Liquid-xenon detectors and their applications

Nuclear Instruments and Methods in Physics Research A299 (1990) 191-194 North-Holland

Liquid-xenon detectors and their applications P. Belli, R. Bernabei and S. d'Angelo

Dipartimento di Fisica, II Umoersità di Roma, and Istituto Nazionale di Fisica Nucleare, Sez. di Roma II, Rome, Italy

L. Aridreanelli, F. Brorizini, A. Bucclieri, A. Iricicchitti and D . Prosperi Dipartimento di Fisica, I Unioersità di Roma, and Istituto Nazionale at Fsica Nucleare, Sez di Roma,

Rome, Italy

We present a brief summary of the properties of liquid xenon as a detector medium and its applications in physics. Some prelirmnary results from a liquid-xenon scintillation counter that we have assembled are also reported . 1. Radiation detection by liquid xenon A survey of the main properties of liquid xenon as regards scintillation, ionization and proportional scintillation can be found in ref. [1]. When scintillation occurs, the emission wavelength is [2] X = 178 ± 1 rim and the bandwidth is AX = 14 ± 2 rim at 160 K (with < 0.01% contamination of N2 , H2 and rare gases) ; the absorption atomic transitions could be estimated from the values measured in gas (147 and 130 rim) and in solid (145 and 129 rim) [3]. Liquid xenon could offer an energy resolution comparable with that of NaI(TI) [4], a high tolerance to radiation damage and a fast time response : t fasi = 3 ns and ts,ow = 20-30 ns [5]. In table 1 we compare the characteristic quantities of liquid xenon with those of other scintillation media. The quoted scintillation efficiency and energy resolution are very similar to those of NaI(TI) up to EY - 100 keV [6]; nonuniformities in light collection could be responsible of the decrease in energy resolution at higher energies . However, the correlation between light response and the presence of im-

Table 1 Comparison of some characteristics of scintillation media [1]

Refractive index Decay time [ns] Rad. length [cm] (d E/d x) m,  [MeV/cm] Energy res. [%] at 1 .275 MeV y 01/e

Liquid xenon

Nal(TI)

BaF2

BGO

1 .41 3, 25 2 .8 at b.p .

1 .85 230 2 .59

1 .56 0.6, 620 2.05

2.15 60,300 112

3 .9

4 .85

6.6

9 .0

-15 1 .1 ±0 .2

5 0 .5

6.2 0.34

11 .5 0 .23

purities such as N2, H 2 O, hydrocarbons, etc. [7] is not well known. UV photomultipliers (with e.g . MgF2, LiF2 windows) or common photomultipliers together with a wave shifter or a readout system with liquid/solid photocathodes [8] could, in principle, be suitable to collect such far-UV scintillation light. In table 2 the main ionization properties of liquid xenon are summarized and compared with those of liquid argon and warm liquids . In liquid xenon the electron avalanche process occurs typically at - (1-2) x 10 6 V/cm field. Large ionization chambers have been realized by Masuda et al . [9] and by Barabash et al. [10] . The first ones obtained a FWHM energy resolution of 7.14% (EY [MeV]) -1/2 up to - 2 kV/cm gridcathode electric field, while the energy resolution for the second apparatus is reported in table 2. The spatial resolution achieved is better than in the gaseous phase, due to the lower number of fluctuations in primary ionization [1] ; the value is ±15-20 pin for a-particles [11], and ±0 .6 mm for 662 keV -y-rays [12] . Note that the electron drift time critically depends on the temperature [12] . Another process in liquid xenon is proportional scintillation [13] . In this case, both ionization and scintillaTable 2 Comparison of some ionization characteristics of liquids [1]

W [eV/pair]

od [10 5 cm/s] at 15

kV/cm Energy res. [%] at 1 .836 MeV y Relative output [mip]

