with a subsequent emission of vibrational bands by molecular ions of the impurities. These bands usually lie between 2000 and 4500 A. On the first source and on the use of an appropriate wavelength fluorescent converter are based the traditional helium scintillation counters.
The gas chamber and the outgassing method have been described in a previous paperT). In the present experiment the helium was introduced in the scintillation chamber through a charcoal and a copper trap at liquid nitrogen temperature. Other gases could be introduced, as controlled amounts of impurities down to a minimum value of 1 0 - 4 T o r r . Pressures above 1 Torr have been measured with a capsule dial gauge, below l Torr with a Pirani gauge. At the lowest values, below 5 × 1 0 - 3 Torr, we measured the pressure in a small chamber and then the gas was expanded in the scintillation cell. The sensitivity of the Pirani gauge to the different gases has been taken into account. A Z4~Am alpha source was positioned in the chamber. The light pulse following an alpha particle emission, has been detected using a quartz window, 10 m m thick, and a photomultiplier E M I 6255 S. The photosensitivity of the apparatus is nearly constant between 3000 and 4000A, where it reaches the maximum value and falls to 10% of its maximum value at 1950 and 5950 A. To detect light or near ultraviolet, a reflector of aluminum or aluminum coated by smoked Mg was positioned in the cell. To detect the vacuum ultraviolet, the reflector and the internal surface of the window were coated, by vacuum deposition, with a wavelength shifter, as p-quaterphenyl (QP), tetraphenylbutadiene (TPB) and diphenylstilbene (DPS). The voltage pulse following an alpha particle detection, was clipped at the photomultiplier output by an RC-network having a differential time constant of 20 psec at the maximum, and then measured by a pulse height analyser. To evaluate the intensity of very slow components (decay time longer than 20psec) the current at the output of the photomultiplier was measured by means of a picoammeter.
A variety of impurity effects are reported in the literature1), especially for H e - N 2 mixtures s); however, systematic data are missing and many of these phenomena are not yet understood. The main features of scintillations produced by ionizing particles are: light yield, decay time, emission spectrum and their dependence from helium pressure and impurities content. In the present paper we report some experimental results concerning the mechanism and the performances of helium scintillation counters. A careful study on the linearity and energy resolution of' these counters has been recently published6). In section 5 are given other informations about the factors determining the energy resolution. 195
196
V. G U A Z Z O N I
A N D M. P I G N A N E L L I
The decay times were measured by means of a 543 Tektronix oscilloscope. In these last measurements the larger value of the differential time constant was 100 psec.
effects. The gases of the second group, also at very low concentrations (less than 10-7) are very active in increasing the emission between 2000 and 4500 A. TABLE 1
3. Vacuum ultraviolet emission As was shown by Bennet ~) in helium of high purity the emission in the near ultraviolet or in the visible is negligible. The radiation emitted is confined in the vacuum ultraviolet region. In a previous paper+), we have shown that the decay time of this radiation is pressure dependent and that above 1 arm can be given by the relation:
where p is the helium pressure in atm. This relation shows that above 20-30 atm the scintillation time is determined by the colour shifter (5-6 nsec for QP), below by helium pressure. The present data can give a satisfactory explanation of the results on the output dependence from the pressure reported by Esterling and Lipman a) for a 3He counter clipped at 150 nsec. 4. Impurity emission 4.1. L o w CONCENTRATIONS We have studied the helium scintillation, without wavelength shifter, at pressures ranging from 1 to 65 atm in mixtures with: a. H2, H20, CH+, ethylene, ethyl alcohol; b. N2, 02, CO, CO2, NO, N20. Impurities of the first group produce only quenching !
[
lo
~
o
oo.,
IQ
5_
\ \
a
10 +
41
atrn
10 a
1-0 2 CO 2
pressure
Impurity contained in helium
N~
z (psec) = 1.7/p 2 ,
,
1
10 t
(Torr)
Fig. 1. Scintillation decay time o f He-CO2 mixture vs CO2 pressure, at four He pressures indicated.
02 CO co2 NO N~O N2+NO
2
3
4
5
k Pressure at Decay time Wavelength ( n s e c ' T o r r ) q u e n c h i n g at q u e n c h i n g interval point (Torr) point (nsec) (A)
15 32 17 19 66 23 380
0.32 p2. 0.11 0.40pt.5" 0.80 0.42 0.23 0.40
40 p - 2 , 300 40p-z.5* 24 150 100 950
3900-4200 3050-5100 2000-2800 3400-3850 2000--2800 3400-3800 2000-2800
* p is t h e H e pressure in atm.
