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First observation of the scintillation light from solid Xe, Kr and Ar with a CsI photocathode
Nuclear Instruments and Methods in Physics Research A 353 (1994) 55-58
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
ELSEVIER
First observation of the scintillation light from solid Xe, Kr and Ar with a Csl photocathode E. Aprile
a , *,
A. Bolotnikov
a,
D. Chen
a, F. XU a,
V. Peskov
b
a Physics Department, Columbia University, New Yor1y NY 10027, USA b Fermi National Accelerator Laboratory, Batavia, IL 60510, USA
Abstract We have demonstrated that the scintillation light from solid Xe, Kr and Àr can be recorded by means of a CsI photocathode directly coupled with the solid. The signal obtained from the CsI photocathode inside solid rare gases is 20-100% higher than that previously observed with CsI photocathodes inside the same liquids . This may open new applications of solid rare gas scintillating detectors with such a readout.
1. Introduction
2. Experimental setup
The use of the scintillation light from liquid and solid rare gases for calorimetry in high energy physics has been suggested by several authors [1,2]. The main advantages of condensed rare gas scintillating detectors are: high stopping power, fast response and radiation hardness. We have previously reported the successful operation of CsI photocathodes in liquid xenon (LXe), liquid krypton (LKr) and liquid argon (LAr) [3]. A very good energy resolution was achieved for ot-particles with energy of 5.5 MeV: 8.0% FWHM in LAr, 5.2% FWHM in LKr and 2.8% FWHM in LXe. The quantum efficiency (QE) of the CsI photocathode was: 10% in LAr, 17% in LKr and 31% in LXe. These results were independently verified by further measurements with a calibrated external source of UV light. A theoretical explanation based on the band structure of the CsI photocathode and the liquid rare gases was suggested. It remains of interest to check the properties of CsI photocathodes coupled with solid rare gases. A solid rare gas detector offers the advantage of being mechanically more stable and less sensitive to contamination from impurities . In addition, the higher electron mobility is favorable when charge collection is involved ; it can result in a high photoelectron yield. Here we summarize our preliminary results from experiments in this direction.
As in our previous work, we used two experimental setups : one with an alpha source (24'Am) installed inside the test chamber and the other with a calibrated mercury lamp and a set of narrow band filters as an external UV light source (see Fig. 1) . The details are described in Ref. [3]. For the present experiments, the test chamber was equipped with a window to visually monitor the formation of the solid. The test chamber contains two parallel plate electrodes . One of the two plates is always a CsI photocathode, while the other one is either a stainless steel plate with the alpha source deposited on its center, or a stainless steel mesh to transmit the light from the external light source . The gap between anode and cathode was 4 mm for the measurements with ot-particles, and 2.5 mm for the measurements with the lamp. The photocathode plate is connected to the high voltage feedthrough, and the other plate is connected to a charge sensitive preamplifier mounted directly on the top flange of the chamber. The signal from the preamplifier is further amplified before being fed into a multichannel analyzer . The CsI photocathode was produced at Fermilab, using vacuum deposition of CsI vapor at high temperature . The details of the production procedure are described in Ref. [4]. Photocathodes with a CsI layer of thickness 50 nm and 500 nm on a stainless steel substrate were used. The chamber was connected to the Columbia purification system to reduce the impurity level in the filling gas to less than 1 ppb OZ equivalent [5]. The method of obtaining LXe, LKr and LAr is described in Ref. [3] . The solid phase for all three gases was usually obtained through
* Corresponding author .
0168-9002/94/$07 .00 © 1994 Elsevier Science B.V . All rights reserved SSDI0168-9002(94)00842-6
Ila. SCINTILLATION DETECTORS
56
E. Aprile et al. /Nucl. Instr. and Meth . i n Phys . Res. A 353 (1994) 55-58
direct crystal growth by cooling the chamber with liquid nitrogen (LN Z). Different cooling rates and procedures were tried in an attempt to achieve a uniform solid crystal with the least imperfections. 3. Measurements For the measurements with the alpha source, both the ionization charge and the scintillation light signals produced by alpha irradiation of the solid rare gases were recorded as a function of the electric field, by reversing the polarity of the voltage on the photocathode plate (see Ref. [3] for details) .
