Variation of scintillation decay in liquid argon excited by electrons and alpha particles

NUCLEAR INSTRUMENTS

AND METHODS

150 ( 1 9 7 8 )

5 6 1 - 5 6 4 , (~

NORTH-HOLLAND

P U B L I S H I N G CO

VARIATION OF SCINTILLATION DECAY IN LIQUID ARGON EXCITED BY ELECTRONS AND ALPHA PARTICLES SH1NZOU KUBOTA, MASAHIKO HISHIDA and AKIRA NOHARA*

Rtkkyo Umverstty, Nis;-lkebukuro, Tokyo, 171, Japan Recewed 26 July 1977 The scintillation decay of liquid argon exoted by 2°7B1 internal converston electrons and by 21°po 7-particles has been stud~ed by a single-photon counting technique The scmtdlatlon decay exhibits a fast component of about 5 ns and a slow component which decay over several ~ts The relative mtensmes of the fast component mcrease with increasing lomzat~on density, which ts d~rectly contrary to the effect m orgamc scintillators A brief cons~deratlon is gwen on the liquid argon scmtdlatlon light output for electrons and ~z-part~cles

1. Introduction There has been considerable interest recently in the luminescence of condensed states of rare gases for use in the detection of ionizing charged particles ~) and for possible use as ultraviolet lasers2). Work by Northrop and Nobles 3) suggested that argon, krypton and xenon all gave a greater light output in the liquid phase and had shorter decay times. There have been several measurements of decay times of the liquid argon scintillation3'4), but the decay curve ts lacking. Very recently, Suemoto et al. 5) observed the luminescence decay of liquid argon to be (1.15___0.15)~s and found that the time integrated intensity of the fast component is very low compared to that of the slower component. We report here on a study of scintillation decay curves of liquid argon excited by 2°7Bi internal conversion electrons and 21°po ~z-particles. The results show that the decay curves depend on the mode of excitation. In addition evidence for the non-existence of ionization quenching in liquid argon is obtained.

2. Experimental procedure To observe the decay curve, a stainless-steel vessel with a glass window was used. A 2~°po or (5.3 MeV) source, or 2°7Bi internal conversion electron source, which emits electrons with energies of 1.05, 0.976, 0.55 and 0.48 MeV, was deposited chemically on the center of the vessel. The radioactive decay rates for the different particles were less than 100 cps. The inner surface of the windQw

Present address

Ta~syo-Se~mel Ltd, Tokyo, Japan

was coated with POPOP to shift the UV liquid argon scintillation to the visible wavelength region. POPOP was used because of its fast fluorescence decay time of 0.93 ns 6). For the decay curve measurements the singlephoton counting technique w a s usedT). The scintillation output from the window was divided into two parts. A time-to-amplitude converter (Ortec model 437) was started by a trigger pulse from a photomultiplier (RCA 8575) that collected a large fraction of the scintillatio~a light. A much smaller portion of the scintillation passed through a glass fiber into a photomultiplier (DuMont 56AVP) cooled to about - 4 0 ° C . A trigger pulse from this photomultiplier was used to stop the time-to-amplitude converter. The dark current of this stop photomultiplier is less than 150 cps. The pulses from the time-to-amplitude converter were fed into a multichannel analyzer where the decay curve was accumulated. Ortec models 453 and 463 constant fraction timing discriminators were used to derive timing signals from the start and the stop photomultipliers, respectively. Typical resolving times obtained from this measurements were 2.0 ns for the standard deviation of the Gaussian apparatus functionsS). The vessel and the gas filling system were baked at about 120°C for more than 24 h. The ultin, ate vacuum obtained was about 2 × 1 0 -7 tort. The outgassing rate was less than 10 -8 torr.l/s. Ultra high purity argon (obtained from Toshiba Co. Ltd. and containing 2 ppm N2; 0.5 ppm 02; 0.2 ppm CmH,; H2 and CO2 were not detected) was liquified into the vessel cooled with liquid oxygen refrigerant maintaining 94 K, corresponding to an argon pressure of 2 atm.

