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Development of a liquid-xenon photon detector––towards the search for a muon rare decay mode at Paul Scherrer Institute
Cryogenics 44 (2004) 223–228 www.elsevier.com/locate/cryogenics
Development of a liquid-xenon photon detector––towards the search for a muon rare decay mode at Paul Scherrer Institute q S. Mihara a,*, T. Doke b, T. Haruyama c, K. Kasami c, A. Maki c, T. Mitsuhashi a, T. Mori a, H. Nishiguchi a, W. Ootani a, K. Ozone a, R. Sawada a, S. Suzuki b, K. Terasawa b, T. Yoshimura b a International Center for Elementary Particle Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Advanced Research Institute for Science and Engineering, Waseda University, 1-6-1 Waseda, Shinjuku-ku, Tokyo 113-0032, Japan High Energy Accelerator Research Organization, Institute of Particle and Nuclear Studies, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan b
c
Received 26 August 2003; received in revised form 8 December 2003; accepted 8 December 2003
Abstract We are developing a new type of photon detector in preparation of an experiment to search for muons decaying into positrons and gamma rays. In the experiment, the photon detector will utilize liquid xenon (Xe) as the scintillation material because of its fast response, large light yield, and high density. The scintillation light emitted in liquid Xe will be observed directly by positioning photomultipliers (PMTs) in the liquid without using a transmission window. In order to determine proper experimental procedures and to study the detector response to gamma rays, we constructed a prototype utilizing a 100 l volume of liquid Xe. The current status and future prospects of detector development are reported in this article. 2004 Elsevier Ltd. All rights reserved. Keywords: Liquid xenon; Photon detector; Muon rare decay; Pulse-tube refrigerator
1. Introduction We are developing a new type of photon detector using liquid xenon as detection material for a muon rare-decay search experiment. Contrary to the previous noble liquid detectors that use an ionization signal, we adopted a method for observing the incoming particles not by the ionization signal, but by the scintillation light. It is possible thus to construct a gamma detector with very fast response, which is an essential ingredient for rare-decay search experiments. We constructed a small prototype with a 2.3 l active volume to start the detector R&D, and performed an experimental proof of the operation principle [1]. Subsequently, we constructed a large prototype using about a 100 l volume of liquid xenon in order to verify the stable operation of a
q
Translation of article originally published in Cryogenic Engineering (Journal of Cryogenic Association of Japan) Vol. 38 (2003), pp. 94–99. * Corresponding author. Fax: +81-3-3814-8806. E-mail address: [email protected] (S. Mihara). 0011-2275/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2003.12.002
similar size detector to the final one and to evaluate the detector performance. Using this prototype we have conducted several kinds of tests. In this article we report on the detector R&D status using a large prototype and describe future prospects of our project.
2. l fi ec search at Paul Scherrer Institute The decay mode that we are searching for is the twobody decay of a muon into a positron and a gamma ðl ! ecÞ. Because the lepton-flavor number is not conserved in this mode, the decay is forbidden in the framework of the Standard Model in which the neutrino mass is supposed to be zero. Even introducing the neutrino mass, as observed by experiments, does not increase the branching ratio above the level where we can reach with the current experimental techniques. However, if we suppose Super Symmetric particles that are expected to exist beyond the Standard Model, the branching ratio can be as large as we can observe in the laboratory [2–4]. Thus, observing this decay mode must directly lead to a new physics beyond the Standard
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Model. Up to now, the upper limit of the branching ratio of the decay mode has been reported to be 1.2 · 1011 [5]. We are planning to search the decay with improved sensitivity, by more than a few orders, by introducing a liquid-xenon photon detector. The experiment will be carried out at Paul Scherrer Institute (PSI) in Switzerland, where the most intense muon beam in the world is available. A proposal of the experiment was submitted in 1999 [6] and approved in the same year. By now, an international collaboration consisting of researchers from Japan, Italy, Switzerland, and Russia has been formed, and preparation of the experimental setup is in progress. An engineering run is planned to be conducted in the year 2004. After that we will continue data acquisition for about one year. Consequently, long-term operation for one year of the photon detector is indispensable to the success of the experiment.