The decay time of this emission depends only on the partial pressure of the impurity, as shown in fig. 1, and results inversely proportional to this pressure: z = k / p i * , where k is a constant caracteristic of each impurity, whose values are given in the column 2 of the table 1. In fig. 2 is given the pulse amplitude dependence for He-CO2 mixture, at four different helium pressures, vs CO2 pressure. Similar curves, for He-N2 and He-O2 mixture, are reported by Baldin et al. 8) and by Koch9), and have been found in the present experiment for all gases of the second group. Previously no correct interpretation has been given for want of complete knowledge of impurities content and temporal dependenceS'9). The decrease in light emission at high values of the impurity pressure is determined by collisional quenchingt). The position of the maximum and the decrease at very low impurity pressure, where, as shown in fig. 1, the decay time becomes very long, is determined only by the choice of the clipping time. It is possible to measure the intensity of light emission, independently from the clipping time, measuring the integrated current at the output of the photomultiplier. In this case the decrease vs low concentrations is very slow as shown by the dotted line of the fig. 2. This last measurement is practically equivalent to one with an infinitely large clipping time. The pressure of the * This decay time dependence h a s been tested down to a minim u m value o f 1 5 - 2 0 nsec.
ON
THE
PERFORMANCE
OF
I
HELIUM
SCINTILLATION
[
i
i
• X
He + C02 1
__ ......
_ .
s~-
~ ~--J~
COUNTERS
~
~
r
1.85 5
• =o
197
15 41
atm
arm atm atrn
0.5
'0 4
10 3
10 2 C02
10 ~
1
10
PRESSURE (TORR)
Fig. 2. Pulse amplitude (arbitrary units) from scintillations in He-CO2 mixture, at four He pressures, vs CO2 pressure. The pulse was clipped at 17/~sec. See text for the dotted line. - - 7 " He + N2
0.5
r
__
10 -1
I
I
10
102
10 ~
Nz PRESSURE (TORR)
Fig. 3. L i g h t y i e l d in t h e q u e n c h i n g r e g i o n for H e - N 2 m i x t u r e a t t h e H e p r e s s u r e s i n d i c a t e d in the figure. T h e c u r v e s are n o r m a l i z e d
at the lowest N2 pressure. irapurity at quenching point (defined as the point at which the light yield is lowered at half of its maximum value) results nearly independent from the helium pressure, except for He-N2 and He-CO mixtures, for which the approximate dependences are given in column 3 of the table 1 (fig. 3). The amplitude of the light pulses is nearly independent from helium pressure in He-CO2 and He-O2 mixtures below 40 atm, as shown in fig. 4. Above this pressure is remarkable a decrease, probably due to the electron-ion recombination that inhibits the charge exchange mechanism responsable of the excitation of the impurity molecules. The light intensity is instead inversely proportional to the helium pressure in He-N 2 and He-CO mixtures. An intermediate dependence has been found for He-NO, H e - N 2 0 and for the ternary mixture He-N2-NO. This last is the only one ternary mixture of practical interest that we have
found. In fig. 4 are given the relative outputs of the mixtures studied; the output obtained for pure helium and QP converter corresponds without any decrease above 40 atm, very closely to that of He-CO2 mixture. By means of optical filters, the wave length range, in which falls the relevant part of the emission spectrum has been found. These data for the different mixtures are given in the last column of table 1. The curves of fig. 4 are not corrected for the sensitivity of the apparatus for the different wavelengths. In spite of the poor sensitivity below 2800 .A, a bigger output has been obtained for He-N2-NO mixture. Unfortunately the decay time of this mixture is relatively long (1-2/tsec). A lower limit of the decay time of the other mixtures studied can be given by the decay time at the pressure of the quenching point (column 4) and results of the order of few tens of nsec.
198 4.2.
P. G U A Z Z O N I
AND
MIXTURES WITH NOBLE GASES
The noble gases (Ne, A, Kr, Xe), at very low concentrations in helium, do not produce noticeable effects. At high concentrations (of the order of 10%) and using a wavelength shifter, may produce a relevant enhancement of the radiation emitted, as has been already shown by other authors1°). In this case however the nuclear composition of the "living t a r g e t " is consistently altered. Frequently in order to increase the light output and the stopping power of the counter, the mixture He-(10%)Xe has been used. In the literature 9"~1) are reported some measurements of scintillation decay times for the visible and near ultraviolet emission in Xe. No data are available for (He + Xe + colour shifter) counters. The decay time of these counters shows a complex dependence from He and Xe pressures. At low Xe pressure (below 0.4 atm) the decay time is strongly dependent from the pressure of both the gases; at higher Xe pressures (between 0.4 and 1.6 atm) the dependence by He pressure is very poor; above 1.6 atm of Xe the decay time becomes insensitive also to Xe pressure and results about 100 nsec [ref. 6) and literature cited there]. The last condition is that of practical interest. A decay time 100 nsec long is not very fast and is of the order of that of the emissions studied in the section 4.1. 4.3.
