Recombination luminescence in the scintillation of high pressure argon gas induced by alpha particles

N U C L E A R I N S T R U M E N T S AND METHODS

150 ( 1 9 7 8 )

517-521

, (~) N O R T H - H O L L A N D PUBLISHING CO

RECOMBINATION LUMINESCENCE IN THE SCINTILLATION OF HIGH PRESSURE ARGON GAS INDUCED BY ALPHA PARTICLES SATOSH1 KONNO and TAN TAKAHASHI

Radtatton Laboratory, The lnstttute of Physwal and Chemtcal Research, Wako-sht, Sattama, 351, Japan Received 29 August 1977 The dependence of the scmtdlation light y~eld induced by alpha particles upon the electric field has been stud~ed for argon gas m the pressure range of 5-25 atm Simultaneously with the observatmn of the sclntdlatlon, the ion current extracted from the alpha tracks by the field was measured The results show that the light yield decreases wtth the electric field The electric field effect on the scintillation can be explained by assuming that a part of the scmtillatton ~s due to luminescence from recombination of tons which Js affected by the field The observed hght yield also depends on the pressure and chppmg ttme constant of the pulse amphfier It Is shown that the intensity of recombmauon luminescence increases w~th pressure and accounts for as much as 37% of the total scmtdlation light yield at 25 atm The pressure dependence of the recombination luminescence can be interpreted by the Jaffe theory and the two-body recombination mechamsm

1. Introduction The gas scintillation counter has often been used for the detection of charged particles and the gas proportional scintillation counter has become an important nuclear detector in recent years. To understand extensively the mechanisms of these counters, it is essential to study the electric field effect on the scintillation light yield. In an earlier paperi), one of the authors showed that the light yield of high pressure helium gas scintillation induced by alpha particles decreased when an electric field was applied perpendicularly to the alpha-particle tracks. It was also shown that such an effect was not observed at pressures less than 3 atm 2) and the light yield was constant for helium gas when an electric field E was applied up to E/p-1.0 V/cm.torr, where p is the gas pres:sure. From these observations, the existence of a field dependent component of scintillation is expected for all h~gh pressure rare gases. In this paper, we report experimental results on ~Lhe electric field effect for high pressure argon gas scintillation by changing the gas pressure and clipping time for the output pulse from the photomultiplier.

the aluminlum electrodes. The two rectangular electrodes ( 2 0 m m × 2 0 m m ) were fixed 6 . 8 m m apart and an electric field perpendicular to the alpha-particle tracks was applied. The emitted light was observed at right angles to the alpha tracks through a glass window by using a photomultiplier. At pressures greater than 4 atm of argon, the range of the alpha particle was well inside the region of the uniform electric field. Usually, the inner surface of the window and electrodes were coated with sodium salicylate to a thickness of about 1 mg/cm 2. The gas filling system consists of two charcoal traps made of stainless steel and stainless steel pipes which lead the argon gas to the vessel. As the light yield is sensitive to impurities in the present experiment, the vessel and gas filling system were baked out for more than 8 h at about 150°C and 300°C respectively, and evacuated to about 1 × 10 -6 torr before filling Reagent grade argon ~ was then slowly fed into the vessel through two charcoal traps in series cooled by a dry icealcohol mixture. The output pulse from the photomultiplier (56 AVP) was clipped by the RC network, which consisted of the photomultiplier load resistance 2. Experimental procedure and the stray capacitance In the measurement of The cylindrical vessel used for the study of ar- the light yield of the scmtillation, clipping times of gon gas scintillation was almost the same as de- 50 ~s, 0.5 tts, and 20 ns were used. These output scribed in ref 1. Alpha particles from a 2~°Po pulses were sent through a pulse shaper into a source were collimated by a Teflon collimator 0.5 mm thick which has a hole of - 0 . 3 mm di- *~ Purchased from Takachlho Trading C o , the nominal purity ameter, in order to prevent them from reaching of the gas being 99 999%

518

S

KONNO

AND

T

TAKAHASHI

400-channel pulse height analyzer The ion current produced by the alpha particles was measured by the use of vibrating-reed electrometer.

70 6 0 a.