3. Liquid-xenon photon detector Detectors using a noble liquid, like argon, krypton, or xenon, have been developed starting around the middle of the 70Õs, and have been adopted in several particle physics experiments [7,8]. When a liquid is used as the detection material, there is an advantage of handling large-scale detectors by simply constructing a vessel for the liquid. Currently, several noble liquid detectors are under operation, or being constructed, like a timeprojection chamber with liquid argon [9], an electromagnetic calorimeter with liquid krypton [10], and a dark-matter search detector with liquid xenon [11]. Our liquid-xenon detector employs a different technique for signal read out from those detectors mentioned above. Of course, some detectors utilizing scintillation light emitted in liquid xenon have been developed, but our detector is characterized by the photomultiplier (PMT) layout to detect the light. The PMTs are located on all walls of the active volume. In addition, the PMTs are placed in a liquid. Thus, we will be able to achieve excellent detector performance with a fast response by observing a sufficient amount of scintillation light without any possible loss caused by transmission windows. It is also an important feature that we can avoid any difficulty accompanied by transmission windows when constructing a cryostat. The properties of liquid xenon are listed in Table 1. Xenon has characteristic features that the temperature band of the liquid phase is quite narrow and the latent heat for solid is rather small. Because of these features, the phase transition to the solid occurs rather easily. In the case of solidification of xenon during detector operation, damage on the PMTs immersed in xenon is concerned. Hence, it is quite important to achieve stable temperature control of the liquid for avoiding a rapid
Table 1 Properties of liquid xenon Property
Unit
Saturation temperature Saturation pressure Latent heat (for liquid) Latent heat (for solid) Specific heat Density Thermal conductivity Viscosity Surface tension Expansion coefficient Temperature/pressure at triple point
T(K) P(MPa) q(J/kg) · 103 q0 (J/kg) · 103 Cp (J/kgK) · 103 q(kg/m3 ) · 103 j(W/mK) l (Pa-s) · 104 r(N/m)·103 b(1/K) · 103 Tt (K)/Pt (MPa)
164.78 0.100 95.8 1.2 0.3484 2.947 0.108 5.08 18.46 2.43 161.36/0.0815
temperature variation in the whole volume of the detector. As described above, the PMTs used to observe scintillation light are placed in a liquid. Because of this, the PMT should be operational at around 160 K and the heat load from its electronics circuit should be minimized. In addition, the PMT window is required to transmit vacuum ultraviolet light because the wavelength of the xenon scintillation light ranges over 174 ± 10 nm. A new PMT has been developed, satisfying all of these requirements, in collaboration with Hamamatsu Photonics. The PMT employs Rb–Cs–Sb as a photocathod material, whose quantum efficiency does not change strongly, even at low temperature. The body of the PMT is made of a metal tube by which a pressure proof up to 0.3 MPa can be archived. The metal channel dynode structure of the PMT helps to reduce the length to be 32 mm. The l ! ec decay search experiment at PSI needs not only a very intense muon beam, but also a large coverage of the detector acceptance in order to achieve the best sensitivity for the small branching ratio that could not be measured before. For this purpose, we are designing a liquid-xenon photon detector with about 800 l of liquid xenon (weights 2.4 ton). The larger is the size of the detector, the more attention we must pay to avoid scintillation light absorption before being observed by the PMTs. It is known that the scintillation light emission mechanism in liquid xenon is a nonreversible reaction [12], resulting in being free from selfabsorption. Thus if any absorption in the liquid occurs, that must be due to some impurity contained in the liquid, such as water or oxygen, which must be removed for reliable detector operation. We conducted several tests using a large prototype in order to establish the detector operating conditions and to verify the required techniques for long-term stable operation under similar circumstances to that for the final detector.
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Fig. 1. Schematic view of the large prototype. Three sets of cosmic-ray trigger counters are also shown.
