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A self-triggered liquid xenon drift chamber by the use of proportional ionization or proportional scintillation
N U C L E A R I N S T R U M E N T S AND METHODS
160 ( 1 9 7 9 )
239-246 ; ©
N O R T H - H O L L A N D PUBLISHING CO.
A S E L F - T R I G G E R E D L I Q U I D XENON D R I F T C H A M B E R BY T H E USE OF P R O P O R T I O N A L I O N I Z A T I O N OR P R O P O R T I O N A L S C I N T I L L A T I O N M. MIYAJIMA
National Laboratory ./'or High Energy Physics, Oho-machi, Tsukuba-gun, lbaraki, Japan K. MASUDA, Y. HOSHI, T. DOKE
Science and Engineering Research Laboratory, Waseda University, Sh#!juku-ku, Tokyo, Japan T. TAKAHASHI, T. HAMADA
Institute qf Pl~vsical and Chemical Research, Wako-shi, Saitama, Japan S. KUBOTA, A. NAKAMOTO
Deparmwnt 01 Physics, St. Paul's University, Nishi-lkebukuro, Tokyo, Japan and E. SHIBAMURA
Saitama College of Health, Urawa-shi, Saitama, Japan Received 2 June 1978 A liquid xenon drift chamber with an electron drift space of 13 mm in length has been constructed and the spatial resolution of the drift chamber was investigated by using alpha particles. In this experiment, the scintillation light directly produced by an alpha particle was used as trigger signal. To detect the drifted electrons, a proportional counter was used with proportional ionization or proportional scintillation mode. The best value of the spatial resolution (r.m.s.), achieved in the drift chamber, was about 20 ~m for both modes. From analytical considerations it is concluded that the main part of the resolution comes from the position uncertainty caused by the finite range of alpha particles in liquid xenon and the finite size of the alpha source. This shows that, if such a drift chamber is used for minimum ionizing particles, a spatial resolution of less than z 10/~m can be expected.
1. Introduction In recent years, many attempts to use liquid rare gas as detector m e d i u m were made. A large size liquid argon calorimeter, being used in the experiments of particle physics, is one o f its successful applicationsl). A n o t h e r expected application is that to a position sensitive detector. In 1968, AIvarez 2) suggested that liquid argon or liquid xenon can be used as detector m e d i u m for a high precision position sensitive detector in the field of particle physics, because of its high electron mobility and high density. After that, Derenzo et al. investigated the possibility of electron multiplication in liquid rare gases and found that it occurs only in liquid xenon3). This fact shows that liquid xenon can be used as detector m e d i u m for multiwire proportional chambers. Derenzo et al. tried to construct some liquid xenon multiwire chambers and found that, when the spacing of wires was taken narrow as well as the distance between the wire and cathode to achieve high spatial resolution, it
was technically difficult to make a field around the wire high enough to enhance electron multiplication4). As a result, they stopped developing the liquid xenon position sensitive detector and constructed a liquid argon multi-strip ionization chamberS). The intrinsic position accuracy of the chamber for m i n i m u m ionizing particles was estimated to be + 2 0 / ~ m . However, because such an ionization chamber requires a n u m b e r of expensive low noise amplifiers, its application to particle physics is not practical. On the other hand, Dolgoshein et al. 6) proposed another position sensitive detector named " t w o - p h a s e d e t e c t o r " , in which both phases of liquid and gas are used. In the non-horizontal case, however, this m e t h o d is not applicable. In order to overcome these difficulties, we tested the possibility of a liquid xenon drift chamber, which does not need narrow wire spacing to achieve high position accuracy, on the basis of the fact that the electron drift velocity in liquid xenon
240
M.
MIYAJIMA
for an electric field larger than 3 k V / c m is almost constant7). In this experiment, a simple drift chamber with a fixed electron drift space was constructed. It consists of an alpha source plate, a guide ring and a proportional counter to detect the drifted electrons. For measurements of the electron drift time, the scintillation pulse directly produced by the alpha particle was used as a trigger signal. In the course of the test, we found the socalled "proportional scintillation" 8) near the anode wire of the proportional counter. The proportional scintillation pulse can also be used as arrival signal of drifting electrons. In this paper, we describe the position accuracy achieved in this liquid xenon drift chamber when it was operated in both modes, proportional ionization and proportional scintillation, as well as the details of the performance of proportional ionization and proportional scintillation.
2. Experimental apparatus In this experiment, a simple type of liquid xenon drift chamber which is composed of an electrode assembly and a photomultiplier as shown in fig. 1 was used. The electrode assembly consists of a source plate, a guide ring and a proportional counter. The source plate is a square brass plate of 10 m m × 10 m m with a cusp of 1 m m high along the edge except the upper-side. A 2~°Po source of 1 m m in diameter is deposited on the central part of the source plate. The cathode of the proportional counter is a cylindrical hole of 6 m m in diameter and 30 m m long made by drilling a brass block of 8 m i n x 10 m i n x 3 0 m m and has an aperture of 5 m m wide and 30 m m long opened to electrons drifting from the source plate. As the anode wire D2
Pl
et al.
P.M. HAMAMATSU TV Pyrex '
R329
glass J
t~-'---J W. I .S.
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of the proportional counter, a tungsten wire of 4/~m in diameter was used. In addition, another window of 6 m m × 1 0 m m is opened on the top side of the brass block to detect proportional scintillation emitted from near the anode wire. To keep the electric field in the electron drift space uniform, a square guide ring of 10 m m × 10 mm and 3 m m wide made of stainless steel of 1 m m in thickness was inserted between the source plate and the proportional counter. These electrodes are set up on a Pyrex glass plate of 3 mm thick with a geometrical arrangement, which gives an electron drift space of 13 ram, as shown in fig. 1. Equipotential lines between the source plate and the anode wire under typical experimental conditions are shown in fig. 2. The electrode assembly
K
W =
2.8kV
/ / ///~
///,,/~
amp.