For the measurements with the Hg lamp the photocurrent from the CsI photocathode was recorded as a function of the electric field. The intensity of the UV light source was calibrated with several commercial photodiodes, with known quantum efficiencies . This allows us to independently estimate the absolute value of the QE [3]. In all the experiments, the measurements were first done with the rare gas in liquid phase. This gave us a reference of the purity level of the filling gas, based on our previous results with the liquids [3]. Without opening the chamber, the next experiment was made with the solid phase to measure the photoelectron yield. Then again a run in the liquid phase was made to check whether the purity level had changed. When working with solid rare gases,
Mercury UV Lamp
Fig. 1. Schematic of the experimental setup with : (a) an
241
,\m a-source, (b) with mercury lamp.
57
E. Aprile et al. /NucL Instr. andMeth. in Phys. Res. A 353 (1994) 55-58 5 1600
Y
Csl Photocathode in Solid Xenon
Am 5.5 MeV 241
=
_ 241
-
n 1200 O
N Û N
Ô
Ô
m E
Z 400
Z
.I 200
Test Pulse AI 400
I ' 600
I 800
u 1000
Channel Number Fig. 2. Typical pulse height spectrum of the scintillation light from 241Am 5.5 MeV a-particles in solid Xe . due to the lower vapor pressure, high voltage breakdown occurred earlier than in the liquid phase. This prevented us from reaching high electric fields . 4. Results The data in solid Xe were obtained with the 500 nm thick CsI photocathode. Fig . 2 shows a typical pulse height spectrum of the scintillation light produced in solid Xe by the alpha particles. Compared to the same data measured in the liquid phase at the same electric field and electronics gain, the peak position is higher. Results obtained in different runs made with liquid and solid Xe are summarized in Fig . 3. One can see that the results obtained with liquid Xe are reproducible in the different runs and in good agreement with our previous results [3]. In the case of the solid, results are different depending on the procedure 5
ô N C
2
Û a)
3
"
W
800
E
Am 5.5 MeV a Particles in Krypton
2
o
1
06
5
o " " o
o lonutahon charge signal in Liquid Photoelectron Signal in Solid Photoelectron Signal in Liquid
10
Electric Field
15
20
Fig. 4. Number of electrons collected as a function of the electric field from the CsI photocathode for 5.5 MeV 24 'Am alpha particles in liquid and solid Kr. followed to form the solid. For a given solid sample the results were stable with time with a fluctuation less than a few percent. The light signal from solid Xe was highest (closed squares in Fig. 3) when we condensed the xenon gas into solid with a LNZ bath . If the Xe gas was first condensed into liquid with a bath of alcohol and LN Z mixture (about -1100C), then cooled down to form solid by changing the bath to LN Z , the observed light signal was lower (closed circles in Fig. 3) . Despite the variation in the data from different solidification procedures, the measured light signal was however always a factor of 50-100% higher than that measured in the liquid phase. The variation in the measured photoelectron signal is related to the quality of the solid. In our experiments, the solid structure was always polycrystalline and in some cases with many defects. The energy resolution for the light produced by the alpha particles in solid Xe at the maximum applied electric field of 2 kV/cm, was typically 10-12%, dominated by
'Am 5.5 MeV a Particles Photoelectron Signal " in Xenon 24
4
8 O
3
W
Ô 2
25
(kV/cm)
a
r
A
O O
6
0
E Z
u "
0
0
1
2
Photoelectron Signal Photoelectron Signal
Electric Field
3
(kV/cm)
in In 4
Liquid
0 0
0 â
Solid
0 2 o "
5
Fig. 3. Number of electrons collected as a function of the electric field from the Csl photocathode for 5.5 MeV 2441Am alpha particles in liquid and solid Xe .