3. Experimental results When the window and electrodes were not coated with the wavelength shifter, scintillation was scarcely observed at pressures less than 25 atm The pulse height markedly increased when the inner surface of the window and electrodes were coated with sodium salicylate, and the pulse height spectrum had a peak for pressures htgher than 4atm. The full width at half maximum of the peak was about 25% for the pressure range of 5-21 atm. The electric field effect on the scintfllatton hght was investtgated at field strengths up to 880 V/cm applied between the electrodes, and the saturauon curve of the ion current was also measured Before and after the measurement where the electric field was apphed, pulse height spectra were taken under the field-free condmon, and the ratio of the peak channel number wtth field to that without field was calculated The decrease m light yield amounted to 19% at 17atm and about 37% at 25 atm for a clipping ttme of 50/zs. In fig 1 are shown the curves of scintillation light yield vs applied electric field, together wnh the saturation curves of the ion current. The experimental results are summarized in table 1. Repeated measurements at 17 atm showed that the percentage of decrease m the hght yield for complete ion collection was (19_+ 1)% for a clipping time of 50/~s and (8 4_+0 5)% for a chppmg time of 20 ns. These values were used for the normahzation of the theorettcal curves in the discussion From the relation between the field strength E

"E

~3

c]

.o

5atrn

50

3.o

[3atm

/

I 0

×--

j S 05

A ~-

I0

_1

_c

o-

Electrm F,eld Strength I 0 [kV/cm] 8

o-

D F

x--

20

g ~,

¢3

F 3(

-/5--

F~g 1 Collected ion current and percentage of hght y~eld vs electric field T h e pressures and are as follows (A) 5 arm, 20 n s , (B) 13 atm, 20 20ns,(D) 21atm, 20ns.(E) 17atm, 50us,(F)

decrease m the chppmg umes n s , (C) 17 arm, 2 1 a t m 50/~s

and the reciprocal of ion current t, the radius of the iomzed column of the alpha-pamcle track was esumated by using the Jaffe theory. Fag. 2 illustrates the relation between 1/t and /(x), where x = [(bk/2 D) E sin 0] 2 and/(x) = e' jrcH~1~0x)/2. In thts formula, E sm 0 ms the component of the field perpendicular to alpha-particle tracks, D the diffusion coefficient of tons, k their average mobihty, b an inmal mean-square radtus of a Gaussian radial dlstrtbution of the lomzed column, H~'(jx) the Hankel function, and j the imaginary unit. According to the Jaffe theory, there should be a lin-

TABLF 1

Elecmc field effect on the a l p h a - p a m c l e - m d u c e d scmullat~on T h e hght yield ~s dw~ded into two c o n s u t u e n t s L e and L r, where L e ~s due to exc~taUon per u m t path length of alpha track and L r is due to recombmaUon per u m t path length T h e observed decrease m hght yield can be expressed as Lr/(Le+Lr), and L r / L e was calculated from the observed value of L r / ( L e + L r) Chppmg u m e Pressure (atm)

5 13 17 21 25

20 ns

50 ~s

L r/(L e + L r)

L r/L e

L r/(L e + Lr)

Lr/Le

(%)

(%)

(%)

(%)

1 0_+03 55_+06 84_+05 14 _+2 21 _+4

I 0_+03 64_+08 92_+06 16 ___2 27 _+4

30_+06 115+_12 19 +1 30 _+4 37 _+4

30_+06 13 _+2 24 +__2 42 _+6 59 +_6

RECOMBINATION

xlO"[ s Ol 4 ok '~

30

7

2.0 f I0 O0

o

JTo..~,.~ ~ x ~ ./x-

J

05

J

to

f (x) Fig 2 Relation between l / t and f ( x ) at 13atm and 17atm Sansfactory hnearlty is obtained for b0 = 3 × 10- 3 cm

ear relation between 1/i and f(x), if a proper value of b is chosen. As shown in fig. 2, a linear relation between 1/i and f(x) was obtained for b = 2 × 1 0 - 4 c m at 17atm The value o f b at 1 atm, b0, is found to be about 3× 10 -3 cm from the relation b = bo/p, where p is the pressure in atm. The saturation current I can also be obtained by extrapolating f(x)--,O 3). The pulse shape of the scintillation for E = 0 was observed by an oscilloscope and the rise time of ~.he scintillation pulse was found to be less than 80 ns, the decay time was about 200 ns for the pressure range investigated. ,4.