Fig. ]. Schematic cross-section of a simple type of liquid xenon drift chamber. D] : source plate, D2: guide ring, K: cathode of the proportional counter, W: anode wire. The size of electrodes is given in ram.
Dl=-6 kV D2= -2.4kV K=O kV
/ /
~. to
A
Fig. 2. Equipotential lines between the source plate and the anode wire in a typical case.
LIQUID XENON DRIFT CHAMBER
is put on the bottom of a cylindrical stainless steel chamber of 60 m m in diameter and 25 m m high and fixed to the chamber with five feedthroughs. The top of the chamber is sealed off by a Pyrex glass plate of 80 m m in diameter and 8 m m thick to detect the direct scintillation and the proportional scintillation through it. Two parts of 10 m m × 2 0 m m on the surface of the Pyrex glass window (just above the source and above the proportional counter) are coated by sodium salicylate as a wavelength shifter, since the wavelength of scintillation from liquid xenon lies in the region of ultraviolet which is outside the photo-sensitive region of the photo-cathode of the photomultiplier (Hamamatsu Television R-329). As a cryostat, which liquefies gaseous xenon and keeps the chamber at a temperature around 106°C, a Dewar vessel containing a liquid mixture of n-hexane and cyclo-hexane, cooled by liquid nitrogen, was used. This liquid mixture was chosen to avoid electric breakdown between the feedthroughs and the chamber. The gas filling procedure is as follows: the chamber and the gas filling system are at less than 2 × 10 7 tort after having been baked out at 130°C for more than 50 h. After that, the outgassing rate of the chamber and gas filling system is assured to be less than 6 × 10 ~ torr.l/s. The gaseous xenon purified by the previously developed purifier 9) is fed into the chamber and liquefied until the liquid xenon fills the entire space of it. Spectra of ionization signals obtained from the anode were measured by changing the anode potential while the potential between the cathode and the source plate was unchanged. The signals correspond to a part of charges produced by an alpha particle or to charges induced by positive ions produced as a result of electron multiplication around the anode wire. A charge sensitive preamplifier was used to observe these signals from the
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241
anode and pulse shaping of the differential and integral time constant of 2/~s was done at the main amplifier. The photomultiplier was used to detect the direct scintillation and the proportional scintillation. The signal was taken from the 10th dynode of the photomultiplier and was shaped with a main amplifier of differential and integral time constant of 0.1/zs. For the m e a s u r e m e n t of the drift time of electrons produced by alpha particles, a time-to-pulseheight converter (TPC) was used. The start of the TPC was triggered by the anode signal of the direct scintillation. As the stop signal, the pulse shaped with an amplifier of differential time constant of 0.1/Ls and integral time constant of several tens ns were used in the ionization or proportional ionization mode and the output pulses directly obtained from the photomultiplier were used in the proportional scintillation mode. A block diagram of the electronic circuits is shown in fig. 3. Two discriminators of leading edge mode were used for the start (trigger) and stop signals, respectively. 3. Performance of the liquid xenon proportional counter During the experiment, the cathode of the proportional counter was grounded and the applied voltages of the source plate and the guide ring were kept at - 6 kV and - 2 . 4 kV, respectively. Thus, an average electric field of 5.4 kV/cm was given for the electron drift space. In this field strength, the free electrons drifting along the electric field are estimated to be about 5% of the ion pairs produced by an alpha particle. After drifting the fixed distance (13 ram), these electrons flow into the proportional counter in which a positive voltage larger than 0.5 kV is applied to the anode wire. Fig. 4 shows the pulse height spectra of 2mPo alpha particles for the various applied voltages to
I.
Fig. 3. Block diagram of electronic circuits for measurements of drift time.
242
M. MIYAJIMA et al.
PI
1oo
g
ANODE WIRE
28kV 30kV / ~L~kV
16kV
J
1o
16
10o OAIN
Fig, 4. Pulse height spectra of 21°po alpha particles versus charge gain at various anode voltages for ionization signals.
the anode when the wire of 4 #m in diameter was used as anode wire and fig. 5 shows the dependence of the peak position in each pulse height spectrum on the applied voltage. As seen from the figure, the pulse height nearly saturates at the applied voltage of 1 kV and begins to increase at about 1.5 kV as the result of the occurrence of electron multiplication. At around 3 kV, the pulse height becomes 200 times higher than the saturated value in the ionization region. But measurements at the voltage higher than 3 kV are difficult, since the anode wire is sometimes broken probably due to electrical or mechanical reasons. The rise time of the electron induced pulse is about 500 ns at the ionization region and becomes fast ( - 100 ns) with an increase of electron multii
i
PI
/
ANODE WIRE
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/
.