00
2
I 4
Electric Field
Photocurrent In Photocurrent In I 6
(kV/cm)
I 8
Liquid Solid 10
Fig. 5. Photocurrent from the CsI photocathode in liquid and solid Kr for experiments using the mercury lamp . Ila. SCINTILLATION DETECTORS
58
E. Aprile et al. /Nucl. Instr. and Meth . to Phys . Res. A 353 (1994) 55-58 5
ô v)
2
4
24t Am o o
the liquid phase. The observed increase in the signal can
5.5 MeV a Particles in Argon
be attributed to the following factors: 1) increase of the
Ionization Charge Signal In Liquid Photoelectron Signal in Liquid Photoelectron Signal in Solid
CsI QE, 2) increase in the intensity of the light produced
0
by ot-particles in solid rare gases because of the lower WPh value in the solid phase. The WPh value for solid rare gases
3
has not been precisely measured . However it is expected
Û N
w
b N
2
that the WPh value does not change significantly from the liquid to the solid phase. We can draw the same conclusion
0
from our own measurements because the measured ioniza-
E
tion charge yield is almost the same for both the liquid and
zl
4t
solid phases . The major contribution to the increase of the 5
to
15
Electric Field (kV/cm)
Fig. 6. Number of electrons collected as a function of the electric field from the CsI photocathode for 5.5 MeV cles in liquid and solid Ar.
241
Am alpha parti-
photoelectron signal should therefore come from the in-
crease of the QE of the CsI in the solid phase. The independent measurements done with the Hg lamp further strengthen this assumption . If we attribute the increase of
the photoelectric signal purely to the increase of the CsI QE, one can then estimate that the absolute value of this
QE in the case of solid rare gases reaches a rather high the electronic noise. In the liquid phase, much higher fields
value. For example, in the case of solid Xe the QE is
observed [3].
necessary to confirm this expectation .
could be applied, and a much better energy resolution was
estimated to be around 60%. Further measurements are
Similar experiments were repeated with Kr and At . To
avoid the charging up problem at the lower cryogenic temperature of LKr and LAr [3], a photocathode with 50
nm thick Csl layer was used . The light signal obtained in these experiments with LAr and LKr was lower than that
measured previously due to the lower QE of the thin CsI photocathode [3]. In Fig. 4 we present the results obtained for the scintillation light produced by ot-particles in solid
and liquid Kr . Again one can see that in the solid phase the photocathode signal is higher than that in liquid.
Typical results obtained with the Hg lamp in liquid and
solid krypton are shown in Fig. 5. The photocurrent in solid is - 20-30% higher than that in liquid, in agreement with the results obtained using the alpha source.
Fig. 6 shows the light and charge signals produced by
ot-particles in solid and liquid At as a function of the
6. Conclusion Our preliminary results show the possibility of using
the CsI photocathode as a photon readout for solid rare gas scintillating detectors that is flexible in design, uniform in
response, inexpensive, capable of working at high rate and has a large area and a high quantum efficiency . Additional
advantages of solid rare gases are: higher electron mobil-
ity, less sensitivity to mechanical vibrations and change in the impurity level. With further research and development work, we expect that a solid rare gas scintillator coupled to a CsI photocathode can find applications in calorimetry in high energy physics, nuclear medicine and astrophysics .
electric field. One can clearly see that for a fixed voltage the signal from the CsI photocathode in solid Ar is 100% higher than that in the liquid .