CUMULATIVE EFFECTS
As said in section 4.1 the gases of the first group, also at very low concentrations (few ppm), produce a
M. P I G N A N E L L I
quenching of the emissions studied in the sections 4.1 and 4.2. We have studied all the possible ternary mixtures of helium with two gases of the second group of section 4.1. For a given ternary mixture He-A-B we have found a light emission intermediate between that of the two binary mixture He-A and He-B, except in the case of He-H2-NO, as shown above. In principle, it is possible to accumulate the radiation in the vacuum ultraviolet, deriving from the energy relised by the metastable state He(2aS) to that in the near ultraviolet or in the visible, determined by the destruction of He ions by impurity molecules4). However, the transparency to the second kind of radiation of the colour shifter, necessary to detect the first, must be taken into account. The transmission of QP, DPS and TPB results satisfactorily for the radiations emitted by He-N z mixture. The radiation of He-N2-NO being in the ultraviolet is partly detected by the colour shifter. It is so possible to increase the light output of a helium scintillation counter, based on the use of a colour shifter, introducing N2 or N2-NO at pressure of the order of 0.1 Tort. The introduction of CO2 in a ( H e + colour shifter) counter does not produce any noticeable effect. If the light yield of a (He + CO2) or a (He + QP) counter is taken as 1, the relative outputs of ( H e + DPS), ( H e + 0 . 1 T o r r of N 2 + 0 . 1 T o r r of NO) and (He+0.1 Tort o f N 2 +0.1 Torr of N O + D P S ) counters at l0 atm of helium, are 2.2 : 2 : 2.6 respectively. At this pressure the decay times in ( H e + D P S ) and (He-N2-NO) are respectively 17 nsec and 1.8/~sec. The decay time of a ( H e - N 2 - N O + D P S ) counter results composited by these two decay times, having the fast component about the 30% of the light yield. Such a counter, being its output bigger than that of any other (pure helium or h e l i u m + l o w concentration impurity) counter, can be usefully employed in a fastslow electronic system.
1 0 - -
T-
.c0 o I
g.
\
5. Energy resolution A drastic limitation of scintillation helium counters derives from the poor energy resolutions obtained. In the literature are reported only these two data (both for 5.45 MeV alpha particles) : 19% by Esterling and Lipman 2) and 12% by us in a previous experiment~).
- _N2o01- \ ~C0~0.I~ 10 He
100 pressure
(at rn)
Fig. 4. Pulse height dependence from He pressure, for various
mixtures. The impurities contained in the mixture and their pressures (Torr) are indicated. The clipping time was 10 psec.
As with any other scintillation the resolution is determined by two factors: I. Geometrical efficiency;
disuniformity
in
light
detection
ON THE P E R F O R M A N C E
OF H E L I U M S C I N T I L L A T I O N
2. Statistical fluctuations associated with the number of photoelectrons produced. It is possible to discriminate between the two contributions because only the second is energy dependent. Obviously the first factor depends on the shape and the nature of the reflector used, dimensions and position of the window, etc. In this experiment many shapes of reflectors has been tested, two of them are shown in fig. 5. The dashed zone, inside the reflector, represents the cone irradiated by ~ particles. The lines A and B indicate the ends of particles tracks for two different pressures. The light collection, for these two different track lengths, deduced from the mean value of pulse distribution, shows a difference of the order of 3-4%. To deduce the difference in light collection between central and lateral track, we changed the impurities concentration at fixed pressure (fixed c~ particle range). From the change in light pulse amplitude, so obtained, it is possible to discriminate 5) the geometrical factor from the statistical factor. The
REFLECTOR
~
~
I
QUARTZ ~ / WINDOW' L. " COLLIMATED o4. PARTICLES I
E
~
~
,
I
0
~
PHOTOTUBE I
~,~
[
1
Gaseous medium
He He H e + 10% Xe H e + 10% Xe H e + 0 . 2 Torr (N2-NO)* He + 0.2 Torr (Ne-NO)* He + 0.2 Torr (N2-NO)* He+0.07 Torr COo
Colour shifter
Energy resolution
QP DPS QP DPS none QP DPS none
11 8.3 6 4.7 8.8 9 7.2 11.5
(%)
6. Conclusions
l
R
TABLE 2
obtained with different mixtures and wavelength shifters are given in table 2. These data have been collected by 5 MeV e particles, at pressures ranging between 6 and 18 atm and using a clipping time of 10/~sec.