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10
i
0
20
32
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I
~ so
PI ANODE WIRE
,4Mm
i
i
1 2 WIRE VOLTAGE ( k V )
o
o'.s
i'.o (RELATIVE
1'.s PULSE
HEIGHT)
2'0 I
Fig. 6. Energy resolution (fwhm%) versus reciprocal pulse height for ionization signals.
plication near the anode wire. In the ionization region, also, the pulse height resolution is gradually improved with an increase of the applied voltage, that is, with an increase of pulse height, but it becomes rapidly worse in the higher voltage region. Such a tendency of the resolution is shown against the reciprocal of the pulse height in fig. 6. The performance of the proportional counter as shown above was reproducible, whenever gaseous xenon was purified by the purifier as described before. Nevertheless, the electronegative ion pumping is always necessary to increase the anode voltage to more than about 2 kV without electric discharge9). Since liquid xenon is a highly luminous material and the decay time of scintillation light from it is very fast (several ns), we used the direct scintillation light produced by alpha particles in liquid xenon to trigger the time analysing electronic systems as described in section 2. The decrease in the pulse height of direct scintillation due to the existence of the electric field is negligibly small.* In the course of this experiment, the proportional scintillation was detected by the photomultiplier used for the direct scintillation. Fig, 7 shows photographs of the direct scintillation and the proportional scintillation at the anode voltage of 1.6 kV and 3.0 kV. In these figures, the first pulses correspond to the direct scintillation and the second to the proportional scintillation. From these figures it is clear that the pulse height of the proportional scintillation increases with the anode potential. A
|
3
Fig. 5. Charge gain versus applied anode voltage.
* For minimum ionizing particles, however, the scintillation light will be reduced to one third by applying an electric field as found in the case of conversion electronsm).
LIQUID
XENON
photograph of the proportional scintillation pulse shown in fig. 8 was taken at the output stage of the photomultiplier with the time scale of 200 ns/div and shows that the rise time is about 100 ns. This fact promises the possibility of the use of the proportional scintillation pulse in measurements of the drill, time of electrons, instead of the proportional ionization pulse. Fig. 9 shows typical pulse height spectra due to alpha particles. The resolution of the proportional scintillation showed the tendency to become better at increasing anode potential. The square of the fwhm against the reciprocal of the pulse height is plotted in fig. 10. A limiting resolution of better than 15% was obtained for alpha particles. As mentioned before, the number of electrons participating in the production of proportional scintillation is only 5% of the number of ion pairs pro-
DRIFT
243
CHAMBER
Fig. 8. Oscilloscope photograph of the proportional scintillation at the anode of the photomultiplier. The anode wire voltage is 2.8 kV. The time scale is 200 n s / d i v .
PS ANOOE WIRE 4 jam mlOO
2 5k~ 3.2k"V 2 6 kV
3.0kV
S w [a.i iv-
1.6 KV
so 10kV
1
,~
10
RELATIVE LIGHT GAIN
IO0
Fig, 9. Pulse height spectra of 21°po alpha particle versus relative light yield at various anode voltages lk~r proportional scintillation signals.
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ps
v b
Fig. 7. Oscilloscope photographs of the scintillation at the output of the main amplifier, In both photographs, the first pulses correspond to the direct scintillation and the second to the proportional scintillation. (a) The anode voltage is 1.6 kV, The time scale is 2 l~s/div. (b) The anode voltage is 3.0 kV. The time scale is the same as in (a), and the gain of the main a m plifier is half of that in (a).
0
2
4
6 B 10 (RELATIVE PULSE HEIGHT)-~
12
Fig. 10. Square of the energy resolution versus the reciprocal pulse height for proportional scintillation signals.
244
M. M I Y A J 1 M A
PS ANODE WIRE
4 J~
D1 =-6.0 kV D2=-2.4 100
z
_~ 10 ..J
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1 WIRE
VOLTAGE
(kV)
Fig. ]1. Relative light gain vs applied anode voltage.
The variation of the spatial resolution (expressed by r.m.s.) thus obtained for the applied anode voltage is shown in fig. 13. These spatial resolutions are obtained by assuming 3x IOs cm/s as the electron drift velocity. As seen from the figure, the resolution in the use of the proportional ionization pulse as stop signal is worse than that in the use of the proportional scintillation pulse over the whole region of the applied anode voltage. In both cases, the resolution in the ionization region is worse than that in the electron multiplication region, but in the region of the applied voltage larger than 2.8 kV, the resolution becomes worse again• Therefore, the best resolution is given at about 2.8 kV. The best value of the spatial resolution was _+23/~m for the proportional ionization mode and _--19/~m for the proportional scintillation mode. The existence of the optimum resolution in the proportional ionization mode is explained as follows. In the ionization chamber region, the spatial resolution is poor, because of the low signal-tonoise ratio and in the higher voltage region, the rapid deterioration of spatial resolution is caused
duced by an alpha particle, and so a comparatively large fluctuation in the number of electrons which are free from recombination with ions is expected. Therefore, we consider that such a fluctuation gives the above limiting value. If conversion electrons are used as radiation, a better limiting resolution may be achieved by a proportional scintillation counter. At present, such an experiment is in progress, by using an improved proportional scintillation counter. Fig. 11 shows the light gain of the proportional scintillation plotted versus the applied anode voltage. The proportiGnal scintillation begins at the anode potential of about 0.5 kV, reaches the same pulse height as that due to direct scintillation at about 2 kV and increases rapidly with an increase in the anode potential as shown in fig. 11.
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4. Measurement of electron drift time
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In order to investigate the limit of the spatial resolution of the liquid xenon drift chamber, the drift time of electrons produced by alpha particles was measured on a drift space of fixed length. Measurements of the time were made by a TPC which was triggered (or started) by the pulse due to the scintillation delayed by about 6/~s and was stopped by the pulse of proportional ionization or proportional scintillation. Fig. 12 shows typical pulse height spectra from the TPC when the proportional ionization or the proportional scintillation was used as stop signal.
et al.
