High pressure gas scintillation counters II

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H I G H P R E S S U R E GAS S C I N T I L L A T I O N C O U N T E R S H J. G. JENKIN* and R. E. SHAMU** Agstralian National University, Canberra

Received 30 November 1964 A gas scintillation counter which has a high resolution is described. For a filling of about 30 atm xenon and about 100 arm helium the spread in pulse heights of 14.5 MeV recoil alpha particles was observed to be less than 5 ~o. The linearity of scintillation response to alpha particles was studied for several mixtures of helium and xenon by bombarding the counter with mono-energetic neutrons. For a mixture of 42 atm helium and 8 arm xenon the pulse height of the maximum energy helium recoils was found to be directly proportional to energy for the

neutron energy range 1 to 8 MeV, a result consistent with earlier work. However, for mixtures containing 34 atm xenon, the response was observed to be nonlinear for neutron energies less than 4 MeV. The scintillation response of helium-xenon mixtures to electrons and alpha particles was studied as a function of helium pressure. The results suggest that the response of a helium-xenon mixture to a charged particle can depend on the dE/dx of the particle.

1. Introduction O f the many applications o f noble gas scintillators, one o f the most i m p o r t a n t is the use o f a helium scintillator as an analyzer o f polarized neutrons. F o r this application the helium can be used in either its gaseous ~) or its liquid z) state. The obvious advantage o f the liquid is that it has a higher density and therefore a higher efficiency for scattering neutrons. O n the other hand, it appears that larger scintillation pulses and smaller spreads in pulse height can be obtained more readily with the gas3), primarily because in this state the helium can be mixed with xenon+). Thus, gaseous helium scintillators can be used in experiments involving low energy neutrons ( ~ 1 MeV) and in experiments in which more t h a n one group o f neutrons is present. For the latter case a good resolution may allow the polarization o f individual neutron groups to be measured. The purpose o f the present paper is to describe a method for constructing a high-resolution counter, and to report measurements o f the scintillation response o f various high-pressure helium-xenon mixtures + to electrons and alpha particles. I n section 2 the gas scintillation counter is described. The resolution obtained with the counter is discussed in section 3. The measurements o f scintillation response are reported in section 4. M u c h o f the present work is an extension o f work described in a publication 3) which shall be referred to as I. Some o f the results o f the present work have been

mentioned already in another paper by the present authorsS). A review o f noble gas scintillators has been published by M u r r a y ' ) .

* Now at A.E.R.E., Harwell, Berks, England. ** Now at Argonne National Laboratory, Argonne, Ill. U.S.A. + The helium used in the present work was found to contain nitrogen and oxygen in quantities less than 65 ppm and 15 ppm, respectively. The xenon was reported by the suppliers to contain nitrogen and krypton, both in quantities less than about 20 ppm. No attempt was made to remove these impurities.

2. Description of the counter The high pressure cell o f the counter used in the present work, which is similar to a cell described in I, is shown in fig. I. The inside o f the stainless steel cell was made highly reflecting by: 1. polishing the steel to a mirror-like finish, 2. evaporating onto the steel a film o f aluminium which was just opaque to visible light and 3. coating the aluminium film with a layer o f M g O about 0.5 m m thick. The M g O was covered by a layer o f diphenylstilbene, a wavelength shifter, which varied in thickness from about 100/zg/cm 2 on the flat surface opposite the glass w i n d o w to about 300 pg/cm 2 on the cylindrical surface near the window. This variation in thickness was obtained by evaporating the diphenylstilbene from a boat located on the axis o f the cell and about 1 c m below the gasket surface. A layer o f diphenylstilbene about 25 pg/cm 2 thick was employed on the inside o f the glass window. Teflon was used as the gasket material between the steel cell and the glass. STAINLESS

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Fig. l. The cell of the high-pressure gas scintillation counter. 116