o4 S O U R C
~
geometrical factor so obtained results also of the order of 3-4%. Comprehensively, taking into account longitudinal and lateral contribution, the geometrical factor must be not larger than 5%. This value is supported by the energy resolutions obtained with the very efficient He-Xe mixture. This geometrical factor results smaller than that extimated in6). The energy resolutions
* Equal content of N2 and NO.
J
~
199
COUNTERS
2
3
I S cm
Fig. 5. Two arrangements of reflector-window geometry used for energy resolution studies.
The experimental data collected in the present paper, show that, beside some kinds of impurity that must be avoided, as the cracking products of diffusive pumps (H2, C H 4 , etc.), other impurities can be usefully introduced in helium scintillation counters. Indeed can be obtained counters with helium practically pure, from a point of view of nuclear composition (concentration of impurities of the order of 10-s), whose luminous output is similar or larger than that of a traditional helium counter using a colour shifter. The use of an impurity is profitable for the reproducibility and stability, being avoided the poor reproducibility of colour shifter coatings and the poisoning effects over time due to its presence. The decay time of a helium counter, containing impurities is suitably fast; a typical value is that of He-CO z mixture: about 50nsec. The decay times found for the impurity emission are of the order of the magnitude of that
200
P. G U A Z Z O N I AND M. P I G N A N E L L I
reported in the literature for He-N2 mixture, however can be reached with impurity concentrations much smaller of those previously quoted1'9). Actually the Nz concentration, in He at 1 atm, necessary to have a decay time of 25 nsec is 0.1% and not 5%9). To achieve a rise time of few nsec the best solution is a helium counter at high pressure (above 30 atm) associated to a fast colour shifter as diphenylstilbene. In the present work some improvements in the crucial point of the energy resolution has been obtained. However further work is needed to achieve very satisfactory performances. In fact we have shown above that the energy resolution for 5 MeV alpha particles is determined mainly by the statistical factor. At lower values of the energy expended in the counter, as for measurements of helium nucleus recoils in a polarization experiment or in high-energy coherent interaction3), this factor may result'prohibitively large. From this point of view is very important a high
luminuous response as that of a (He-NE-NO+DPS) counter. References ~) J. B. Birks, The theory" and practice of scintillation counting (Pergamon Press, Oxford, 1964) ch. 14. 2) R . J . Esterling and N. H. Lipman, Rev. Sci. Instr. 36 (1965) 493. 3) E. Fiorini, M. Pignanelli and A . J . Herz, Report CERN] ECFA (1966) WG2/US-SG4. 4) p. Guazzoni and M. Pignanelli, Phys. Letters .28 A (1968) 432. 5) W. R. Bennet, Ann. Phys. 18 (1962) 367. 6) G. L. Morgan and R. L. Walter, Nucl. Instr. and Meth. 58 (1968) 277. v) p. Guazzoni and M. Pignanelli, Nuovo Cimento 40 B (1965) 454. s) S. Baldin, V. V. Gabrilovsky and F. E. Chukreev, Atomnaya Energiya 3 (1957) 331. 9) L. Koch, Thesis (Paris, 1959). 10) j. A. Northrop and J. C. Gursky, Nucl. Instr. 3 (1958) 207. 11) R. Henck and A. Coche, 1EEE Trans. Nucl. Sci. 14 (1967) 478.
© NORTH-HOLLAND PUBLISHING CO.
O N T H E P E R F O R M A N C E OF H E L I U M S C I N T I L L A T I O N C O U N T E R S P. GUAZZONI and M. PIGNANELLI lstituto di Fisica dell" Universit?t, Milano, Italy
and lstituto Nazionale Fisica Nucleare, Milano, Italy
Received 3 March 1969 A systematic study of the mechanism of helium scintillation counters is reported. The decay time and light yield in scintillations produced by ~ particles in pure helium or helium containing other gases, at low concentrations, has been measured.
Some mixtures, as (He-CO2) and (He-N~-NO) can have practical interest. The factors determining the energy resolution are discussed.