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3. Experimental results In fig. 1 typical scintillation decay curves are presented for excitation by electrons of energies about 1 MeV (whose energies are selected by the start timing dtscriminator, corresponding to K and L conversion electrons of energies of 1.05 and 0.976 MeV, respectively), and by 21°Po at-particles (5.3 MeV). The two curves are normalized at the peaks in intensities at zero time. It is apparent that the scintillation decays exhibit fast decay and slow decay components. The apparent decay ttme of the fastest component is about 5 ns, which does not depend appreciably on the mode of excitation. The figure evidently shows that the relative intensity of the slow component excited by electrons is larger than that excited by ~z-particles. This effect ~s dtrectly contrary to the effect in organic scintillators, m which the relative intensity of the slow component increases with increasing specific iontzatton densityT'9). In an auxiliary experiment, pulse-height distributions were measured from mixed a 2~°po and 2°7Bi source under conditions such that the photomultiplier current was integrated over a period of 300/zs. From the pulse height data we find the ratio L~/L~ to be 5.7_+°6, where L~ and L~ are the i

i

pulse heights for 5.3 MeV (=E=) ~z-particles and for electrons of energy (E~) about 1 MeV, which are composed of 1.06 MeV L conversion electrons and 0.976 MeV K conversion electrons. This value implies that the intensities per unit energy are in the ratio (L~/E~)(L~/EB) -1 = 1.08+°1104, and that the total scintillation output is proportional to the particle energy dissipated m the liquid argon. Using these data, we show in fig. 2 the decay curves normalized m area to intensities in the ratio E~/E/~ =5.3. The data were corrected for a background resulting from chance coincidences of start and stop pulses. It can be seen that the shapes of the two curves are approximately independent of the mode of excitation over the range of time from about 0 5 to 5 ps. The apparent decay times for this ttme range are (1.1 +__0.1)/zs and (1.2 +_ 0.1)/ts for electrons and ~z-particles, respectively. Following Bollinger and ThomasT), the decay reformation ms presented in table 1 by giving the fraction of the scintillation that is emitted with time delays greater than several specified limtts. In order to show the remarkable difference of the decay on the mode of excitation between decays in liquid argon and those in an organic scintillator, the decays for stilbene measured by Bollinger and

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300 400 500 Time ( ns ) Fig 1 T i m e dependence o f scmtdlatton lntenstty for hqu]d argon T h e points given at t~mes greater t h a n about 100 n s were obtained by averaging the data for several c h a n n e l s Each o f the two curves was m e a s u r e d m a t~me of about 80 m m

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Fig 2 T i m e dependence of t h e intensity for the slow compon e n t o f lmqmd argon scintillation T h e points g~ven at times greater than about 0 5/~s were obtained by averaging the data for several c h a n n e l s

VARIATION

OF S C I N T I L L A T I O N

TABLE 1 Percentage of light emitted at t~mes greater than t(ns) but less than T(~ts) after the time of excitation of hqmd argon and of a stflbene scintillator. Here, T is the measured time range of 5 ~s and 75 ~s for hquid argon and stflbene, respectively

t(ns) 0 20 50 100 200 500 1000 2000 5000 10000 750OO

Llqmd argon (this exp.) Mode of excitation electrons a~-partlcles 100 66 60 55 50 38 24 10 0

100 38 23 17 14 11 74 34 0

Stflbene (ref 7) Mode of excitation electrons o~-part~cles 100 15 86 68 5.4 40 32

13 0

100 54 44 38 32 24 19

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Thomas 7) are also shown in table I. As shown in fig. 2 and table I, the fast component is enhanced for u-particle excitation compared with that for electrons in liquid argon. This effect makes pulseshape discrimination feasible. 4. Discussion The liquid argon scintillation consists of a narrow band UV continuum due to transitions from the excited molecule Ar~ to the repulsive ground state1°). Martin 11) and Molchanov ~2) suggested that this excited molecule is created either by selftrapping of a free exciton or by recombination of a self-trapped hole and a free electron. According to experiments on the effect of electric fields on the liquid argon scintillation by one of the authors~3), it was shown that 67% of the produced emission is due to the recombination process of holes and electrons and 33% to the emission from self-trapped excltons. Therefore, it is emphasized here that a large fraction of the scintillation is due to the recombination process. It is certain that the rate of recombination is rapid in the case of or-particle excitation compared with that for fast electron excitation, because of its high specific ionization density. Thus we conclude that the enhanced fast component excited by ~x-particles is due to the emission from Ar~ which is created by the recombination, process at an initial stage of high densities of electrons and self-trapped holes. In this case the decay is essentially limited by the decay time of Ar~.