4. Detector R&D using a prototype 4.1. Prototype A schematic view of the large prototype is shown in Fig. 1. This prototype consists of 228 PMTs mounted on a rectangular frame (Fig. 2) and a liquid xenon vessel with a vacuum insulation layer. There are four 241 Am alpha sources on the frame for PMT calibration. For detector operation, the vessel is filled with liquid xenon, so that the PMT array can be immersed in liquid. For the liquefaction and recondensation of xenon, a liquidnitrogen cooling pipe and a pulse-tube refrigerator are equipped. To trigger cosmic-ray events, three sets of plastic scintillation counters are located above and below the vessel. The advantages of using a pulse-tube refrigerator are the following. There is no mechanical moving part in the low-temperature side. Consequently, the refrigerator does not produce an inevitable noise for the detector, and long-term operation can be easily achieved. In addition, operation is safe and easy because no liquid coolant is used for cooling. We are developing a pulsetube refrigerator for use in the xenon detector [13], which has been proved to have 65 W cooling power with a 2.2 kW compressor. During detector operation, the following heat load will enter and/or appear in the liquid. Heat generation from the resistors on the electronics circuit of the PMTs, heat conduction through high-voltage and signal cables (456 coaxial cables of 3 m length) for the PMTs, heat conduction through the inner vessel supports and service ports, and radiation through the multilayer insulation. The heat generation from the resistors and conduction through the cables are estimated to be 18 and 10 W, respectively. The others are obtained to be 24 W in total
Fig. 2. Construction of a photomultiplier (PMT) array consisting of 228 PMTs installed in the cooling chamber.
based on the evaporation rate of liquid xenon and the extracooling power of the refrigerator, which is consistent with the design value. It is important to know how heat conduction occurs from the resistors to liquid, since many PMTs are located in the liquid. Bubbling due to nucleate boiling may disturb the light transmission in the liquid. For clarifying this, another experiment has been carried out [14] to investigate the heat-conduction characteristics while boiling. According to this result, the heat flux from the resistors is so small that they are cooled by natural convection, and probably do not disturb light observations in the liquid. 4.2. Operation As a first step, the vessel is evacuated before filling gas xenon of 0.2 MPa and cooling down to around 165 K
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be reproduced well with wavelength-dependent absorption by 10 ppm water. Since the xenon fed in the vessel is purified before liquefaction, it is quite unlikely that an impurity like water is contained in the xenon. Such an impurity is presumed to be contained in the vessel without being evacuated by a vacuum pump, and is exuded in the liquid after the volume is filled with liquid xenon. We introduced a xenon circulation system, as shown in Fig. 3, in order to remove such an impurity. Xenon is pumped from the bottom of the vessel, purified, and returned to the vessel from the top. Liquid xenon pumped from the bottom evaporates immediately around the exit. The heat of evaporation is used to cool the returned xenon in a heat exchanger. A diaphragm pump is employed for circulating xenon, whose maximum flow rate is 25 l/min and maximum outlet pressure is 150 kPa. The flow rate is controlled with a valve near to the pump to be about 10 cc/min in the liquid. It is not easy to estimate the exact efficiency of the heat exchanger, but when we circulate xenon with this flow rate, additional cooling power of 55 W is needed for liquefying xenon after passing the purifier at room temperature. This exceeds the surplus cooling power of the pulse-tube refrigerator. Thus, for stabilizing the liquid xenon during the purification process liquid nitrogen is also used for refrigeration. Supposing that any impurity in the vessel does not exude in the vessel to the liquid under constant circulation rate, we can conclude that the amount of impurity in the liquid will decrease with a time constant of s that is determined by the total volume of the xenon and the circulation speed. Because the absorption is inversely proportional to the amount of impurity, the absorption length will increase with the same time constant. Consequently, when we observe the scintillation light at a
Heat Exchanger
4.3. Xenon purification and performance evaluation Even if we start a measurement immediately after completing liquefaction, the remaining water (or oxygen) in the vessel disturbs the scintillation light transmission in the liquid by absorption. This depends on the initial condition inside of the vessel. We could actually observe only 1/4 of the expected amount of scintillation light for cosmic-ray events. In this data, the light-yield dependence on the incident position of cosmic rays can
Xe Tank
using the liquid-nitrogen cooling pipe and refrigerator(pre-cooling). It takes about 24 h for this step. Next, liquefaction continues with supplying xenon continuously for two days (liquefaction). Then, the detector enters into a stable operation mode only with the refrigerator. During this period, the cold-head temperature is controlled to be constant (164–165 K) with the help of a heater equipped in the cold head. Thus, we have achieved a stable retention of the liquid xenon, which enables PMT operation without any problem. An unexpected pressure increase can be avoided by forcing flow of liquid nitrogen in the cooling pipe (above 0.13 MPa) and by a pressure relief valve with a maximum pressure limit of 0.25 MPa. When supplying xenon to the vessel, impurities such as water, oxygen, and hydrocarbons are removed by passing the gas in a metal getter purifier for noble gas. If any contamination of water exists, as described above, the absorption of the scintillation light will not be negligible, resulting in a deterioration of the detector performance. Consequently, the water contamination level should be reduced below the ppm level. The most efficient method for removing water is to bake the detector during evacuation. However, our detector contains PMTs inside, whose temperature must not be higher than 70 C. For this reason, the purification of xenon with circulation, as described later, is necessary for removing any impurities in the vessel after completing liquefaction. After detector operation is finished, xenon is recovered to a storage tank (285 l volume). Recovery is achieved by cooling the storage tank with liquid nitrogen. For accelerating xenon evaporation in the vessel, the refrigerator is turned off, nitrogen gas is put in to brake the insulation vacuum, and a heater placed at the bottom of the vessel is excited. Usually, it takes 2 days to recover all of the xenon and three days to warm up the detector to room temperature. Long-term operations for more than one month have been executed by now several times, and the pulse-tube refrigerator is proved to be able to retain the 100 l volume of the liquid xenon in a stable manner. Needless to say, all of the system is made closed-circuit, and we have succeeded to perform operation without losing expensive xenon.