I. Introduction
2. Experimental apparatus
In recent applications the gas scintillators are often used as "living targets", in which the gaseous medium serves not only as scintillator but also as target for nuclear physics experiment. Well known is the use of 3He or 4He targets in neutron detectors or polarization analysersl). In view of the interest of helium targets in very high energy nuclear interactions2'3), a systematic study of helium scintillation counters has been undertaken. In a previous paper 4) we pointed out that there are two main sources for radiative processes in helium: dlestruction of metastable atomic states and subsequent vacuum ultraviolet emission, or charge exchange processes between helium ions and molecules of other gases contained in helium as impurities, with a subsequent emission of vibrational bands by molecular ions of the impurities. These bands usually lie between 2000 and 4500 A. On the first source and on the use of an appropriate wavelength fluorescent converter are based the traditional helium scintillation counters.
The gas chamber and the outgassing method have been described in a previous paperT). In the present experiment the helium was introduced in the scintillation chamber through a charcoal and a copper trap at liquid nitrogen temperature. Other gases could be introduced, as controlled amounts of impurities down to a minimum value of 1 0 - 4 T o r r . Pressures above 1 Torr have been measured with a capsule dial gauge, below l Torr with a Pirani gauge. At the lowest values, below 5 × 1 0 - 3 Torr, we measured the pressure in a small chamber and then the gas was expanded in the scintillation cell. The sensitivity of the Pirani gauge to the different gases has been taken into account. A Z4~Am alpha source was positioned in the chamber. The light pulse following an alpha particle emission, has been detected using a quartz window, 10 m m thick, and a photomultiplier E M I 6255 S. The photosensitivity of the apparatus is nearly constant between 3000 and 4000A, where it reaches the maximum value and falls to 10% of its maximum value at 1950 and 5950 A. To detect light or near ultraviolet, a reflector of aluminum or aluminum coated by smoked Mg was positioned in the cell. To detect the vacuum ultraviolet, the reflector and the internal surface of the window were coated, by vacuum deposition, with a wavelength shifter, as p-quaterphenyl (QP), tetraphenylbutadiene (TPB) and diphenylstilbene (DPS). The voltage pulse following an alpha particle detection, was clipped at the photomultiplier output by an RC-network having a differential time constant of 20 psec at the maximum, and then measured by a pulse height analyser. To evaluate the intensity of very slow components (decay time longer than 20psec) the current at the output of the photomultiplier was measured by means of a picoammeter.
A variety of impurity effects are reported in the literature1), especially for H e - N 2 mixtures s); however, systematic data are missing and many of these phenomena are not yet understood. The main features of scintillations produced by ionizing particles are: light yield, decay time, emission spectrum and their dependence from helium pressure and impurities content. In the present paper we report some experimental results concerning the mechanism and the performances of helium scintillation counters. A careful study on the linearity and energy resolution of' these counters has been recently published6). In section 5 are given other informations about the factors determining the energy resolution. 195
196
V. G U A Z Z O N I
A N D M. P I G N A N E L L I
The decay times were measured by means of a 543 Tektronix oscilloscope. In these last measurements the larger value of the differential time constant was 100 psec.
effects. The gases of the second group, also at very low concentrations (less than 10-7) are very active in increasing the emission between 2000 and 4500 A. TABLE 1
3. Vacuum ultraviolet emission As was shown by Bennet ~) in helium of high purity the emission in the near ultraviolet or in the visible is negligible. The radiation emitted is confined in the vacuum ultraviolet region. In a previous paper+), we have shown that the decay time of this radiation is pressure dependent and that above 1 arm can be given by the relation:
where p is the helium pressure in atm. This relation shows that above 20-30 atm the scintillation time is determined by the colour shifter (5-6 nsec for QP), below by helium pressure. The present data can give a satisfactory explanation of the results on the output dependence from the pressure reported by Esterling and Lipman a) for a 3He counter clipped at 150 nsec. 4. Impurity emission 4.1. L o w CONCENTRATIONS We have studied the helium scintillation, without wavelength shifter, at pressures ranging from 1 to 65 atm in mixtures with: a. H2, H20, CH+, ethylene, ethyl alcohol; b. N2, 02, CO, CO2, NO, N20. Impurities of the first group produce only quenching !
[
lo
~
o
oo.,
IQ
5_
\ \
a
10 +
41
atrn
10 a
1-0 2 CO 2
pressure
Impurity contained in helium
N~
z (psec) = 1.7/p 2 ,
,
1
10 t
(Torr)
Fig. 1. Scintillation decay time o f He-CO2 mixture vs CO2 pressure, at four He pressures indicated.