5. Discussion For all three rare gases tested, Xe, Kr and Ar, we observed that in the solid phase the photoelectron signal from the CsI photocathode is 20-100% higher than that in
References [I] M. Chen et al ., Nucl. Instr. and Meth. A 267 (1988) 43 . [2] T. Ypsilantis, presented at the Conf. on New Technologies for Supercolliders, Erice, Italy, 1991 . [3] E. Aprile et al., Nucl . Instr. and Meth . A 338 (1994) 328. [4] D.F . Anderson et al ., Nucl . Instr. and Meth . A 323 (1992) 626. [5] E. Aprile et al ., Nucl . Instr. and Meth . A 300 (1991) 343.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
ELSEVIER
First observation of the scintillation light from solid Xe, Kr and Ar with a Csl photocathode E. Aprile
a , *,
A. Bolotnikov
a,
D. Chen
a, F. XU a,
V. Peskov
b
a Physics Department, Columbia University, New Yor1y NY 10027, USA b Fermi National Accelerator Laboratory, Batavia, IL 60510, USA
Abstract We have demonstrated that the scintillation light from solid Xe, Kr and Àr can be recorded by means of a CsI photocathode directly coupled with the solid. The signal obtained from the CsI photocathode inside solid rare gases is 20-100% higher than that previously observed with CsI photocathodes inside the same liquids . This may open new applications of solid rare gas scintillating detectors with such a readout.
1. Introduction
2. Experimental setup
The use of the scintillation light from liquid and solid rare gases for calorimetry in high energy physics has been suggested by several authors [1,2]. The main advantages of condensed rare gas scintillating detectors are: high stopping power, fast response and radiation hardness. We have previously reported the successful operation of CsI photocathodes in liquid xenon (LXe), liquid krypton (LKr) and liquid argon (LAr) [3]. A very good energy resolution was achieved for ot-particles with energy of 5.5 MeV: 8.0% FWHM in LAr, 5.2% FWHM in LKr and 2.8% FWHM in LXe. The quantum efficiency (QE) of the CsI photocathode was: 10% in LAr, 17% in LKr and 31% in LXe. These results were independently verified by further measurements with a calibrated external source of UV light. A theoretical explanation based on the band structure of the CsI photocathode and the liquid rare gases was suggested. It remains of interest to check the properties of CsI photocathodes coupled with solid rare gases. A solid rare gas detector offers the advantage of being mechanically more stable and less sensitive to contamination from impurities . In addition, the higher electron mobility is favorable when charge collection is involved ; it can result in a high photoelectron yield. Here we summarize our preliminary results from experiments in this direction.
As in our previous work, we used two experimental setups : one with an alpha source (24'Am) installed inside the test chamber and the other with a calibrated mercury lamp and a set of narrow band filters as an external UV light source (see Fig. 1) . The details are described in Ref. [3]. For the present experiments, the test chamber was equipped with a window to visually monitor the formation of the solid. The test chamber contains two parallel plate electrodes . One of the two plates is always a CsI photocathode, while the other one is either a stainless steel plate with the alpha source deposited on its center, or a stainless steel mesh to transmit the light from the external light source . The gap between anode and cathode was 4 mm for the measurements with ot-particles, and 2.5 mm for the measurements with the lamp. The photocathode plate is connected to the high voltage feedthrough, and the other plate is connected to a charge sensitive preamplifier mounted directly on the top flange of the chamber. The signal from the preamplifier is further amplified before being fed into a multichannel analyzer . The CsI photocathode was produced at Fermilab, using vacuum deposition of CsI vapor at high temperature . The details of the production procedure are described in Ref. [4]. Photocathodes with a CsI layer of thickness 50 nm and 500 nm on a stainless steel substrate were used. The chamber was connected to the Columbia purification system to reduce the impurity level in the filling gas to less than 1 ppb OZ equivalent [5]. The method of obtaining LXe, LKr and LAr is described in Ref. [3] . The solid phase for all three gases was usually obtained through
* Corresponding author .
0168-9002/94/$07 .00 © 1994 Elsevier Science B.V . All rights reserved SSDI0168-9002(94)00842-6
Ila. SCINTILLATION DETECTORS
56
E. Aprile et al. /Nucl. Instr. and Meth . i n Phys . Res. A 353 (1994) 55-58
direct crystal growth by cooling the chamber with liquid nitrogen (LN Z). Different cooling rates and procedures were tried in an attempt to achieve a uniform solid crystal with the least imperfections. 3. Measurements For the measurements with the alpha source, both the ionization charge and the scintillation light signals produced by alpha irradiation of the solid rare gases were recorded as a function of the electric field, by reversing the polarity of the voltage on the photocathode plate (see Ref. [3] for details) .