HIGH PRESSURE GAS SCINTILLATION

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recoil alpha particles. The resolution was essentially the same when neutrons with a much lower energy, e.g. 5 MeV, were used. It is believed that the good resolution obtained in the present work can be attributed primarily to three factors: 1. the large light output of the gas mixture, 2. the high reflectance of the coating inside the cell and 3. the distribution of the diphenylstilbene. The first factor reduced the pulse height spread by decreasing Es :22.6 MW CORRECTED COUNTS the statistical spread associated with the photomulti----G j¢~ T ~ I E T EVACUATED - plier. The second factor reduced this spread by in-XENON r~ creasing the light output and, what is more important, I.Z by minimizing the variation in response from place to 0 rJ place inside the cell. It is clear, for example, that if the coating inside the cell were instead totally absorbing, the response at a given point inside the cell would depend on the solid angle subtended by the window. The last factor also reduced the variation in response with position. The effect on this variation of the thickness of diphenylstilbene on the window was reported in I. The diphenylstilbene layer on the cell wall may have reduced this variation, also. It is known that a 300pg/cm 2 thick layer has a smaller conversion efficiency than a 100pg/cm 2 thick layerT). Thus the distribution of diphenylstilbene on the cell wall may o have compensated for the fact that the diphenylstilbeneo 2o 40 60 I0O 120 CHANNEL NUMBER MgO-aluminium combination was not a perfect Fig. 2. A pulse-height distribution resulting from the interaction reflector. of 22.6 MeV neutrons with alpha particles. Corrections for the An attempt was made to improve the resolution neutron background (tritium gas target evacuated) and the by decreasing the thickness of diphenylstilbene both counter background (xenon only in the counter) are shown and, on the cell wall and on the glass window. A decrease of together with a wall effect correction, have been applied to the spectrum. The peak at channel 43 is caused by disintegration 25 ~ did not change the pulse-height spread appreciably. It is worthwhile to point out that the major difference particles from the He4(n,d)T reaction, The remaining pulses are due to alpha-particle recoils produced by elastic scattering between the present counter and that described in I see ref. 5). is that for the former the diphenylstilbene layer on the cylindrical surface near the window is about The scintillations were viewed through the window 10 times thicker. The much-improved resolution obtained in the present work is evidence that the pulsewith an EMI 9536B photomultiplier. height spread does indeed depend on the diphenyl3. Resolution stilbene distribution on the wall. The resolution of the counter was studied by bombarding it with 22.6 MeV neutrons from the T(d,n) 4. Response to electrons and alpha particles reaction. A pulse height distribution for a filling of The linearity of scintillation response to alpha partiabout 30 atm xenon and 100 atm helium is shown in cles was studied for several mixtures of helium and fig. 2. The dosed circles in this figure correspond to xenon by bombarding the counter with mono-energetic pulses caused by the interaction of neutrons with neutrons. This technique entailed measuring the pulse helium. The peak at channel 43 is due to disintegration height of the maximum-energy recoil alpha particles particles from the He+(n,d)T reaction. The remaining as a function of the incident neutron energy. The monopulses are caused by alpha-particle recoils produced by energetic neutrons were obtained from the T(p,n) and elastic scattering. The tail of the recoil spectrum, i.e. the the T(d,n) reactions. The voltage pulses from the EMI pulses above channel 108, is consistent with a spread photomultiplier were amplified by a Tennelec pre(fwhm) of less than 5~o in the pulse heights of the amplifier and a Franklin amplifier and sorted by a I

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R I D L pulse-height analyzer. The width o f the pulses was limited to about 2 #s by delay-line clipping. The linearity o f the amplifying and analyzing system was checked with the aid o f a Franklin precision pulse generator and was found to be linear to about 1 ~o for the range o f pulse heights o f interest. The results o f these linearity studies are shown in figs. 3 and 4. I n these figures and in fig. 5 the helium partial pressures were determined by subtracting the xenon pressure from the total pressure and therefore are only approximate. It is seen that for the mixture o f 8 atm xenon and 42 atm helium the alpha-particle pulse height is directly proportional to its energy. This i