Getter
226
Circulator Pump Fig. 3. Xenon circulation scheme for purification in the large prototype.
S. Mihara et al. / Cryogenics 44 (2004) 223–228
Cosmic-ray events
3
Alpha particle events 2.6
d = 7.6 cm d = 11.6 cm
2.4 2.5
2.2
relative peak
relative landau peak position
227
2
1.5
2 1.8 1.6 1.4 1.2 1
1 0
50
100
150
200
250
300
0.8
0
50
time [hours]
100
150
200
250
300
time [hours]
Fig. 4. Change in the light yield for cosmic-ray events (left). The change in the light yield for alpha events, observed by PMTs located 7.6 and 11.6 cm from the alpha sources is also shown (right).
fixed distance (d) from a light source, the amount of light is t d N ðd; tÞ ¼ N1;d exp exp ; ð1Þ k0 s and asymptotically reach the maximum of N1;d . Here, k0 means the initial absorption length of light when purification is started. The changes in the light yield for cosmic-ray events and for alpha events are shown in Fig. 4. In both figures data are plotted with normalization with the initial light yield as a function of the purification time up to 300 h. By fitting the three different data sets, we obtained time constants of 350 h with an accuracy of 3%. They agreed with each other. We continued purification for another 300 h. Then, the absorption length of the scintillation light in liquid xenon was estimated using a-particle data, resulting in 100 cm as a lower limit with more than a 95% confidence level, which satisfies the minimum requirement for the l ! ec experiment. This absorption length was found to correspond to 100 ppb water equivalent contamination in liquid xenon by using our simulation.
5. Future prospects We have proved that it is possible to operate our xenon detector using a 100 l volume of xenon in a stable manner for long duration and that the detector performance will be improved by removing any impurity in the xenon. As a next step, we have started to construct the final detector for the l ! ec experiment based on the experiences obtained in large prototype studies. The heat load for the final detector, with 800 l xenon and 1000 PMTs, is estimated to be 130 W at 165 K. Currently, the fabrication of a new pulse-tube refrigerator with high cooling power is ongoing to meet the heat load.
In addition, we are planning to develop a new purification method not in the gas phase, but in the liquid phase, in order to improve the purification efficiency. This will be realized by using a low-temperature liquid pump and molecular sieves. Xenon transfer in the molecular sieves will be done in the liquid phase, enabling us to increase the circulation speed drastically. While the current gas phase method needs about one month for purification, the new method is expected to reduce the duration of this period by two orders of magnitude. This is quite important in the initial stage of the final detector operation. The development of a liquid-phase purification system will be initiated with a small test chamber to check the reliability of each component. After that, the system will be mounted on the large prototype and/or final detector for verifying the real performance of the system. Acknowledgements We wish to thank many people for their suggestions and support, especially Prof. Y. Matsubara in Nihon Univ., who provided his simulation program for the pulse-tube refrigerator design together with valuable comments. We thank the counter-experimental-hall and cryogenic groups in IPNS, KEK for indispensable support to perform our tests. This work has been supported by a Grant-in aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. References [1] Mihara S et al. IEEE TNS 2002;49:588. [2] Barbieri R, Hall LJ. Phys Lett B 1994;338:212. [3] Barbieri R et al. Nucl Phys B 1995;445:219. [4] Hisano J, Nomura D. Phys Rev D 1999;59:116005, and references therein.