LUMINESCENCE

Under this assumption, L e can be regarded as being proportional to the energy density deposited by the alpha particle Thus, L e can be assumed to be proportional to the pressure (p). The pressure dependence of Lr is more complicated because L~ is associated with inter-atomic (molecular) processes. If the column of the alpha track is uniformly ionized within the column of radius b, dL r/dt is thought to be proportional to ~rb2(dn/dt) = -~zzrb 2 n 2 for the two-body recombination, e.g., Ar2+ + e - - , A r * + A r . Here, n is the volume density of ions in the column, cz is the recombination coefficient, and Ar* denotes an argon atom in the excited state. From the differential equation d n / d t = -~zn 2, n is found to be n , / ( l + u n , O, where n, is the initial density of ions. Integrating - c ~ r c b 2 n ~ / ( l + ~ n , t ) 2 over t, L r is found proportional to p - 2 n2 [~T~/(1 + c~n, T~)], where TI is the upper limit of the integration. Since n, is about 2.5 times larger than the density of the excated species (nex)5), L,/L~ oc p-2 n2, [~T,/(1 +~n, T,)J/p-2 n~, ~-

2.5 a T 1 p3/(1 + a T l p3),

* It m~ght be inquired that amphficatton of primary sclntdlanon would mask the field-reduced decrease m the hgha yield, and L e would be smaller than the observed value Such a compensanon seems to be very small, although we cannot exclude completely such a poss]bfltty In th~s paper, the m i n i m u m hght yield observed was defined as L e

(1)

where a as a constant. T~ can be regarded as a measure of the clipping time. For sufficiently large clipping times, Lr/L~ would be independ07

Discussion

The scintillation light yield can be divided into lwo constituents Le and Lr, where L e is the scintdlation light yield due to excitation per unit path length of alpha track and L~ is due to recom bination per unit length~.4). Under the condition of complete aon collection, L~(i= I) is zero*. The value of L J ( L e + L ~ ) can be obtained from the experimental data. The second and fourth columns of table 1 show this ratio m percentage. Lr(i--0 ) for the field-free condition is expressed simply as L r hereafter. Naturally, both Le and L~ depend on pressure. If the mechanism of the scintillation due to excitation does not change appreciably with pressure, the product (mean value of Le)x(range of the alpha particle) will be independent of the pressure.

519

iI

06 05 L~e

50~s /

04

! I I I

I I I I I

/

I /

05 I

I

I I

20ns

II l

OZ I

/

I

O I O0

r

0

5

I0

15

2~0

2r5

Pressure Eatm] Fig 3 Pressure dependence of L r / L e Sohd and dotted lines represent eq (1) normahzed to the experimental values at 17 atm for chppmg rimes of 20 ns and 50~ts © and × show experimental values for chppmg nines of 20 ns and 50 ~ts respecnvely

520

s

KONNO AND T

TAKAHASHI

I 5~Xe_ I0

ent of pressure, if the recombination continues untd all ions in the column are lost by this process. If the clipping time TI is less than 1 #s, Lr/Le would be approximately proportional to p3, since ~z is of the order of 10 -6 cm3/sorless *. >From the observed values of Lr/(Le+Lr), Lr/Le can be obtained. The experimental values Lr/Le are shown in table 1 and fig. 3. In fig. 3 the solid 0 and dotted hnes represent eq. (1), normahzed to L r / L e = 0.092+_0.006 and 0.24+_0.02 at 17 atm for clipping time constants of 20 ns and 50/.ts respec5I L 012 I I i J tively (table 1). 0% 04 06 The foregoing discussion is based on the asE/p [V/cm-Torr] sumption that the recombination time is longer than the thermalization of secondary electrons. Fig 4 Electric field effect on the scmtdlatlon hght yield for The time required for an electron having an ener- unpunfied argon gas of 10 arm for a chppmg time of 50 ~s and gy of 10 eV to lose this by elastic collisions with that of xenon gas obtained by Dolgoshem et al 7) argon atoms to 0.2 eV is 1.8× 10-3S at 1 torr6). Thus, the thermahzation time is estimated to be elastic scattering of electrons in the gaseous meabout 10 8 s at 17 atm. The recombination tame is diumlL~2). The amplification of primary scintillaestimated to be of the order of 10 7s at 17 atm tion occurred at lower field strengths for unpurifrom n = n,/(l+~n, t)t. Actually, the pressure de- fled argon than for purified argon. This might be pendence agreed well with eq. (1) at relatively high explained by the fact that the first excited level of pressure (fig 3). A slight dewatlon at pressures the tmpurity (e.g. N2) is lower than that of argon. less than about 10 atm might be due to the diffusion of ions or the amplification of the primary The authors wish to acknowledge valuable disscintillation (see fig. 1, curve A). Furthermore, the cuss~ons with Drs. T. Hamada and T. Doke. We concept of "plasma on the track" 4) might be nec- are grateful to Drs. S. Kubota and T. Katou for essary for the full understanding of the present their advice and useful comments. experiment. The electric field effect on the scintillation light y~eld of unpurified argon gas of 10 atm and that References of xenon gas obtained by Doigoshein et al. 7) are I) T Takahash], J Phys Soc Japan 24 (1968) 561 shown in fig. 4. 2) S Kubota, T Takahashl and T Doke, Phys Rev 165 (1968) 225 The light yield decreased at low electric field 3) j W Boag, Radtatton dostmetry (eds F H Attlx and W and then increased The decrease of hght yield can C Roesch, Academic Press, New York, 1966) 2nd ed vol be explained by the prevention of electron-ion re2, ch 9, p 32 combination as described above. The increase at 4) B A Dolgoshem, V N. Lebedenko, A M Rozozhm, B U Rodlonov and E N Shuvalova, Soy Phys JETP 29 high field, which is the basic property of gas scm(1969) 619 tdlat~on applied to the gas proportional scintillation L Platzman, Radiation btology and medwme (ed W. D counterS), may be explained by the formation of 5) R Claus, Addison-Wesley, New York, 1958) ch 2, p 53 excited species by energetic electrons 9J°) and/or 6) G L Bragha, G M de 'Munan and G Mambnam, Comlby the emission of electromagnetic radiation in the taro Nazlonale Energla Nuclear (1965) RT/F1 (65) p 61