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Fig. 3. Linearity curves for the gas scintillation counter filled with mixtures of helium and xenon. The ordinates refer to the pulse heights for the maximum-energy helium recoils relative to those for alpha particles from a Po source. The dashed lines below 4 MeV axe extrapolations of the straight lines which fit the higher energy data. The energy intercept of each dashed line is shown in parentheses.

result is in agreement with that obtained in I for a similar gas filling*. However, it is seen that for the mixtures with about 34 atm xenon the pulse height is a linear function o f energy only above about 4 MeV neutron energy (2.6 M e V alpha-particle energy). Such a result indicates that for these mixtures the response may depend on the differential energy loss o f the particle. The dashed lines below about 4 MeV are extrapolations o f the straight lines which fit the data above this energy. The value o f the energy intercept o f each extrapolation is indicated. The value o f this intercept is a measure o f the departure o f the corresponding curve from a directly proportional relationship and therefore is a measure o f the nonlinearity o f response. It is noted that these values increase with an increase in helium pressure. It should be pointed out that the recoil-particle pulse heights which were measured were affected by multiple scattering in the gas. However, multiple scattering effects can account for neither the sign nor the magnitude o f the observed departures from linearity. In view o f the results mentioned above it appeared worthwhile to investigate as a function o f helium pressure the scintillation response o f helium-xenon mixtures to particles with a wide range o f values o f dE/dx. Thus, measurements o f pulse height vs helium * Studies carried out at the Institut fiir Kernphysik, Frankfurt, 8). also confirm the result obtained in L

HIGH PRESSURE GAS SCINTILLATION

pressure were made for 0.5I MeV electrons, 5.3 MeV alpha particles and 29 MeV alpha particles. A xenon pressure of 34 atm was employed for these studies. The average values of dE/dx in xenon for these particles are approximately in the ratio 1:25:10 for 0.51 MeV electrons: 5.3 MeV alphas: 29 MeV alphas. Several techniques were employed to obtain these different types of particles. The electrons were photoelectrons which were produced by the 0.5! MeV gamma rays from a Na 2z source. The 5.3-MeV alpha particles were obtained from a polonium source which was placed inside the counter. The 29 MeV alpha particles were produced by bombarding the counter with 22.6 MeV neutrons. These last-mentioned particles were shown to correspond to xenon disintegrations by measuring their number as a function of xenon pressure. To determine their energy the pulse-height scale of the R I D L analyzer was calibrated with the aid of the 14.5 MeV alpha recoils. The observed Q-value of these disintegrations identified them as (n,g) reactions. In fig. 5, plots of the measured response vs helium pressure are shown for these electrons and alpha particles. The data labelled "15 MeV Xe disintegration particles" will be discussed later. For each curve the data is normalised to the pulse height for the pure xenon filling. The most interesting feature of this data is that the response curves are considerably different. Furthermore, the curves differ in a way which is related to the average dE/dx values of the particles. Thus it appears that for the conditions of the present work the scintillation response of a helium-xenon mixture to a charged particle depends on the dE/dx of the particle. It is seen, also, that the response to the 29 MeV alphas increases appreciably compared to that for the 5.3 MeV alphas for the addition of even a small amount of helium, e.g. 10 atm, to the 34 atm of xenon. Such a behaviour is consistent with the linearity data shown in figs. 3 and 4. It should be pointed out that approximately one-half the observed decrease in response to the 5.3 MeV alphas can be attributed to a geometric effect. As the helium pressure is increased more light is absorbed by the 0.25 mm diameter wire on which the source is deposited because the alpha range is decreased. The observed dependence of scintillation response on ionization density can be employed as a crude method of particle identification. The data in fig. 5 which is labelled "15 MeV xenon disintegration particles" refers to pulses which were distinguished as a "knee" in the pulse-height spectrum of the xenon disintegrations. The response curve for these particles is seen to be very