02 CO co2 NO N~O N2+NO
2
3
4
5
k Pressure at Decay time Wavelength ( n s e c ' T o r r ) q u e n c h i n g at q u e n c h i n g interval point (Torr) point (nsec) (A)
15 32 17 19 66 23 380
0.32 p2. 0.11 0.40pt.5" 0.80 0.42 0.23 0.40
40 p - 2 , 300 40p-z.5* 24 150 100 950
3900-4200 3050-5100 2000-2800 3400-3850 2000--2800 3400-3800 2000-2800
* p is t h e H e pressure in atm.
The decay time of this emission depends only on the partial pressure of the impurity, as shown in fig. 1, and results inversely proportional to this pressure: z = k / p i * , where k is a constant caracteristic of each impurity, whose values are given in the column 2 of the table 1. In fig. 2 is given the pulse amplitude dependence for He-CO2 mixture, at four different helium pressures, vs CO2 pressure. Similar curves, for He-N2 and He-O2 mixture, are reported by Baldin et al. 8) and by Koch9), and have been found in the present experiment for all gases of the second group. Previously no correct interpretation has been given for want of complete knowledge of impurities content and temporal dependenceS'9). The decrease in light emission at high values of the impurity pressure is determined by collisional quenchingt). The position of the maximum and the decrease at very low impurity pressure, where, as shown in fig. 1, the decay time becomes very long, is determined only by the choice of the clipping time. It is possible to measure the intensity of light emission, independently from the clipping time, measuring the integrated current at the output of the photomultiplier. In this case the decrease vs low concentrations is very slow as shown by the dotted line of the fig. 2. This last measurement is practically equivalent to one with an infinitely large clipping time. The pressure of the * This decay time dependence h a s been tested down to a minim u m value o f 1 5 - 2 0 nsec.
ON
THE
PERFORMANCE
OF
I
HELIUM
SCINTILLATION
[
i
i
• X
He + C02 1
__ ......
_ .
s~-
~ ~--J~
COUNTERS
~
~
r
1.85 5
• =o
197
15 41
atm
arm atm atrn
0.5
'0 4
10 3
10 2 C02
10 ~
1
10
PRESSURE (TORR)
Fig. 2. Pulse amplitude (arbitrary units) from scintillations in He-CO2 mixture, at four He pressures, vs CO2 pressure. The pulse was clipped at 17/~sec. See text for the dotted line. - - 7 " He + N2
0.5
r
__
10 -1
I
I
10
102
10 ~
Nz PRESSURE (TORR)
Fig. 3. L i g h t y i e l d in t h e q u e n c h i n g r e g i o n for H e - N 2 m i x t u r e a t t h e H e p r e s s u r e s i n d i c a t e d in the figure. T h e c u r v e s are n o r m a l i z e d
at the lowest N2 pressure. irapurity at quenching point (defined as the point at which the light yield is lowered at half of its maximum value) results nearly independent from the helium pressure, except for He-N2 and He-CO mixtures, for which the approximate dependences are given in column 3 of the table 1 (fig. 3). The amplitude of the light pulses is nearly independent from helium pressure in He-CO2 and He-O2 mixtures below 40 atm, as shown in fig. 4. Above this pressure is remarkable a decrease, probably due to the electron-ion recombination that inhibits the charge exchange mechanism responsable of the excitation of the impurity molecules. The light intensity is instead inversely proportional to the helium pressure in He-N 2 and He-CO mixtures. An intermediate dependence has been found for He-NO, H e - N 2 0 and for the ternary mixture He-N2-NO. This last is the only one ternary mixture of practical interest that we have
found. In fig. 4 are given the relative outputs of the mixtures studied; the output obtained for pure helium and QP converter corresponds without any decrease above 40 atm, very closely to that of He-CO2 mixture. By means of optical filters, the wave length range, in which falls the relevant part of the emission spectrum has been found. These data for the different mixtures are given in the last column of table 1. The curves of fig. 4 are not corrected for the sensitivity of the apparatus for the different wavelengths. In spite of the poor sensitivity below 2800 .A, a bigger output has been obtained for He-N2-NO mixture. Unfortunately the decay time of this mixture is relatively long (1-2/tsec). A lower limit of the decay time of the other mixtures studied can be given by the decay time at the pressure of the quenching point (column 4) and results of the order of few tens of nsec.