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[5] Brooks ML et al. Phys Rev Lett 1999;83:1521. [6] Mori T, et al. Research Proposal to PSI. R-99.05.1, May 1999. [7] Schinzel D. Nucl Instr Meth A 1998;419:217. [8] Doke T, Masuda K. Nucl Instr Meth A 1999;420:62. [9] Cavanna F. NOON 2000. In: Proceedings of the Conference. World Scientific; 2001. p. 122.
[10] Jeitler M. Nucl Instr Meth A 2002;478:404. [11] Suzuki S. Bull Phys Soc Jpn 1997;53:181. [12] Doke T. Port Phys 1981;12:9. [13] Haruyama T et al. In: Proceedings of ICEC19, Narasa, 2003, p. 613. [14] Haruyama T. Adv Cryog Eng 2002;47B:1499.
Development of a liquid-xenon photon detector––towards the search for a muon rare decay mode at Paul Scherrer Institute q S. Mihara a,*, T. Doke b, T. Haruyama c, K. Kasami c, A. Maki c, T. Mitsuhashi a, T. Mori a, H. Nishiguchi a, W. Ootani a, K. Ozone a, R. Sawada a, S. Suzuki b, K. Terasawa b, T. Yoshimura b a International Center for Elementary Particle Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Advanced Research Institute for Science and Engineering, Waseda University, 1-6-1 Waseda, Shinjuku-ku, Tokyo 113-0032, Japan High Energy Accelerator Research Organization, Institute of Particle and Nuclear Studies, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan b
c
Received 26 August 2003; received in revised form 8 December 2003; accepted 8 December 2003
Abstract We are developing a new type of photon detector in preparation of an experiment to search for muons decaying into positrons and gamma rays. In the experiment, the photon detector will utilize liquid xenon (Xe) as the scintillation material because of its fast response, large light yield, and high density. The scintillation light emitted in liquid Xe will be observed directly by positioning photomultipliers (PMTs) in the liquid without using a transmission window. In order to determine proper experimental procedures and to study the detector response to gamma rays, we constructed a prototype utilizing a 100 l volume of liquid Xe. The current status and future prospects of detector development are reported in this article. 2004 Elsevier Ltd. All rights reserved. Keywords: Liquid xenon; Photon detector; Muon rare decay; Pulse-tube refrigerator
1. Introduction We are developing a new type of photon detector using liquid xenon as detection material for a muon rare-decay search experiment. Contrary to the previous noble liquid detectors that use an ionization signal, we adopted a method for observing the incoming particles not by the ionization signal, but by the scintillation light. It is possible thus to construct a gamma detector with very fast response, which is an essential ingredient for rare-decay search experiments. We constructed a small prototype with a 2.3 l active volume to start the detector R&D, and performed an experimental proof of the operation principle [1]. Subsequently, we constructed a large prototype using about a 100 l volume of liquid xenon in order to verify the stable operation of a
q
Translation of article originally published in Cryogenic Engineering (Journal of Cryogenic Association of Japan) Vol. 38 (2003), pp. 94–99. * Corresponding author. Fax: +81-3-3814-8806. E-mail address: [email protected] (S. Mihara). 0011-2275/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2003.12.002
similar size detector to the final one and to evaluate the detector performance. Using this prototype we have conducted several kinds of tests. In this article we report on the detector R&D status using a large prototype and describe future prospects of our project.
2. l fi ec search at Paul Scherrer Institute The decay mode that we are searching for is the twobody decay of a muon into a positron and a gamma ðl ! ecÞ. Because the lepton-flavor number is not conserved in this mode, the decay is forbidden in the framework of the Standard Model in which the neutrino mass is supposed to be zero. Even introducing the neutrino mass, as observed by experiments, does not increase the branching ratio above the level where we can reach with the current experimental techniques. However, if we suppose Super Symmetric particles that are expected to exist beyond the Standard Model, the branching ratio can be as large as we can observe in the laboratory [2–4]. Thus, observing this decay mode must directly lead to a new physics beyond the Standard
224
S. Mihara et al. / Cryogenics 44 (2004) 223–228
Model. Up to now, the upper limit of the branching ratio of the decay mode has been reported to be 1.2 · 1011 [5]. We are planning to search the decay with improved sensitivity, by more than a few orders, by introducing a liquid-xenon photon detector. The experiment will be carried out at Paul Scherrer Institute (PSI) in Switzerland, where the most intense muon beam in the world is available. A proposal of the experiment was submitted in 1999 [6] and approved in the same year. By now, an international collaboration consisting of researchers from Japan, Italy, Switzerland, and Russia has been formed, and preparation of the experimental setup is in progress. An engineering run is planned to be conducted in the year 2004. After that we will continue data acquisition for about one year. Consequently, long-term operation for one year of the photon detector is indispensable to the success of the experiment.