ANODE WIRE 4 ~m W = 2.8kV D1 =-6.0 kV D2 =- 2.4 kV Start : S Stop : P[
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1 nsec per channel
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40 6O CHANNEL NUMBER Fig. 12. Pulse height spectra from the TPC. The upper spect r u m was obtained by u s i n g t h e proportional ionization pulse as stop signal and the lower was obtained by using the proportional scintillation pulse•
LIQUID
ANODE WIRE
4~arn
XENON
o:S -PS
D I = - 6.0 kV D2 =- 2.4 kV
10o
E E
50
0
1 2 WIRE VOLTAGE ( kV )
3
Fig. 13. Spatial resolution expressed by r.m.s, v e r s u s anode voltage.
by an increase in the spread of the pulse height distribution. For the proportional scintillation mode, on the other hand, there exists an optimum resolution but the variation of the spatial resolution for applied voltages is small, because the signal-to-noise ratio is comparatively large in the low applied voltage and the spread of the pulse height distribution in the higher voltage region is small compared with that in the proportional ionization mode. 5. Spacial resolution in the liquid xenon drift chamber The use of liquid rare gas as detector m e d i u m of a position sensitive chamber for m i n i m u m ionizing particles is expected to reduce significantly the limiting factors of spatial resolution, such as fluctuation in primary ionization, influence of diffusion during drift of electrons and delta-ray effect, because its density is several hundred times higher than that of gas. The theoretical estimation by Alverez for such a case shows that a spatial resolution of the order of several micrometers can be achieved2). In the present experiment, the best spatial resolution of about _+20/~m was obtained with the proportional ionization mode or with the proportional scintillation mode using alpha particles. This value is worse than the theoretical one as mentioned above. We consider that this is caused by the use of alpha particles emitted from the radiation source with finite size. Namely, (1) the fluctuation in the position of the center of charge along the track of the alpha particle, having a range of about 45/,,m in liquid xenon, emitted from the source of 1 m m in diameter produces an
DRIFT
CHAMBER
245
uncertainty of the particle position, (2) the difference in the toal drift path length (the path between the source plate and the anode wire of the proportional counter) of electrons, which depends on the location of alpha particle emission, effects the fluctuation in the electron drift time and (3) the geometrical error in setting the anode wire and the source plate also produces an uncertainty of the particle position for the radiation source with a finite size. The position uncertainty due to (1) is estimated to be 13/am (r.m.s.) assuming uniform ionization along the track of the alpha particle, and assuming uniform distribution of the alpha particle emitter in the source, the uncertainty due to (2) to be 13/~m (r.m.s.) using the lines of electric field obtained from the equipotential lines as shown in fig. 2 and the uncertainty due to (3) to be about 6/zm (r.m.s.). The attainable spatial resolution in the use of an alpha particle source is estimated to be -=_19.3/2m from the above uncertainties. This value is in good agreement with the best spatial resolution obtained in the liquid xenon drift chamber. This fact shows that the assumption made above is real and, in addition, the uncertainty of particle position due to fluctuation in primary ionization and diffusion during electron drift is small as compared with the best spatial resolution of ± 2 0 / ~ m . This is consistent with the predicted resolution as mentioned before. Since the difference between the calculated spatial resolution due to an alpha particle source with finite size and that experimentally obtained is small, the spatial resolution in the application of a liquid xenon drift chamber to m i m i n u m ionizing particle detection is expected to be of the order of several micrometers except for multiple scattering effect. From such a viewpoint, we are now constructing a multiwire liquid xenon drift chamber of 50 mm × 50 m m × 2 m m , in which the wire spacing is 2-4 ram, in order to test the spatial resolution as expected above by particles with a m o m e n t u m above 3 G e V / c normally incident on the detector where the effect due to the multiple scattering is less than 1/~m. 6. Conclusion In the self-triggered liquid xenon drift chamber, the best spatial resolution of +_20~m in both modes - the proportional ionization and the proportional scintillation modes - was obtained by using an alpha particle source of finite size. The result of the analyses for the best spatial resolution
246
M. M I Y A J I M A et al.
thus obtained shows that the main part of the resolution is determined by the size effect of the alpha source and the range of alpha particles in liquid xenon. The spatial resolution due to fluctuation in primary ionization and diffusion during electron drift is of the order of several micrometers as theoretically predicted. This means that the application of the liquid xenon drift chamber to a high precision position detector for minimum ionizing particles is promising. The gain of the proportional scintillation, found in the course of the experiment, is reproducible and the spatial resolution obtained in the proportional scintillation mode is better than that in the proportional ionization mode. A liquid xenon drift chamber operated with such a proportional scintillation mode is applicable to a gamma camera or to a two-dimensional position detector for annihilation gamma rays. References 1) W. J. Willis and V. Radeka, Nucl. Instr. and Meth. 120
2) 3)
4) 5) 6) 7)
8) 9)
10)
(1974) 221; C. Cerri et al., Nucl. Instr. and Meth. 141 (1977) 207; D. Hitlin et al., Nucl. Instr. and Meth. 137 (1976) 225; C. W. Fabjan et al., Nucl. Instr. and Meth. 141 (1977) 61. L. W. Alvarez, Lawrence Radiation Laboratory, Physics Note No. 672 (1968). S. E. Derenzo et al., Lawrence Radiation Laboratory, UCRL-20118 (1970); R. A. Muller et al., Phys. Rev. Lett. 27 (1971) 532. S. E. Derenzo et al., Lawrence Berkeley Laboratory Report No. LBL-1791 (April 1973). S. E. Derenzo et al., Nucl. Instr. and Meth. 122 (1974) 319. B. A. Dolgoshein et al., JETP Lett. 11 (1970) 351; B. A. Dolgoshein et al., Sov. J. Particles Nucl. 4 (1973) 70. E. Shibamura, A. Hitachi, T. Doke, T. Takahashi, S. Kubota and M. Miyajima, Nucl. Instr. and Meth. 131 (1975) 249. C. A. N. Conde and A. J. P. L. Policarpo, Nucl. Instr. and Meth. 53 (1967) 7. M. Miyajirna, K. Masuda, A. Hitachi, T. Doke, T. Takahashi, S. Konno, T. Hamada, S. Kuhota, A Nakamoto and E. Shibamura, Nucl. Instr. and Meth. 134 (1976) 403. S. Kubota, A. Nakamoto, T. Takahashi. T. Hamada, E. Shibamura, M. Miyajima, K. Masuda and T. Doke, Phys. Rev. BI7 (1978) 2762.