DECAY

563

According to an earlier experimentS), the decay curve of Ari~, free from the recombination process and measured by applying an electric field, can be described as the sum of two exponential decays with decay times of (5.0+_0.2)ns and (0.86_0.03)/as. Following Mulliken t4) and LorentzlS), we infer that the fast and slow components correspond to t~+...~l~; and 3X+~--,IX~ transitions, respectively. Thus we think that our observed two apparent decay times (5 ns for the fastest decay and 1.15/as for the time range from 0.5 to 5/as), which do not appreciably depend on the mode of excitation, reflect the deays of the above two excited states. It is very interesting to compare the decay curves of liquid argon with those of organic scintillators. It is well known that in an organic scintillator6'7'9) the relative Intensities of the slow component increase with increasing specific ionization density when the amplitudes are normalized at their peaks in the form of fig. 1 (see table 1). Birks 9) simply explained the variation of decay curves in an organic scintillator by assuming that only the fast scintillation component is subject to non-radiative ionization quenching and that the delayed slow scintillation component originates from a process which is not subject to the main "static" ionization quenching. Thus the heavy charged particles produced much less l i g h t than electrons for the same energy loss in an organic scintillator. In the case of liquid argon, we find a remarkable difference in that the relative intensity of the slow component decreases with increasing specific Ionization density. A plausible explanation of this difference is ~6) that the liquid argon scintillation is not subject to non-radiative ionization quenching as in the gas phase 9). Extending this explanation to the scintillation intensity, it is to be expected that the scintillation light output is practically proportional to the particle energy dissipated in the liquid argon over a wide range of specific ionization densities. The present experiment shows that this is the case. The problem of the scintillation response to heavy charged particles is still of interest. The authors wish to thank Dr. T. Doke, Dr. T. Takahashl and Mr. H. Murakami for their stimulating discussions throughout this work. They are also grateful to Dr. T. A. King for reading the manuscript and for his critical comments. Finally,

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one of the authors (S.K.) thanks Nishina Memorial Foundation for the financial support o f this experiment.

References 1) A Lanslart, A Seigneur, J Morett~ and J Morucc~, Nucl Instr and Meth 135 (1976) 47 2) N G Basov, E M Balashov, O V Bogdankevltch, V A Dandychev, G N Kashmkov, N P Lantzov and D D Khodkevltch, J Luminescence 1, 2 (1970) 834 3) j A Northrop and R Noble, Nucleomcs 14, no 4 (1956) 36 IRE Trans Nucl Sc~ NS-5, no 4 (1956)59 4) R A Giles and E J Burge, Rev Scl Instr 34 (1963)709 5) T Suemoto, Y Kondo and H Kanzakl, Phys Lett 61A (1977) 131 6) j B Blrks, Photophyslcs o! aromattc molecules (J Wdey, New York, 1970)

7) L M Bolhnger and G E Thomas, Rev Scl Instr 32 (1961) 1044 8) S Kubota, M Hlshlda and K Gen, J Phys C (1978) 9) j B. Blrks, The theory and ~racttce of scmtdlatton counting (Pergamon Press, New York, 1964). 10) j Jortner, L Meyer, S A Rice and E G. Wdson, J Chem Phys 42 (1965) 4250 11) M Martin, J Chem Phys 54 (1971)3289 12) A G Molchanov, Sov Phys USPEKHI 15 (1972) 124 13) S Kubota, A Nakamoto, T Takahashl, T Hamada, E Shlbamura, M Mlyajlma, K Masuda and T Doke, Phys Rev 17B (1978), March 15th (m press) 14) R S Mulhken, J Chem Phys 52 (1970)5170, Rad Res 59 (1974) 357 15) D C Lorents, Phys~ca 82C (1976) 19 16) The writers are indebted to Dr T Doke who first suggested to them th~s explanation of the above phenomena 17) A Sayres and C S Wu, Rev Scl lnstr 28 (1957) 758