3. Liquid-xenon photon detector Detectors using a noble liquid, like argon, krypton, or xenon, have been developed starting around the middle of the 70Õs, and have been adopted in several particle physics experiments [7,8]. When a liquid is used as the detection material, there is an advantage of handling large-scale detectors by simply constructing a vessel for the liquid. Currently, several noble liquid detectors are under operation, or being constructed, like a timeprojection chamber with liquid argon [9], an electromagnetic calorimeter with liquid krypton [10], and a dark-matter search detector with liquid xenon [11]. Our liquid-xenon detector employs a different technique for signal read out from those detectors mentioned above. Of course, some detectors utilizing scintillation light emitted in liquid xenon have been developed, but our detector is characterized by the photomultiplier (PMT) layout to detect the light. The PMTs are located on all walls of the active volume. In addition, the PMTs are placed in a liquid. Thus, we will be able to achieve excellent detector performance with a fast response by observing a sufficient amount of scintillation light without any possible loss caused by transmission windows. It is also an important feature that we can avoid any difficulty accompanied by transmission windows when constructing a cryostat. The properties of liquid xenon are listed in Table 1. Xenon has characteristic features that the temperature band of the liquid phase is quite narrow and the latent heat for solid is rather small. Because of these features, the phase transition to the solid occurs rather easily. In the case of solidification of xenon during detector operation, damage on the PMTs immersed in xenon is concerned. Hence, it is quite important to achieve stable temperature control of the liquid for avoiding a rapid
Table 1 Properties of liquid xenon Property
Unit
Saturation temperature Saturation pressure Latent heat (for liquid) Latent heat (for solid) Specific heat Density Thermal conductivity Viscosity Surface tension Expansion coefficient Temperature/pressure at triple point
T(K) P(MPa) q(J/kg) · 103 q0 (J/kg) · 103 Cp (J/kgK) · 103 q(kg/m3 ) · 103 j(W/mK) l (Pa-s) · 104 r(N/m)·103 b(1/K) · 103 Tt (K)/Pt (MPa)
164.78 0.100 95.8 1.2 0.3484 2.947 0.108 5.08 18.46 2.43 161.36/0.0815
temperature variation in the whole volume of the detector. As described above, the PMTs used to observe scintillation light are placed in a liquid. Because of this, the PMT should be operational at around 160 K and the heat load from its electronics circuit should be minimized. In addition, the PMT window is required to transmit vacuum ultraviolet light because the wavelength of the xenon scintillation light ranges over 174 ± 10 nm. A new PMT has been developed, satisfying all of these requirements, in collaboration with Hamamatsu Photonics. The PMT employs Rb–Cs–Sb as a photocathod material, whose quantum efficiency does not change strongly, even at low temperature. The body of the PMT is made of a metal tube by which a pressure proof up to 0.3 MPa can be archived. The metal channel dynode structure of the PMT helps to reduce the length to be 32 mm. The l ! ec decay search experiment at PSI needs not only a very intense muon beam, but also a large coverage of the detector acceptance in order to achieve the best sensitivity for the small branching ratio that could not be measured before. For this purpose, we are designing a liquid-xenon photon detector with about 800 l of liquid xenon (weights 2.4 ton). The larger is the size of the detector, the more attention we must pay to avoid scintillation light absorption before being observed by the PMTs. It is known that the scintillation light emission mechanism in liquid xenon is a nonreversible reaction [12], resulting in being free from selfabsorption. Thus if any absorption in the liquid occurs, that must be due to some impurity contained in the liquid, such as water or oxygen, which must be removed for reliable detector operation. We conducted several tests using a large prototype in order to establish the detector operating conditions and to verify the required techniques for long-term stable operation under similar circumstances to that for the final detector.
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225
Fig. 1. Schematic view of the large prototype. Three sets of cosmic-ray trigger counters are also shown.