160 ( 1 9 7 9 )
239-246 ; ©
N O R T H - H O L L A N D PUBLISHING CO.
A S E L F - T R I G G E R E D L I Q U I D XENON D R I F T C H A M B E R BY T H E USE OF P R O P O R T I O N A L I O N I Z A T I O N OR P R O P O R T I O N A L S C I N T I L L A T I O N M. MIYAJIMA
National Laboratory ./'or High Energy Physics, Oho-machi, Tsukuba-gun, lbaraki, Japan K. MASUDA, Y. HOSHI, T. DOKE
Science and Engineering Research Laboratory, Waseda University, Sh#!juku-ku, Tokyo, Japan T. TAKAHASHI, T. HAMADA
Institute qf Pl~vsical and Chemical Research, Wako-shi, Saitama, Japan S. KUBOTA, A. NAKAMOTO
Deparmwnt 01 Physics, St. Paul's University, Nishi-lkebukuro, Tokyo, Japan and E. SHIBAMURA
Saitama College of Health, Urawa-shi, Saitama, Japan Received 2 June 1978 A liquid xenon drift chamber with an electron drift space of 13 mm in length has been constructed and the spatial resolution of the drift chamber was investigated by using alpha particles. In this experiment, the scintillation light directly produced by an alpha particle was used as trigger signal. To detect the drifted electrons, a proportional counter was used with proportional ionization or proportional scintillation mode. The best value of the spatial resolution (r.m.s.), achieved in the drift chamber, was about 20 ~m for both modes. From analytical considerations it is concluded that the main part of the resolution comes from the position uncertainty caused by the finite range of alpha particles in liquid xenon and the finite size of the alpha source. This shows that, if such a drift chamber is used for minimum ionizing particles, a spatial resolution of less than z 10/~m can be expected.
1. Introduction In recent years, many attempts to use liquid rare gas as detector m e d i u m were made. A large size liquid argon calorimeter, being used in the experiments of particle physics, is one o f its successful applicationsl). A n o t h e r expected application is that to a position sensitive detector. In 1968, AIvarez 2) suggested that liquid argon or liquid xenon can be used as detector m e d i u m for a high precision position sensitive detector in the field of particle physics, because of its high electron mobility and high density. After that, Derenzo et al. investigated the possibility of electron multiplication in liquid rare gases and found that it occurs only in liquid xenon3). This fact shows that liquid xenon can be used as detector m e d i u m for multiwire proportional chambers. Derenzo et al. tried to construct some liquid xenon multiwire chambers and found that, when the spacing of wires was taken narrow as well as the distance between the wire and cathode to achieve high spatial resolution, it
was technically difficult to make a field around the wire high enough to enhance electron multiplication4). As a result, they stopped developing the liquid xenon position sensitive detector and constructed a liquid argon multi-strip ionization chamberS). The intrinsic position accuracy of the chamber for m i n i m u m ionizing particles was estimated to be + 2 0 / ~ m . However, because such an ionization chamber requires a n u m b e r of expensive low noise amplifiers, its application to particle physics is not practical. On the other hand, Dolgoshein et al. 6) proposed another position sensitive detector named " t w o - p h a s e d e t e c t o r " , in which both phases of liquid and gas are used. In the non-horizontal case, however, this m e t h o d is not applicable. In order to overcome these difficulties, we tested the possibility of a liquid xenon drift chamber, which does not need narrow wire spacing to achieve high position accuracy, on the basis of the fact that the electron drift velocity in liquid xenon
240
M.
MIYAJIMA
for an electric field larger than 3 k V / c m is almost constant7). In this experiment, a simple drift chamber with a fixed electron drift space was constructed. It consists of an alpha source plate, a guide ring and a proportional counter to detect the drifted electrons. For measurements of the electron drift time, the scintillation pulse directly produced by the alpha particle was used as a trigger signal. In the course of the test, we found the socalled "proportional scintillation" 8) near the anode wire of the proportional counter. The proportional scintillation pulse can also be used as arrival signal of drifting electrons. In this paper, we describe the position accuracy achieved in this liquid xenon drift chamber when it was operated in both modes, proportional ionization and proportional scintillation, as well as the details of the performance of proportional ionization and proportional scintillation.
2. Experimental apparatus In this experiment, a simple type of liquid xenon drift chamber which is composed of an electrode assembly and a photomultiplier as shown in fig. 1 was used. The electrode assembly consists of a source plate, a guide ring and a proportional counter. The source plate is a square brass plate of 10 m m × 10 m m with a cusp of 1 m m high along the edge except the upper-side. A 2~°Po source of 1 m m in diameter is deposited on the central part of the source plate. The cathode of the proportional counter is a cylindrical hole of 6 m m in diameter and 30 m m long made by drilling a brass block of 8 m i n x 10 m i n x 3 0 m m and has an aperture of 5 m m wide and 30 m m long opened to electrons drifting from the source plate. As the anode wire D2
Pl
et al.
P.M. HAMAMATSU TV Pyrex '
R329
glass J
t~-'---J W. I .S.