198 4.2.
P. G U A Z Z O N I
AND
MIXTURES WITH NOBLE GASES
The noble gases (Ne, A, Kr, Xe), at very low concentrations in helium, do not produce noticeable effects. At high concentrations (of the order of 10%) and using a wavelength shifter, may produce a relevant enhancement of the radiation emitted, as has been already shown by other authors1°). In this case however the nuclear composition of the "living t a r g e t " is consistently altered. Frequently in order to increase the light output and the stopping power of the counter, the mixture He-(10%)Xe has been used. In the literature 9"~1) are reported some measurements of scintillation decay times for the visible and near ultraviolet emission in Xe. No data are available for (He + Xe + colour shifter) counters. The decay time of these counters shows a complex dependence from He and Xe pressures. At low Xe pressure (below 0.4 atm) the decay time is strongly dependent from the pressure of both the gases; at higher Xe pressures (between 0.4 and 1.6 atm) the dependence by He pressure is very poor; above 1.6 atm of Xe the decay time becomes insensitive also to Xe pressure and results about 100 nsec [ref. 6) and literature cited there]. The last condition is that of practical interest. A decay time 100 nsec long is not very fast and is of the order of that of the emissions studied in the section 4.1. 4.3.
CUMULATIVE EFFECTS
As said in section 4.1 the gases of the first group, also at very low concentrations (few ppm), produce a
M. P I G N A N E L L I
quenching of the emissions studied in the sections 4.1 and 4.2. We have studied all the possible ternary mixtures of helium with two gases of the second group of section 4.1. For a given ternary mixture He-A-B we have found a light emission intermediate between that of the two binary mixture He-A and He-B, except in the case of He-H2-NO, as shown above. In principle, it is possible to accumulate the radiation in the vacuum ultraviolet, deriving from the energy relised by the metastable state He(2aS) to that in the near ultraviolet or in the visible, determined by the destruction of He ions by impurity molecules4). However, the transparency to the second kind of radiation of the colour shifter, necessary to detect the first, must be taken into account. The transmission of QP, DPS and TPB results satisfactorily for the radiations emitted by He-N z mixture. The radiation of He-N2-NO being in the ultraviolet is partly detected by the colour shifter. It is so possible to increase the light output of a helium scintillation counter, based on the use of a colour shifter, introducing N2 or N2-NO at pressure of the order of 0.1 Tort. The introduction of CO2 in a ( H e + colour shifter) counter does not produce any noticeable effect. If the light yield of a (He + CO2) or a (He + QP) counter is taken as 1, the relative outputs of ( H e + DPS), ( H e + 0 . 1 T o r r of N 2 + 0 . 1 T o r r of NO) and (He+0.1 Tort o f N 2 +0.1 Torr of N O + D P S ) counters at l0 atm of helium, are 2.2 : 2 : 2.6 respectively. At this pressure the decay times in ( H e + D P S ) and (He-N2-NO) are respectively 17 nsec and 1.8/~sec. The decay time of a ( H e - N 2 - N O + D P S ) counter results composited by these two decay times, having the fast component about the 30% of the light yield. Such a counter, being its output bigger than that of any other (pure helium or h e l i u m + l o w concentration impurity) counter, can be usefully employed in a fastslow electronic system.
1 0 - -
T-
.c0 o I
g.
\
5. Energy resolution A drastic limitation of scintillation helium counters derives from the poor energy resolutions obtained. In the literature are reported only these two data (both for 5.45 MeV alpha particles) : 19% by Esterling and Lipman 2) and 12% by us in a previous experiment~).
- _N2o01- \ ~C0~0.I~ 10 He
100 pressure
(at rn)
Fig. 4. Pulse height dependence from He pressure, for various
mixtures. The impurities contained in the mixture and their pressures (Torr) are indicated. The clipping time was 10 psec.
