Estimation of absolute photon yields in liquid argon and xenon for relativistic (1 MeV) electrons

Nuclear Instruments and Methods in Physics Research A291 (1990) 617-620 North-Holland

617

ESTIMATION OF ABSOLUTE PHOTON YIELDS IN LIQUID ARGON AND XENON FOR RELATIVISTIC (1 MeV) ELECTRONS Tadayoshi DOKE'), Kimiaki MASUDA

2)

and Eido SHIBAMURA 2)

Science and Engineering Research Laboratory Waseda University, Kikuecho-17, Shenjuku-ku, Tokyo 162, Japan zl Saetama College of Health, Kameokubo 519, Urawa-she, Saitama 338, Japan

Received 12 December 1989 Using recent scintillation data of liquid argon and xenon, the numbers of photons emitted in both liquids have been estimated. The ideal photon yields are 5 .1 X 10 ° photons/MeV for liquid argon and 6.8 X 10 ° photons/MeV for liquid xenon. The photon yields due to 1 MeV electrons are 4.0 X 10 ° photons and 4.2 X 10 ° photons for respective liquids. These numbers are almost the same as that of a Nal(TI) crystal . 1. Introduction It has been suggested by several investigators [1-3] that the scintillation photon yield in liquid argon or xenon is very high and comparable to that in a NaI(TI) crystal, but they did not give the exact photon yield for any ionizing radiation. At present, the possibility of using liquid-xenon scintillation calorimeters in SSC- or LHC-experiments is being discussed [4,5] because of their fast time response and high photon yield . This is expected to lead to a good energy resolution . To design such a detector, the exact photon yield should be known, but the absolute measurement of such a photon yield has not been made yet. However, we can estimate it in liquid argon and xenon, using the data obtained for both liquids so far [6-11] . In this paper, the results of estimation for both photon yields are presented for relativistic (1 MeV) electrons and other particles. 2. Ideal photon yield As described in a review paper [3], the scintillation from a liquid rare gas is produced through the following two processes of excited atoms R* and ions R + produced by ionizing radiation : (i) R* : R*+R-R2,

R*2 ->2R+hv, (ii) R+: R+ +R-Rz, Rz +e--R**+R, - R* + heat, R* * R* + R -> R2, R2 ->2R+hv. Here, by means an ultraviolet photon, and the process R* * - R* + heat corresponds to a nonradiative transition . In the above processes, the excited dimer Rz, at the lowest excitation level, should be de-excited to the dissociative ground state by emitting one UV-photon, because the energy gap between the lowest excitation level and the ground level is so large that there exists no decay channel except the above radiative transition . Although this is not yet perfectly confirmed by experiments [2], here we assume that one excited dimer emits one photon. Under this assumption, the average energy required for the production of one photon, WPh, can be written as follows [8,10,11]: WPh = W(1 + Nex/Ni),

where W is the W-value for an electron-ion pair production, and Wex and N, are the number of excited atoms and the number of ion pairs produced by ionizing radiation, respectively . These values for liquid argon and xenon are shown in table 1. The W-values in liquid argon and xenon are experimental ones [13,14] and the

Table 1 WPh-values and photon yields in liquid Ar and Xe and parameters for their estimation

Liquid Ar Liquid Xe

W [eV]

Ref.

Nx /N,

Ref.

WPh [eV]

23.6±0 .3 15 .6±0 .3

[13] [13]

0.21 14 .7

(131 [131

19.5 14.7

0168-9002/90/$03 .50 © 1990 - Elsevier Science Publishers B.V . (North-Holland)

Ideal photon yields [ph/MeV] 5.13 X 10 4 6.80 X 10 4

61 8

T. Doke et al. / Estimation of absolute photon yields in liquid Ar and Xe

values of N,,/N, in both liquids are calculated [13] . The value in liquid argon is in good agreement with that experimentally obtained [15,16]. On the other hand, that in liquid xenon has no comparable experimental value . So it has some uncertainty, but should be smaller than that in liquid argon . Thus the uncertainty of the Wph in liquid xenon estimated from eq . (1) will be 10% at maximum, while for liquid argon the estimation error of the Wph should be a few percent . Table 1 also shows the ideal photon yields for deposition energy of 1 MeV in liquid argon and xenon calculated with eq . (1) . Until now, we have measured the relative photon yields in liquid argon and xenon for various particles . By finding one case for which such ideal photon yields are realized in the measurements, we can estimate the absolute photon yields for all particles .