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Fig. 5. Measurements of the scintillation response of mixtures of 34 atm xenon and increasing amounts of helium to the various particles listed on the figure. The ordinate refers to the respective pulse heights relative to those obtained for the xenon filling. The solid fines are smooth curves through the data points for each kind of particle. similar to that for the 0.51 McV electrons. This behavior suggests that these particles were singly-charged heavy particles, i.e. protons, deuterons, and/or tritons. At an energy of about 15 MeV these particles have an average dE/dx close to that of 0.51 MeV electrons. It is enlightening to compare the response to electrons with that to alphas in terms of light output per MeV. For the filling of 34 atm xenon, for example, the light output per MeV was about 60% larger for the 5.3 MeV alphas than for the photo-electrons. (It is well known that solid scintillators, e.g. stilbene, generally show the opposite behavior). However, for the filling of 34 atm xenon and 105 atm helium, this parameter was about the same for the electrons and the 5.3 MeV alphas. Thus, although fig. 5 suggests that the opposite might be true, adding helium to the xenon tends to equalize the light output per MeV for the two types of particles. The response of xenon to protons, deuterons and alpha particles has been studied by Nobles 9) for the energy range 2 to 5 MeV. A xenon pressure of less than 5 atm appears to have been employed. His measurements indicated: 1. the light output per MeV was the same for these particles and 2. the response was a linear function of energy over the range investigated; however, the extrapolated energy intercept was about 0.5 MeV. His former result does not necessarily disagree with that for xenon reported above since, in the present work, particles with a greater difference in dE/dx were investigated and xenon pressures which were much

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J.G. JENKIN AND R. E. SHAMU

higher were used. On the other hand, his latter result appears to be inconsistent with the present results. The data at the bottom of fig. 3 indicates that the curve of response vs energy has an intercept at zero energy for a counter filling of 8 atm of xenon and 42 arm of helium. The measurements of linearity as a function of helium pressure (figs. 3 and 4) show that adding helium appears to enhance non-linear effects. Thus the present results suggest that the energy intercept would be zero, also, for a filling only of 8 atm of xenon. This apparent discrepancy can be explained if it is assumed that the pulse heights measured by Nobles depended on the length of the track of the exciting particle. He obtained evidence that the pulse height was independent of the track length by measuring the pulse height for Pu 239 alpha particles as a function of gas pressure. However, this evidence is questionable because he neglected to take into account the possibility that the light output of the gas was a function of pressure. Since little is understood about the scintillation mechanism in gases, even at low pressures, no attempt will be made to explain the results of this section in terms of basic processes. Furthermore, the gases which were used were known to contain impurities of nitrogen,

oxygen and krypton in quantities of the order of about 20 ppm. It is well known that an impurity even in this quantity can affect profoundly the properties of the scintillator1°). We wish to thank Mr. J. Pethes for so expertly machining the stainless-steel cell of the counter. The manuscript of this paper was prepared while the authors were at the Rutherford High Energy Laboratory, Chilton, England (R.E.S.) and A.E.R.E., Harwell, England (J.G.J.). The hospitality and cooperation of these laboratories are gratefully acknowledged.

References 1) p. J. Pasma, Nucl. Phys., 6 (1958) 141. 2) J. E. Simmons and R. B. Perkins, Rev. Sci. Instr. 32 (1961) 1173. 3) R. E. Shamu, Nucl. Instr. and Meth. 14 (1961) 297. 4) J. A. Northrop and J. C. G ~ y , Nucl. Instr., 3 (1958) 207. 5) R. E. Shamu and J. G. Jenkin, Phys. Rev. 135 (1964) B99. 6) R. B. Murray, Nuclear Instruments and Their Use, Vol. I (John Wiley & Sons, Inc., New York, 1962) Ch. 2. 7) T. A. Hodges, private communication. s) U. Strohbusch, private communication. 9) R. A. Nobles, Rev. Sci. Instr. 27 (1956) 280. I0) W. R. Bennett, Ann. Phys. 18 (1962) 367.