Liq .Xe
-
t
R1
R2 ; , ~
of the proportional counter, a tungsten wire of 4/~m in diameter was used. In addition, another window of 6 m m × 1 0 m m is opened on the top side of the brass block to detect proportional scintillation emitted from near the anode wire. To keep the electric field in the electron drift space uniform, a square guide ring of 10 m m × 10 mm and 3 m m wide made of stainless steel of 1 m m in thickness was inserted between the source plate and the proportional counter. These electrodes are set up on a Pyrex glass plate of 3 mm thick with a geometrical arrangement, which gives an electron drift space of 13 ram, as shown in fig. 1. Equipotential lines between the source plate and the anode wire under typical experimental conditions are shown in fig. 2. The electrode assembly
K
W =
2.8kV
/ / ///~
///,,/~
amp.
Fig. ]. Schematic cross-section of a simple type of liquid xenon drift chamber. D] : source plate, D2: guide ring, K: cathode of the proportional counter, W: anode wire. The size of electrodes is given in ram.
Dl=-6 kV D2= -2.4kV K=O kV
/ /
~. to
A
Fig. 2. Equipotential lines between the source plate and the anode wire in a typical case.
LIQUID XENON DRIFT CHAMBER
is put on the bottom of a cylindrical stainless steel chamber of 60 m m in diameter and 25 m m high and fixed to the chamber with five feedthroughs. The top of the chamber is sealed off by a Pyrex glass plate of 80 m m in diameter and 8 m m thick to detect the direct scintillation and the proportional scintillation through it. Two parts of 10 m m × 2 0 m m on the surface of the Pyrex glass window (just above the source and above the proportional counter) are coated by sodium salicylate as a wavelength shifter, since the wavelength of scintillation from liquid xenon lies in the region of ultraviolet which is outside the photo-sensitive region of the photo-cathode of the photomultiplier (Hamamatsu Television R-329). As a cryostat, which liquefies gaseous xenon and keeps the chamber at a temperature around 106°C, a Dewar vessel containing a liquid mixture of n-hexane and cyclo-hexane, cooled by liquid nitrogen, was used. This liquid mixture was chosen to avoid electric breakdown between the feedthroughs and the chamber. The gas filling procedure is as follows: the chamber and the gas filling system are at less than 2 × 10 7 tort after having been baked out at 130°C for more than 50 h. After that, the outgassing rate of the chamber and gas filling system is assured to be less than 6 × 10 ~ torr.l/s. The gaseous xenon purified by the previously developed purifier 9) is fed into the chamber and liquefied until the liquid xenon fills the entire space of it. Spectra of ionization signals obtained from the anode were measured by changing the anode potential while the potential between the cathode and the source plate was unchanged. The signals correspond to a part of charges produced by an alpha particle or to charges induced by positive ions produced as a result of electron multiplication around the anode wire. A charge sensitive preamplifier was used to observe these signals from the
sy:q
p$
241
anode and pulse shaping of the differential and integral time constant of 2/~s was done at the main amplifier. The photomultiplier was used to detect the direct scintillation and the proportional scintillation. The signal was taken from the 10th dynode of the photomultiplier and was shaped with a main amplifier of differential and integral time constant of 0.1/zs. For the m e a s u r e m e n t of the drift time of electrons produced by alpha particles, a time-to-pulseheight converter (TPC) was used. The start of the TPC was triggered by the anode signal of the direct scintillation. As the stop signal, the pulse shaped with an amplifier of differential time constant of 0.1/Ls and integral time constant of several tens ns were used in the ionization or proportional ionization mode and the output pulses directly obtained from the photomultiplier were used in the proportional scintillation mode. A block diagram of the electronic circuits is shown in fig. 3. Two discriminators of leading edge mode were used for the start (trigger) and stop signals, respectively. 3. Performance of the liquid xenon proportional counter During the experiment, the cathode of the proportional counter was grounded and the applied voltages of the source plate and the guide ring were kept at - 6 kV and - 2 . 4 kV, respectively. Thus, an average electric field of 5.4 kV/cm was given for the electron drift space. In this field strength, the free electrons drifting along the electric field are estimated to be about 5% of the ion pairs produced by an alpha particle. After drifting the fixed distance (13 ram), these electrons flow into the proportional counter in which a positive voltage larger than 0.5 kV is applied to the anode wire. Fig. 4 shows the pulse height spectra of 2mPo alpha particles for the various applied voltages to
I.
Fig. 3. Block diagram of electronic circuits for measurements of drift time.
242
M. MIYAJIMA et al.
PI
1oo
g
ANODE WIRE
28kV 30kV / ~L~kV
16kV
J
1o
16
10o OAIN
Fig, 4. Pulse height spectra of 21°po alpha particles versus charge gain at various anode voltages for ionization signals.
the anode when the wire of 4 #m in diameter was used as anode wire and fig. 5 shows the dependence of the peak position in each pulse height spectrum on the applied voltage. As seen from the figure, the pulse height nearly saturates at the applied voltage of 1 kV and begins to increase at about 1.5 kV as the result of the occurrence of electron multiplication. At around 3 kV, the pulse height becomes 200 times higher than the saturated value in the ionization region. But measurements at the voltage higher than 3 kV are difficult, since the anode wire is sometimes broken probably due to electrical or mechanical reasons. The rise time of the electron induced pulse is about 500 ns at the ionization region and becomes fast ( - 100 ns) with an increase of electron multii
i
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/
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/
.