3.1 . Liquid argon

In a series of experiments made at the Bevalac, LBL, for measuring the scintillation yields in liquid argon [8-11], we found that the scintillation for relativistic heavy ions from Ne to La gives the maximum yield, which is 1 .4 times larger than that for a-particles (5 .3 MeV) from 210 Po and 1 .27 times larger for conversion electrons (0 .976 MeV) from 207Bi . Fig. 1 shows schematically the photon yield at zero electric field as a function of the LET given by the particles. In the figure,

1 .33

0 .5 0 f .f . . .1

. . . .i

. .d 1

10

,

. ..i

le

. . .i 10

2

Reduction factor

Liquid Ar Liquid Xe

a-particles

1 MeV electrons

0.71 0.75

0.78 0.62

3

. . .i

la,

la s

LET (MeV glc" )

Fig. 1 . Scintillation yield dL/dE as a function of LET. Solid squares represent the experimental results in liquid xenon and open squares those in liquid argon for incident particles of 0.976 MeV electrons (e - ), 40 MeV protons (p), 5.305 MeV and 6.12 MeV a-particles (alpha) and fission fragments (f.f.) . The open circle represents the experimental result in a Nal(TI) crystal for y-rays, whose LET is assumed to be equal to that of 200 keV electrons.

Photon yield for 1 MeV electrons [pli/MeV] 4.0 x 10 4 4.2 x 10 4

the ideal photon yield is taken to be unity in the ordinate, and the photon yields obtained for relativistic heavy ions, including relativistic protons and helium ions, are expressed by a thick solid curve . These yields are given by the relation dL/dE=C'

3. Photon yields for 1 MeV electrons

1 0 .00

Table 2 Reduction factors for a-particles and 1 MeV electrons, and photon yields for 1 MeV electrons

dE/dx

1 + C(dE/dx)

+rla

which was derived in a previous paper [11], with % = 0 .75 and C' = C. In these experiments, the scintillation and ionization were measured simultaneously as a function of the electric field . The increase in the electric field results in a reduction in scintillation, corresponding to an increase in ionization . From this correspondence, we can estimate the number of emitted photons, assuming that one recombination of an ion pair emits one photon . The sum of the photon number and the collected-electron number gives an almost constant value, which is in good agreement with the ideal photon yield in liquid argon, independent of the electric field (for >__ 3 kV/cm) and the type of particles, except for relativistic gold ions. The constant level is given by a thin solid line in the figure . The low photon yields for relativistic light ions and electrons can be explained by "escaping electrons" as described in ref . [11] . For relativistic heavy ions heavier than La, a-particles and fission fragments, the reduction is attributed to the so-called "quenching effect" [11] . The reduction factors from the ideal line for conversion electrons (0 .976 MeV) from 207Bi and a-par210 ticles (5 .3 MeV) from Po are shown in table 2, as well as the absolute photon yield for 1 MeV electrons, which is given by multiplying the ideal photon yield by the reduction factor . 3.2 . Liquid xenon

The scintillation photon yields for 1 MeV electrons [6], 40 MeV protons [17] and fission fragments [6,7] obtained in liquid xenon were measured by using a-particles (6 .12 MeV) from 212 Bi as a standard [6,7] . They are plotted in fig . 1, assuming that the yield for a-particles (6 .12 MeV) is 1 .4(±0 .2) times larger than that in liquid argon . This value was estimated by comparing the energy resolutions obtained for 4x-particles (6 .12

T. Doke et al / Estimation of absolute photon yields in liquid Ar and Xe MeV), using the same chamber for both liquids . In the estimation it is assumed that the energy resolution is determined only by the statistical fluctuation in the number of photoelectrons . The relation between the energy resolution and the number of photoelectrons is well established for scintillation in liquid argon if a fixed source is used [7] . Here we assume that the maximum yield in liquid xenon is also given for relativistic Ne to La ions, as in liquid argon . The ratio of both yields at the flat parts in liquid argon and xenon should be taken to be

61 9

4. Consistency in the photon yield estimation The results of the estimation mentioned above should be consistent theoretically and experimentally. From the following, it will be shown that fig . 1 does not include any contradiction .