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

Liquid xenon

Liquid argon

TMS

TMP

15 .6±0 .3

23 .6±0 .3

< 73

_< 71

15

3

5

4 .9

4 .1

>_ 4

12 .6

4 .6

1

4 .5 >_ 4 1 .1

I. DETECTORS

192

P Bellt et al. / Liquid-xenon detectors

tion (arising from primary electrons accelerated in the electric field) pulses are detected and the spatial resolution is improved [11] . A relevant point when developing liquid-xenon detectors is the purification process [141 ; in fact, impurities such as 02, CO, C02, H 2O and SF6 absorb energy in collisions with excited atoms or in direct photon absorption . This problem - depending on the kind of impurities - could be overcome using very clean materials, degassing procedures and physical adsorption by : (a) fractional distillation ; (b) chemical adsorption by metallic getters, zeolites and activated charcoal ; and (c) current purification for the electronegative impurities . An alternative solution could be the doping of liquid xenon with triethylamine (TEA) or trimethylamine (TMA) [15] . In this case, the collected charge at 1 kV 2t° p.d . increases by a factor of 10 for a from Po and additive multiplication processes are present in proportional scintillation due to the ionization of TEA and TMA by the photons. 2. Some preliminary results from a scintillator prototype We assembled [16] a scmtillator with 118 cm3 (i .e. about 61 I of gaseous xenon at NTP) maximum volume . Considering that at - 1 bar pressure the temperature difference between boiling and melting points is - 4 K, we used a cryogenerator and a temperature controller (Leybold company) to obtain a stability of - 0.5 K. To collect the scintillation light we use UV photomultipliers with a quantum efficiency in the interesting range of - 12%. To avoid thermal stress on the photomultiplier windows (MgF2), they are decoupled in vacuum from the sapphire windows (28 mm in diameter and 2 mm thick) of the detector ; the sapphire used has good UV light transmission (< 60%), high purity (99.98%) and can be used over a large temperature range. All the materials were properly cleaned. The test sources were introduced in the detector. In this preliminary test we used a gas with 99 .998% purity, supplied by Messer Griesheim, and a system for further purification using an H2O trap, an oxtsorb cartridge and a high temperature getter . Moreover, we collected the light only from one side of the detector and the pulse height never exceeded 500 mV . The shape of the light pulses was recorded by a Lecroy 6880A waveform digitizer, that samples the signal at 1.36 X 10 9 samples/s. A typical pulse from a 22 Na source is shown in fig. 1. To study the light collection and the counting efficiency we introduced a - 5000 Bq 22 Na source in the detector at various distances from the photomultiplier; m fig. 2 we show the pulse height H (fig . 2a) and the rate R (fig. 2b) as functions of the distance. Fitting the data with an exponential function one finds in case (a) H(x) = 842 mV X exp(-x/16 mm) and in case (b)

-50

-100

0

Fig. 1.

22

100

200

t (n s)

300

400

500

Na pulse as viewed by our prototype and recorded by a Lecroy 6880A waveform digitizer.

R(x) = 4547 cps X exp(-x/18 mm). The attenuation length observed for H(x) is in agreement with the value reported in ref. [6]. We estimate that the total residual contamination during these measurements was of the

800 600 400 200 0 2500 2000 1500 1000 500 0

0

10

20

30

40

50

distance (mm) Fig. 2. Pulse height (a) and rate (b) as functions of the distance between the source and the photomultiplier.

P. Belli et al. / Liquid-xenon detectors order of 0.1 ppm (without considering residual noble gases, SF, and CF4 contributions: - 13 ppm) . We are now improving the system in order to obtain a higher vacuum (i .e . higher purification level), that - together

with the introduction of a proper reflecting material on the inner detector surface and both photomultipliers could allow us to improve the energy resolution .

3. Applications of liquid-xenon detectors Liquid-xenon

mediate-energy

detectors could be

useful in interhigh-energy

nuclear physics [17], in

calorimetry [18] and in underground physics [19] .

In intermediate-energy nuclear physics, liquid xenon

could be very interesting when realizing photon monitors or y and mo calorimeters . In particular, its decay time could allow fast monitors for very-high-intensity photon beams to be realized ; up to now, mainly Nal(TI)

detectors have been used, but their slow decay time can give serious pileup problems for beam intensities higher than _ 10 5 y/s. Furthermore, it could be of interest when realizing y and mo calorimeters, considering also its high tolerance to radiation damage [18] .

In the last years, some authors have also taken into

account the possibility to realize liquid-xenon electromagnetic calorimeters for high-energy physics and a large interest has grown because of new projects for very-high-energy machines . The main advantages seem to be an e/m ratio very close to 1, linearity up to 10 4 mips, large pulse height, good granularity, fast response and high tolerance to radiation damage [18] .

Finally, liquid xenon could also have applications in underground physics. In fact, due to its characteristics as a scintillation and decay material (136 Xe isotope) it

could allow ßß decay experiments [19] to be realized with a very large number of decaying atoms, but small

size . Furthermore, we have already pointed out [20] the possibility of using such a material to detect "dark matter" candidates interacting by weak v-like vector currents [21], by looking for the scintillation pulse from

the recoil nucleus following the elastic scattering of massive "dark matter" particles.

4. Conclusions In spite of some technical difficulties which arise

when using liquid xenon as a detector medium, the possible fields of application in physics strongly suggest that such development should be continued.

References [11 A. Incicchitti, P. Belli and M. Scafi, INFN/TC 89/09 (1989) ; Nucl . Instr. and Meth . A289 (1990) 236 and refs . therein .

193

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