4. Detector R&D using a prototype 4.1. Prototype A schematic view of the large prototype is shown in Fig. 1. This prototype consists of 228 PMTs mounted on a rectangular frame (Fig. 2) and a liquid xenon vessel with a vacuum insulation layer. There are four 241 Am alpha sources on the frame for PMT calibration. For detector operation, the vessel is filled with liquid xenon, so that the PMT array can be immersed in liquid. For the liquefaction and recondensation of xenon, a liquidnitrogen cooling pipe and a pulse-tube refrigerator are equipped. To trigger cosmic-ray events, three sets of plastic scintillation counters are located above and below the vessel. The advantages of using a pulse-tube refrigerator are the following. There is no mechanical moving part in the low-temperature side. Consequently, the refrigerator does not produce an inevitable noise for the detector, and long-term operation can be easily achieved. In addition, operation is safe and easy because no liquid coolant is used for cooling. We are developing a pulsetube refrigerator for use in the xenon detector [13], which has been proved to have 65 W cooling power with a 2.2 kW compressor. During detector operation, the following heat load will enter and/or appear in the liquid. Heat generation from the resistors on the electronics circuit of the PMTs, heat conduction through high-voltage and signal cables (456 coaxial cables of 3 m length) for the PMTs, heat conduction through the inner vessel supports and service ports, and radiation through the multilayer insulation. The heat generation from the resistors and conduction through the cables are estimated to be 18 and 10 W, respectively. The others are obtained to be 24 W in total
Fig. 2. Construction of a photomultiplier (PMT) array consisting of 228 PMTs installed in the cooling chamber.
based on the evaporation rate of liquid xenon and the extracooling power of the refrigerator, which is consistent with the design value. It is important to know how heat conduction occurs from the resistors to liquid, since many PMTs are located in the liquid. Bubbling due to nucleate boiling may disturb the light transmission in the liquid. For clarifying this, another experiment has been carried out [14] to investigate the heat-conduction characteristics while boiling. According to this result, the heat flux from the resistors is so small that they are cooled by natural convection, and probably do not disturb light observations in the liquid. 4.2. Operation As a first step, the vessel is evacuated before filling gas xenon of 0.2 MPa and cooling down to around 165 K
S. Mihara et al. / Cryogenics 44 (2004) 223–228
be reproduced well with wavelength-dependent absorption by 10 ppm water. Since the xenon fed in the vessel is purified before liquefaction, it is quite unlikely that an impurity like water is contained in the xenon. Such an impurity is presumed to be contained in the vessel without being evacuated by a vacuum pump, and is exuded in the liquid after the volume is filled with liquid xenon. We introduced a xenon circulation system, as shown in Fig. 3, in order to remove such an impurity. Xenon is pumped from the bottom of the vessel, purified, and returned to the vessel from the top. Liquid xenon pumped from the bottom evaporates immediately around the exit. The heat of evaporation is used to cool the returned xenon in a heat exchanger. A diaphragm pump is employed for circulating xenon, whose maximum flow rate is 25 l/min and maximum outlet pressure is 150 kPa. The flow rate is controlled with a valve near to the pump to be about 10 cc/min in the liquid. It is not easy to estimate the exact efficiency of the heat exchanger, but when we circulate xenon with this flow rate, additional cooling power of 55 W is needed for liquefying xenon after passing the purifier at room temperature. This exceeds the surplus cooling power of the pulse-tube refrigerator. Thus, for stabilizing the liquid xenon during the purification process liquid nitrogen is also used for refrigeration. Supposing that any impurity in the vessel does not exude in the vessel to the liquid under constant circulation rate, we can conclude that the amount of impurity in the liquid will decrease with a time constant of s that is determined by the total volume of the xenon and the circulation speed. Because the absorption is inversely proportional to the amount of impurity, the absorption length will increase with the same time constant. Consequently, when we observe the scintillation light at a
Heat Exchanger
4.3. Xenon purification and performance evaluation Even if we start a measurement immediately after completing liquefaction, the remaining water (or oxygen) in the vessel disturbs the scintillation light transmission in the liquid by absorption. This depends on the initial condition inside of the vessel. We could actually observe only 1/4 of the expected amount of scintillation light for cosmic-ray events. In this data, the light-yield dependence on the incident position of cosmic rays can
Xe Tank