4.1 . Reduction factors due to "escaping electrons" Since the quenching mechanism is still not clear, we cannot know whether the reduction factors obtained for high-LET particles are reasonable or not. As for the reduction factors for low-LET particles such as 1 MeV electrons, a new theory for explaining the reduction was presented [11] recently . According to the theory, the reduction is caused by "escaping electrons", and the larger the fraction of "escaping electrons" is, the smaller the reduction factor . The result of the analysis of the curve of the collected charge as a function of the electric field in liquid argon and xenon on the basis of the "Onsager theory" shows that the fraction of "escaping electrons" in liquid xenon is much larger than in liquid argon [18] . Although the application of the "Onsager theory" to liquid rare gases has some weak points, it should give a measure of "escaping electrons" in liquid argon and xenon . As seen from table 2, the reduction factor for 1 MeV electrons in liquid xenon is considerably smaller than that in liquid argon . This is consistent with the above result that the fraction of escaping electrons is higher in liquid xenon .

Lmax(Xe)/Lmax(Ar) = Wph (Ar)/Wph (Xe) = 1 .33 . Thus, the smooth curve with a flat part, shown by a dotted line, can be drawn . In this case, we also assumed the relation dL/dE = 1 .33C(dE/dx) / [1 + C(dE/dx)] +%, where rlo = 0.31, which is obtained from the fraction of "escaping electrons" (- 0 .73) estimated by Takahashi et al . [18] . The flat part of the smooth curve for liquid argon includes an experimental error of ±5% . The error of the flat part for liquid xenon is ± 15%, except for the uncertainty for the Wph-value in liquid xenon, as described in section 2. Each error bar for electrons, protons, a-particles and fission fragments shows the measurement error of the scintillation yields . The reduction factors for the electrons and a-particles (5 .3 MeV) from the ideal level of the photon yield in liquid xenon are 0.62 and 0 .75, respectively. Using these values and the ideal photon yield, the photon yields for a-particles and 1 MeV electrons in liquid xenon can be obtained . The result for 1 MeV electrons (that is, minimum-ionization particles or "mips") is shown, as well as the reduction factors for both particles . As can be seen from the table, the photon yields of both liquid argon and xenon for mips are comparable to that for NaI(TI) crystals (= 4 .3 X 10 4 photons/MeV) measured by Miyajima et al . [19], which is also shown in fig . 1 .

4.2 . Consistency of the assumption for estimation of the photon yield When estimating the photon yield, we assumed that one excited dimer emits one photon . In a recent measurement of scintillation for a-particles in triethylamine- or trimethylamine-doped liquid xenon, we found that the quantum efficiency for photoionization of the molecules is nearly 100%, assuming that the photon yield is given by eq. (1) and the quenching (reduction)

Table 3 Decay times, intensity ratios and photon yields of various scintillators for 1 MeV electron excitation Scintillator

Liquid Ar Ref .

Liquid Xe Ref.

Nal(TI) Ref .

BaFZ

Ref .

BGO Ref.

Fast decay time Tf [ns] Slow decay time T, [ns] Recombination time , [ns]

6 1590 <1

2 .2 27 45

250 -

0 .6 620 -

[21] [21]

300

Fast intensity ratio If /I(oo) Slow intensity ratio Is /I(oo) Recomb . intensity ratio I,/I(oc) Wp , [eV] Total photon number I(oo) [ph/MeV] Photon for 10 ns, I (10 ns) [ph/MeV] Photon for 3 ns, 1 (3 ns) [ph/MeV]

0 .23 0 .77 25 4 .0 x 1()4 7 .6 x 10 3 3 .7 x 10 3

[23] [23] [23] [231 [231

0 .01 0 .25 0 .74 24 4 .2x10 ° 9 .8x10 3 3 .4x10 3

[22] [22] [23] [22,24] [22,24] [241

[21]

1 .0 [19] 23 4 .3x10 ° 1 .7x10 3 5 .1x10 3

0 .24 [21] 0 .76 [21] 120 [21] 8 .5X10' 2 .1x10 3 2 .0x10 3

[21]

1 .0 360 [21] 2.8x10 3 9.2x10 2.8x10

62 0

T. Doke et al. / Estimation of absolute photon yields m liquid Ar and Xe

factor is the same as that in liquid argon (0 .71) [201 . In this case, the quantum efficiency of 100% for photoioni-

zation also means that all of the photons in the ideal

case are observed . In the present analysis we also obtained the reduction factor (0.75) for a-particles in liquid xenon, which is very close to the assumed value.