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I
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i
i
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Fig. 6. Energy resolution (fwhm%) versus reciprocal pulse height for ionization signals.
plication near the anode wire. In the ionization region, also, the pulse height resolution is gradually improved with an increase of the applied voltage, that is, with an increase of pulse height, but it becomes rapidly worse in the higher voltage region. Such a tendency of the resolution is shown against the reciprocal of the pulse height in fig. 6. The performance of the proportional counter as shown above was reproducible, whenever gaseous xenon was purified by the purifier as described before. Nevertheless, the electronegative ion pumping is always necessary to increase the anode voltage to more than about 2 kV without electric discharge9). Since liquid xenon is a highly luminous material and the decay time of scintillation light from it is very fast (several ns), we used the direct scintillation light produced by alpha particles in liquid xenon to trigger the time analysing electronic systems as described in section 2. The decrease in the pulse height of direct scintillation due to the existence of the electric field is negligibly small.* In the course of this experiment, the proportional scintillation was detected by the photomultiplier used for the direct scintillation. Fig, 7 shows photographs of the direct scintillation and the proportional scintillation at the anode voltage of 1.6 kV and 3.0 kV. In these figures, the first pulses correspond to the direct scintillation and the second to the proportional scintillation. From these figures it is clear that the pulse height of the proportional scintillation increases with the anode potential. A
|
3
Fig. 5. Charge gain versus applied anode voltage.
* For minimum ionizing particles, however, the scintillation light will be reduced to one third by applying an electric field as found in the case of conversion electronsm).
LIQUID
XENON
photograph of the proportional scintillation pulse shown in fig. 8 was taken at the output stage of the photomultiplier with the time scale of 200 ns/div and shows that the rise time is about 100 ns. This fact promises the possibility of the use of the proportional scintillation pulse in measurements of the drill, time of electrons, instead of the proportional ionization pulse. Fig. 9 shows typical pulse height spectra due to alpha particles. The resolution of the proportional scintillation showed the tendency to become better at increasing anode potential. The square of the fwhm against the reciprocal of the pulse height is plotted in fig. 10. A limiting resolution of better than 15% was obtained for alpha particles. As mentioned before, the number of electrons participating in the production of proportional scintillation is only 5% of the number of ion pairs pro-
DRIFT
243
CHAMBER
Fig. 8. Oscilloscope photograph of the proportional scintillation at the anode of the photomultiplier. The anode wire voltage is 2.8 kV. The time scale is 200 n s / d i v .
PS ANOOE WIRE 4 jam mlOO
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Fig. 7. Oscilloscope photographs of the scintillation at the output of the main amplifier, In both photographs, the first pulses correspond to the direct scintillation and the second to the proportional scintillation. (a) The anode voltage is 1.6 kV, The time scale is 2 l~s/div. (b) The anode voltage is 3.0 kV. The time scale is the same as in (a), and the gain of the main a m plifier is half of that in (a).
0
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12
Fig. 10. Square of the energy resolution versus the reciprocal pulse height for proportional scintillation signals.
244
M. M I Y A J 1 M A
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The variation of the spatial resolution (expressed by r.m.s.) thus obtained for the applied anode voltage is shown in fig. 13. These spatial resolutions are obtained by assuming 3x IOs cm/s as the electron drift velocity. As seen from the figure, the resolution in the use of the proportional ionization pulse as stop signal is worse than that in the use of the proportional scintillation pulse over the whole region of the applied anode voltage. In both cases, the resolution in the ionization region is worse than that in the electron multiplication region, but in the region of the applied voltage larger than 2.8 kV, the resolution becomes worse again• Therefore, the best resolution is given at about 2.8 kV. The best value of the spatial resolution was _+23/~m for the proportional ionization mode and _--19/~m for the proportional scintillation mode. The existence of the optimum resolution in the proportional ionization mode is explained as follows. In the ionization chamber region, the spatial resolution is poor, because of the low signal-tonoise ratio and in the higher voltage region, the rapid deterioration of spatial resolution is caused
duced by an alpha particle, and so a comparatively large fluctuation in the number of electrons which are free from recombination with ions is expected. Therefore, we consider that such a fluctuation gives the above limiting value. If conversion electrons are used as radiation, a better limiting resolution may be achieved by a proportional scintillation counter. At present, such an experiment is in progress, by using an improved proportional scintillation counter. Fig. 11 shows the light gain of the proportional scintillation plotted versus the applied anode voltage. The proportiGnal scintillation begins at the anode potential of about 0.5 kV, reaches the same pulse height as that due to direct scintillation at about 2 kV and increases rapidly with an increase in the anode potential as shown in fig. 11.
UJ
4. Measurement of electron drift time
< _J UJ
In order to investigate the limit of the spatial resolution of the liquid xenon drift chamber, the drift time of electrons produced by alpha particles was measured on a drift space of fixed length. Measurements of the time were made by a TPC which was triggered (or started) by the pulse due to the scintillation delayed by about 6/~s and was stopped by the pulse of proportional ionization or proportional scintillation. Fig. 12 shows typical pulse height spectra from the TPC when the proportional ionization or the proportional scintillation was used as stop signal.
et al.