For the measurements with the Hg lamp the photocurrent from the CsI photocathode was recorded as a function of the electric field. The intensity of the UV light source was calibrated with several commercial photodiodes, with known quantum efficiencies . This allows us to independently estimate the absolute value of the QE [3]. In all the experiments, the measurements were first done with the rare gas in liquid phase. This gave us a reference of the purity level of the filling gas, based on our previous results with the liquids [3]. Without opening the chamber, the next experiment was made with the solid phase to measure the photoelectron yield. Then again a run in the liquid phase was made to check whether the purity level had changed. When working with solid rare gases,
Mercury UV Lamp
Fig. 1. Schematic of the experimental setup with : (a) an
241
,\m a-source, (b) with mercury lamp.
57
E. Aprile et al. /NucL Instr. andMeth. in Phys. Res. A 353 (1994) 55-58 5 1600
Y
Csl Photocathode in Solid Xenon
Am 5.5 MeV 241
=
_ 241
-
n 1200 O
N Û N
Ô
Ô
m E
Z 400
Z
.I 200
Test Pulse AI 400
I ' 600
I 800
u 1000
Channel Number Fig. 2. Typical pulse height spectrum of the scintillation light from 241Am 5.5 MeV a-particles in solid Xe . due to the lower vapor pressure, high voltage breakdown occurred earlier than in the liquid phase. This prevented us from reaching high electric fields . 4. Results The data in solid Xe were obtained with the 500 nm thick CsI photocathode. Fig . 2 shows a typical pulse height spectrum of the scintillation light produced in solid Xe by the alpha particles. Compared to the same data measured in the liquid phase at the same electric field and electronics gain, the peak position is higher. Results obtained in different runs made with liquid and solid Xe are summarized in Fig . 3. One can see that the results obtained with liquid Xe are reproducible in the different runs and in good agreement with our previous results [3]. In the case of the solid, results are different depending on the procedure 5
ô N C
2
Û a)
3
"
W
800
E
Am 5.5 MeV a Particles in Krypton
2
o
1
06
5
o " " o
o lonutahon charge signal in Liquid Photoelectron Signal in Solid Photoelectron Signal in Liquid
10
Electric Field
15
20
Fig. 4. Number of electrons collected as a function of the electric field from the CsI photocathode for 5.5 MeV 24 'Am alpha particles in liquid and solid Kr. followed to form the solid. For a given solid sample the results were stable with time with a fluctuation less than a few percent. The light signal from solid Xe was highest (closed squares in Fig. 3) when we condensed the xenon gas into solid with a LNZ bath . If the Xe gas was first condensed into liquid with a bath of alcohol and LN Z mixture (about -1100C), then cooled down to form solid by changing the bath to LN Z , the observed light signal was lower (closed circles in Fig. 3) . Despite the variation in the data from different solidification procedures, the measured light signal was however always a factor of 50-100% higher than that measured in the liquid phase. The variation in the measured photoelectron signal is related to the quality of the solid. In our experiments, the solid structure was always polycrystalline and in some cases with many defects. The energy resolution for the light produced by the alpha particles in solid Xe at the maximum applied electric field of 2 kV/cm, was typically 10-12%, dominated by
'Am 5.5 MeV a Particles Photoelectron Signal " in Xenon 24
4
8 O
3
W
Ô 2
25
(kV/cm)
a
r
A
O O
6
0
E Z
u "
0
0
1
2
Photoelectron Signal Photoelectron Signal
Electric Field
3
(kV/cm)
in In 4
Liquid
0 0
0 â
Solid
0 2 o "
5
Fig. 3. Number of electrons collected as a function of the electric field from the Csl photocathode for 5.5 MeV 2441Am alpha particles in liquid and solid Xe .