This shows that there is no contradiction between the experimental results and the assumption made in the

1 MeV electrons in both liquids are estimated to be 4.0 x 10 4 (±10%) photons/MeV and 4.2 x 10 4 (±20%) photons/MeV. A comparison between several scintilla-

tors (liquid Ar and Xe, Nal(TI), Bafz and BGO), shows that the photon yields in liquid argon and xenon for a gate time of several ns are superior to those in other scintillators .

estimation of the photon yield.

References 5. Comparison with crystal scintillators Table 3 shows the comparison of the photon yield

for 1 MeV electrons in liquid argon and xenon, with those in NaI(TI), BaF2 and BGO crystals . The photon

yield in the table is divided into three cases. The first case shows the total number of photons, which can be measured with a shaping amplifier with a time constant of longer than 6 ps . The second and third cases show the photon yields with gate times of 10 and 3 ns,

respectively. These cases are considered to compare the contribution to the scintillation during a short gate time from all components : fast, slow and recombination. W'

in the table is the average energy for a photon produc-

tion due to 1 MeV electrons. In the table, the decay times and their intensity ratios used in the estimation

are also shown. As can be seen from the table, the photon yields of liquid argon and xenon are comparable

to that of Nal(TI) crystals, having the highest photon yield for a gate time longer than 6 Its . On the other

[11 J.A . Northrop, J.C. Gursky and A.E. Johnsrud, IRE Trans. Nucl . Sci . NS-5 (1958) 81 . [2] L. Lavoie, Med. Phys . 3 (1976) 283. [3] T. Doke, Portugal Phys . 12 (1980) 9. [4] M. Chen et al., Nucl . Instr. and Meth . A267 (1988) 43 . [5] M. Chen et al ., Report on Liquid Detectors (Precision EM and Hadron Calorimeters), Proc . ECFA Workshop,

Barcelona, Spain, Sept . 1989, vol. 1, p. 486. [6] N. Funayama et al., Ionizing Radiation 8 (1981) 62 (in Japanese). A. Hitachi et al ., Nucl. Instr. and Meth. 196 (1982) 97. T. Doke et al ., Nucl . Instr. and Meth . A235 (1985) 136. H.J . Crawford et al ., Nucl . Instr. and Meth . A256 (1987) 47 . [10] E. Shibamura et al., Nucl. Instr. and Meth. A260 (1987) 437. [111 T. Doke et al ., Nucl. Instr. and Meth. A269 (1988) 291 . [121 E.E. Huber, D.A . Emmons and R.M . Lerner, Opt. Commun. 11 (1974) 155. This paper shows that the intrinsic

hand, the photon yields of liquid argon and xenon for 3 ns and for 10 ns are respectively 1.9-1 .7 and 3.6-4 .6

times larger than that of BaF2 , which has the shortest

decay time constant in the table. This is due to the contribution to the fast scintillation from the slow com-

ponent . To decide which liquid should be used in the calorimeter, other factors such as radiation length, den-

sity, wavelength of scintillation light, etc. have to be

considered .

6. Conclusion From the analysis of the scintillation data in liquid

argon and xenon obtained so far, the photon yields for

[131 [141 [151 [16] [17] [181 [191 [20] [21] [221 [231 [241

efficiency of scintillation in liquid xenon is 50-5556, which is just equal to that estimated by the method as described in section 2, assuming 100% quantum efficiency for UVphoton emission . M. Miyajima et al., Phys. Rev. A9 (1974) 1438 . T. Takahashi et al., Phys. Rev. A12 (1975) 1771 . S. Kubota et al ., Phys . Rev . B13 (1976) 1649 . T. Doke et al ., Chem . Phys . Lett . 115 (1985) 164. T. Doke et al ., Ann. Report of INS (University of Tokyo 1983) p. 49 . T. Takahashi et al ., Sci. Paper of IPCR 74 (1980) 65 . M. Miyajima et al ., Nucl . Instr. and Meth . 224 (1984) 331. S. Suzuki et al ., Nucl. Instr. and Meth. A245 (1986) 78 ; A. Hitachi et al ., submitted to Phys . Rev. Lett . M. Aguilar-Benitez et al ., Phys . Lett . B204 (1988) 54. S. Kubota et al ., J. Phys . 11 (1978) 2645 . A. Hitachi et al ., Phys . Rev. 27 (1983) 5272. S. Kubota et al ., Phys . Rev. 17 (1978) 2762 .