ANODE WIRE 4 ~m W = 2.8kV D1 =-6.0 kV D2 =- 2.4 kV Start : S Stop : P[
3
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LIQUID
ANODE WIRE
4~arn
XENON
o:S -PS
D I = - 6.0 kV D2 =- 2.4 kV
10o
E E
50
0
1 2 WIRE VOLTAGE ( kV )
3
Fig. 13. Spatial resolution expressed by r.m.s, v e r s u s anode voltage.
by an increase in the spread of the pulse height distribution. For the proportional scintillation mode, on the other hand, there exists an optimum resolution but the variation of the spatial resolution for applied voltages is small, because the signal-to-noise ratio is comparatively large in the low applied voltage and the spread of the pulse height distribution in the higher voltage region is small compared with that in the proportional ionization mode. 5. Spacial resolution in the liquid xenon drift chamber The use of liquid rare gas as detector m e d i u m of a position sensitive chamber for m i n i m u m ionizing particles is expected to reduce significantly the limiting factors of spatial resolution, such as fluctuation in primary ionization, influence of diffusion during drift of electrons and delta-ray effect, because its density is several hundred times higher than that of gas. The theoretical estimation by Alverez for such a case shows that a spatial resolution of the order of several micrometers can be achieved2). In the present experiment, the best spatial resolution of about _+20/~m was obtained with the proportional ionization mode or with the proportional scintillation mode using alpha particles. This value is worse than the theoretical one as mentioned above. We consider that this is caused by the use of alpha particles emitted from the radiation source with finite size. Namely, (1) the fluctuation in the position of the center of charge along the track of the alpha particle, having a range of about 45/,,m in liquid xenon, emitted from the source of 1 m m in diameter produces an
DRIFT
CHAMBER
245
uncertainty of the particle position, (2) the difference in the toal drift path length (the path between the source plate and the anode wire of the proportional counter) of electrons, which depends on the location of alpha particle emission, effects the fluctuation in the electron drift time and (3) the geometrical error in setting the anode wire and the source plate also produces an uncertainty of the particle position for the radiation source with a finite size. The position uncertainty due to (1) is estimated to be 13/am (r.m.s.) assuming uniform ionization along the track of the alpha particle, and assuming uniform distribution of the alpha particle emitter in the source, the uncertainty due to (2) to be 13/~m (r.m.s.) using the lines of electric field obtained from the equipotential lines as shown in fig. 2 and the uncertainty due to (3) to be about 6/zm (r.m.s.). The attainable spatial resolution in the use of an alpha particle source is estimated to be -=_19.3/2m from the above uncertainties. This value is in good agreement with the best spatial resolution obtained in the liquid xenon drift chamber. This fact shows that the assumption made above is real and, in addition, the uncertainty of particle position due to fluctuation in primary ionization and diffusion during electron drift is small as compared with the best spatial resolution of ± 2 0 / ~ m . This is consistent with the predicted resolution as mentioned before. Since the difference between the calculated spatial resolution due to an alpha particle source with finite size and that experimentally obtained is small, the spatial resolution in the application of a liquid xenon drift chamber to m i m i n u m ionizing particle detection is expected to be of the order of several micrometers except for multiple scattering effect. From such a viewpoint, we are now constructing a multiwire liquid xenon drift chamber of 50 mm × 50 m m × 2 m m , in which the wire spacing is 2-4 ram, in order to test the spatial resolution as expected above by particles with a m o m e n t u m above 3 G e V / c normally incident on the detector where the effect due to the multiple scattering is less than 1/~m. 6. Conclusion In the self-triggered liquid xenon drift chamber, the best spatial resolution of +_20~m in both modes - the proportional ionization and the proportional scintillation modes - was obtained by using an alpha particle source of finite size. The result of the analyses for the best spatial resolution
246
M. M I Y A J I M A et al.
thus obtained shows that the main part of the resolution is determined by the size effect of the alpha source and the range of alpha particles in liquid xenon. The spatial resolution due to fluctuation in primary ionization and diffusion during electron drift is of the order of several micrometers as theoretically predicted. This means that the application of the liquid xenon drift chamber to a high precision position detector for minimum ionizing particles is promising. The gain of the proportional scintillation, found in the course of the experiment, is reproducible and the spatial resolution obtained in the proportional scintillation mode is better than that in the proportional ionization mode. A liquid xenon drift chamber operated with such a proportional scintillation mode is applicable to a gamma camera or to a two-dimensional position detector for annihilation gamma rays. References 1) W. J. Willis and V. Radeka, Nucl. Instr. and Meth. 120
2) 3)
4) 5) 6) 7)
8) 9)
10)
(1974) 221; C. Cerri et al., Nucl. Instr. and Meth. 141 (1977) 207; D. Hitlin et al., Nucl. Instr. and Meth. 137 (1976) 225; C. W. Fabjan et al., Nucl. Instr. and Meth. 141 (1977) 61. L. W. Alvarez, Lawrence Radiation Laboratory, Physics Note No. 672 (1968). S. E. Derenzo et al., Lawrence Radiation Laboratory, UCRL-20118 (1970); R. A. Muller et al., Phys. Rev. Lett. 27 (1971) 532. S. E. Derenzo et al., Lawrence Berkeley Laboratory Report No. LBL-1791 (April 1973). S. E. Derenzo et al., Nucl. Instr. and Meth. 122 (1974) 319. B. A. Dolgoshein et al., JETP Lett. 11 (1970) 351; B. A. Dolgoshein et al., Sov. J. Particles Nucl. 4 (1973) 70. E. Shibamura, A. Hitachi, T. Doke, T. Takahashi, S. Kubota and M. Miyajima, Nucl. Instr. and Meth. 131 (1975) 249. C. A. N. Conde and A. J. P. L. Policarpo, Nucl. Instr. and Meth. 53 (1967) 7. M. Miyajirna, K. Masuda, A. Hitachi, T. Doke, T. Takahashi, S. Konno, T. Hamada, S. Kuhota, A Nakamoto and E. Shibamura, Nucl. Instr. and Meth. 134 (1976) 403. S. Kubota, A. Nakamoto, T. Takahashi. T. Hamada, E. Shibamura, M. Miyajima, K. Masuda and T. Doke, Phys. Rev. BI7 (1978) 2762.