00
2
I 4
Electric Field
Photocurrent In Photocurrent In I 6
(kV/cm)
I 8
Liquid Solid 10
Fig. 5. Photocurrent from the CsI photocathode in liquid and solid Kr for experiments using the mercury lamp . Ila. SCINTILLATION DETECTORS
58
E. Aprile et al. /Nucl. Instr. and Meth . to Phys . Res. A 353 (1994) 55-58 5
ô v)
2
4
24t Am o o
the liquid phase. The observed increase in the signal can
5.5 MeV a Particles in Argon
be attributed to the following factors: 1) increase of the
Ionization Charge Signal In Liquid Photoelectron Signal in Liquid Photoelectron Signal in Solid
CsI QE, 2) increase in the intensity of the light produced
0
by ot-particles in solid rare gases because of the lower WPh value in the solid phase. The WPh value for solid rare gases
3
has not been precisely measured . However it is expected
Û N
w
b N
2
that the WPh value does not change significantly from the liquid to the solid phase. We can draw the same conclusion
0
from our own measurements because the measured ioniza-
E
tion charge yield is almost the same for both the liquid and
zl
4t
solid phases . The major contribution to the increase of the 5
to
15
Electric Field (kV/cm)
Fig. 6. Number of electrons collected as a function of the electric field from the CsI photocathode for 5.5 MeV cles in liquid and solid Ar.
241
Am alpha parti-
photoelectron signal should therefore come from the in-
crease of the QE of the CsI in the solid phase. The independent measurements done with the Hg lamp further strengthen this assumption . If we attribute the increase of
the photoelectric signal purely to the increase of the CsI QE, one can then estimate that the absolute value of this
QE in the case of solid rare gases reaches a rather high the electronic noise. In the liquid phase, much higher fields
value. For example, in the case of solid Xe the QE is
observed [3].
necessary to confirm this expectation .
could be applied, and a much better energy resolution was
estimated to be around 60%. Further measurements are
Similar experiments were repeated with Kr and At . To
avoid the charging up problem at the lower cryogenic temperature of LKr and LAr [3], a photocathode with 50
nm thick Csl layer was used . The light signal obtained in these experiments with LAr and LKr was lower than that
measured previously due to the lower QE of the thin CsI photocathode [3]. In Fig. 4 we present the results obtained for the scintillation light produced by ot-particles in solid
and liquid Kr . Again one can see that in the solid phase the photocathode signal is higher than that in liquid.
Typical results obtained with the Hg lamp in liquid and
solid krypton are shown in Fig. 5. The photocurrent in solid is - 20-30% higher than that in liquid, in agreement with the results obtained using the alpha source.
Fig. 6 shows the light and charge signals produced by
ot-particles in solid and liquid At as a function of the
6. Conclusion Our preliminary results show the possibility of using
the CsI photocathode as a photon readout for solid rare gas scintillating detectors that is flexible in design, uniform in
response, inexpensive, capable of working at high rate and has a large area and a high quantum efficiency . Additional
advantages of solid rare gases are: higher electron mobil-
ity, less sensitivity to mechanical vibrations and change in the impurity level. With further research and development work, we expect that a solid rare gas scintillator coupled to a CsI photocathode can find applications in calorimetry in high energy physics, nuclear medicine and astrophysics .
electric field. One can clearly see that for a fixed voltage the signal from the CsI photocathode in solid Ar is 100% higher than that in the liquid .
5. Discussion For all three rare gases tested, Xe, Kr and Ar, we observed that in the solid phase the photoelectron signal from the CsI photocathode is 20-100% higher than that in
References [I] M. Chen et al ., Nucl. Instr. and Meth. A 267 (1988) 43 . [2] T. Ypsilantis, presented at the Conf. on New Technologies for Supercolliders, Erice, Italy, 1991 . [3] E. Aprile et al., Nucl . Instr. and Meth . A 338 (1994) 328. [4] D.F . Anderson et al ., Nucl . Instr. and Meth . A 323 (1992) 626. [5] E. Aprile et al ., Nucl . Instr. and Meth . A 300 (1991) 343.