As with any other scintillation the resolution is determined by two factors: I. Geometrical efficiency;
disuniformity
in
light
detection
ON THE P E R F O R M A N C E
OF H E L I U M S C I N T I L L A T I O N
2. Statistical fluctuations associated with the number of photoelectrons produced. It is possible to discriminate between the two contributions because only the second is energy dependent. Obviously the first factor depends on the shape and the nature of the reflector used, dimensions and position of the window, etc. In this experiment many shapes of reflectors has been tested, two of them are shown in fig. 5. The dashed zone, inside the reflector, represents the cone irradiated by ~ particles. The lines A and B indicate the ends of particles tracks for two different pressures. The light collection, for these two different track lengths, deduced from the mean value of pulse distribution, shows a difference of the order of 3-4%. To deduce the difference in light collection between central and lateral track, we changed the impurities concentration at fixed pressure (fixed c~ particle range). From the change in light pulse amplitude, so obtained, it is possible to discriminate 5) the geometrical factor from the statistical factor. The
REFLECTOR
~
~
I
QUARTZ ~ / WINDOW' L. " COLLIMATED o4. PARTICLES I
E
~
~
,
I
0
~
PHOTOTUBE I
~,~
[
1
Gaseous medium
He He H e + 10% Xe H e + 10% Xe H e + 0 . 2 Torr (N2-NO)* He + 0.2 Torr (Ne-NO)* He + 0.2 Torr (N2-NO)* He+0.07 Torr COo
Colour shifter
Energy resolution
QP DPS QP DPS none QP DPS none
11 8.3 6 4.7 8.8 9 7.2 11.5
(%)
6. Conclusions
l
R
TABLE 2
obtained with different mixtures and wavelength shifters are given in table 2. These data have been collected by 5 MeV e particles, at pressures ranging between 6 and 18 atm and using a clipping time of 10/~sec.
o4 S O U R C
~
geometrical factor so obtained results also of the order of 3-4%. Comprehensively, taking into account longitudinal and lateral contribution, the geometrical factor must be not larger than 5%. This value is supported by the energy resolutions obtained with the very efficient He-Xe mixture. This geometrical factor results smaller than that extimated in6). The energy resolutions
* Equal content of N2 and NO.
J
~
199
COUNTERS
2
3
I S cm
Fig. 5. Two arrangements of reflector-window geometry used for energy resolution studies.
The experimental data collected in the present paper, show that, beside some kinds of impurity that must be avoided, as the cracking products of diffusive pumps (H2, C H 4 , etc.), other impurities can be usefully introduced in helium scintillation counters. Indeed can be obtained counters with helium practically pure, from a point of view of nuclear composition (concentration of impurities of the order of 10-s), whose luminous output is similar or larger than that of a traditional helium counter using a colour shifter. The use of an impurity is profitable for the reproducibility and stability, being avoided the poor reproducibility of colour shifter coatings and the poisoning effects over time due to its presence. The decay time of a helium counter, containing impurities is suitably fast; a typical value is that of He-CO z mixture: about 50nsec. The decay times found for the impurity emission are of the order of the magnitude of that
200
P. G U A Z Z O N I AND M. P I G N A N E L L I
reported in the literature for He-N2 mixture, however can be reached with impurity concentrations much smaller of those previously quoted1'9). Actually the Nz concentration, in He at 1 atm, necessary to have a decay time of 25 nsec is 0.1% and not 5%9). To achieve a rise time of few nsec the best solution is a helium counter at high pressure (above 30 atm) associated to a fast colour shifter as diphenylstilbene. In the present work some improvements in the crucial point of the energy resolution has been obtained. However further work is needed to achieve very satisfactory performances. In fact we have shown above that the energy resolution for 5 MeV alpha particles is determined mainly by the statistical factor. At lower values of the energy expended in the counter, as for measurements of helium nucleus recoils in a polarization experiment or in high-energy coherent interaction3), this factor may result'prohibitively large. From this point of view is very important a high
luminuous response as that of a (He-NE-NO+DPS) counter. References ~) J. B. Birks, The theory" and practice of scintillation counting (Pergamon Press, Oxford, 1964) ch. 14. 2) R . J . Esterling and N. H. Lipman, Rev. Sci. Instr. 36 (1965) 493. 3) E. Fiorini, M. Pignanelli and A . J . Herz, Report CERN] ECFA (1966) WG2/US-SG4. 4) p. Guazzoni and M. Pignanelli, Phys. Letters .28 A (1968) 432. 5) W. R. Bennet, Ann. Phys. 18 (1962) 367. 6) G. L. Morgan and R. L. Walter, Nucl. Instr. and Meth. 58 (1968) 277. v) p. Guazzoni and M. Pignanelli, Nuovo Cimento 40 B (1965) 454. s) S. Baldin, V. V. Gabrilovsky and F. E. Chukreev, Atomnaya Energiya 3 (1957) 331. 9) L. Koch, Thesis (Paris, 1959). 10) j. A. Northrop and J. C. Gursky, Nucl. Instr. 3 (1958) 207. 11) R. Henck and A. Coche, 1EEE Trans. Nucl. Sci. 14 (1967) 478.