--

/

* Recombination coefficient ~z for the reaction Ar2+ + e - ~s 8 5 x 10 7 cm3/s at 300 K according to ref 13 However, a neutral-assisted component which depends on the pressure was found m hehum afterglow experiments [e g , R Deloche et al , Phys Rev A 13 (1976) 1140] No information is available concerning such a process for argon t If the track of the alpha particle is assumed to be uniformly iomzed within the column of radms b = 2 x 10 -4 cm, n] is calculated to be about 8;,< 10 ]2 cm -3 at 17 atm

7) B A Dolgoshem, V N Lebedenko and B U Rodlonov, Soy Phys JETP Lett 6 (1967) 224 8) See e . g . C A N Conde and A J P L Pohcarpo, Nucl Instr a n d M e t h 53 (1967)7,A J P L. Pohcarpo, M A F Alves and C A N Conde, ibld 55 (1967) 105, J R Benett and A J Colhnson, J Phys B2 (1969)571, A J P L Pohcarpo, M A F Alves, M C M dos Santos and M J T Carvalho, Nucl Instr and Meth 102 (1972) 337, H. E Palmer and L A Braby, lbld 116 (1974) 587, A J P L Pohcarpo, M A F Alves, M Salete S C P Leite and M C M dos Santos, lbld 118 (1974) 221, P E Thless and

RECOMBINATION G H. Mdey, IEEE Trans Nucl. Sct NS-21 no. 1 (1974); C A N Conde, M C M Santos, M. Fatima, A Ferre~ra and C A. Sousa, ibld NS-22 no. 1 (1975) 104; H E Palmer, ibid. NS-22 no 1 (1975) 100, A J P L. Pohcarpo, M A F. Alves and M Salete S C P Lelte, Nucl Instr and Meth 128 (1975) 49, H L Beck, J E McLaughhn and K Mdler, IEEE Trans Nucl Scl. NS-23, no 1 (1976) 676, C. A N Conde, L R Ferrelra and M F. A Ferrelra, lbld NS-24, no. 1 (1977) 221, D. F Anderson, W Ku, D D Mitchell, R Novlck and R S Wolff, lbld NS-24, no 1 (1977) 283, R D Andresen, E - A Lelmann, A Peacock, B. G Taylor, G Brownhe and P. Sanford, ibld NS-24, no 1 (1977) 810, R D Andresen, E A Lelmann and A Pea-

LUMINESCENCE

521

cock, Nucl Instr and Meth 140 (1977) 371 9) j p Moruccl and A Lanslart, 1EEE Trans Nucl Scl NS17, no 3 (1970) 95. 101 G L Bragha, G M de'Munan and G Mambnam, Nuovo Clm 41B (1966) 96, ibld 43B (1966) 130, G L Bragha, L Gabba, F Gmslano, G M de'Munan and G Mambrlanl, Nuovo Clm 56B (1968) 283 ll) y A Butlkov, B A. Dolgoshem, V N. Lebedenko, A M Rogozhm and B U. Rodlonov, Sov Phys JETP 30 (1970) 24 12) j D. Jackson, Classtcal electrodynamtcs (J Wiley, New York, 1962) 13) F J Mehr and M A Biondt, Phys Rev 176 (1968) 322