using the liquid-nitrogen cooling pipe and refrigerator(pre-cooling). It takes about 24 h for this step. Next, liquefaction continues with supplying xenon continuously for two days (liquefaction). Then, the detector enters into a stable operation mode only with the refrigerator. During this period, the cold-head temperature is controlled to be constant (164–165 K) with the help of a heater equipped in the cold head. Thus, we have achieved a stable retention of the liquid xenon, which enables PMT operation without any problem. An unexpected pressure increase can be avoided by forcing flow of liquid nitrogen in the cooling pipe (above 0.13 MPa) and by a pressure relief valve with a maximum pressure limit of 0.25 MPa. When supplying xenon to the vessel, impurities such as water, oxygen, and hydrocarbons are removed by passing the gas in a metal getter purifier for noble gas. If any contamination of water exists, as described above, the absorption of the scintillation light will not be negligible, resulting in a deterioration of the detector performance. Consequently, the water contamination level should be reduced below the ppm level. The most efficient method for removing water is to bake the detector during evacuation. However, our detector contains PMTs inside, whose temperature must not be higher than 70 C. For this reason, the purification of xenon with circulation, as described later, is necessary for removing any impurities in the vessel after completing liquefaction. After detector operation is finished, xenon is recovered to a storage tank (285 l volume). Recovery is achieved by cooling the storage tank with liquid nitrogen. For accelerating xenon evaporation in the vessel, the refrigerator is turned off, nitrogen gas is put in to brake the insulation vacuum, and a heater placed at the bottom of the vessel is excited. Usually, it takes 2 days to recover all of the xenon and three days to warm up the detector to room temperature. Long-term operations for more than one month have been executed by now several times, and the pulse-tube refrigerator is proved to be able to retain the 100 l volume of the liquid xenon in a stable manner. Needless to say, all of the system is made closed-circuit, and we have succeeded to perform operation without losing expensive xenon.
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Circulator Pump Fig. 3. Xenon circulation scheme for purification in the large prototype.
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Fig. 4. Change in the light yield for cosmic-ray events (left). The change in the light yield for alpha events, observed by PMTs located 7.6 and 11.6 cm from the alpha sources is also shown (right).
fixed distance (d) from a light source, the amount of light is t d N ðd; tÞ ¼ N1;d exp exp ; ð1Þ k0 s and asymptotically reach the maximum of N1;d . Here, k0 means the initial absorption length of light when purification is started. The changes in the light yield for cosmic-ray events and for alpha events are shown in Fig. 4. In both figures data are plotted with normalization with the initial light yield as a function of the purification time up to 300 h. By fitting the three different data sets, we obtained time constants of 350 h with an accuracy of 3%. They agreed with each other. We continued purification for another 300 h. Then, the absorption length of the scintillation light in liquid xenon was estimated using a-particle data, resulting in 100 cm as a lower limit with more than a 95% confidence level, which satisfies the minimum requirement for the l ! ec experiment. This absorption length was found to correspond to 100 ppb water equivalent contamination in liquid xenon by using our simulation.
5. Future prospects We have proved that it is possible to operate our xenon detector using a 100 l volume of xenon in a stable manner for long duration and that the detector performance will be improved by removing any impurity in the xenon. As a next step, we have started to construct the final detector for the l ! ec experiment based on the experiences obtained in large prototype studies. The heat load for the final detector, with 800 l xenon and 1000 PMTs, is estimated to be 130 W at 165 K. Currently, the fabrication of a new pulse-tube refrigerator with high cooling power is ongoing to meet the heat load.
In addition, we are planning to develop a new purification method not in the gas phase, but in the liquid phase, in order to improve the purification efficiency. This will be realized by using a low-temperature liquid pump and molecular sieves. Xenon transfer in the molecular sieves will be done in the liquid phase, enabling us to increase the circulation speed drastically. While the current gas phase method needs about one month for purification, the new method is expected to reduce the duration of this period by two orders of magnitude. This is quite important in the initial stage of the final detector operation. The development of a liquid-phase purification system will be initiated with a small test chamber to check the reliability of each component. After that, the system will be mounted on the large prototype and/or final detector for verifying the real performance of the system. Acknowledgements We wish to thank many people for their suggestions and support, especially Prof. Y. Matsubara in Nihon Univ., who provided his simulation program for the pulse-tube refrigerator design together with valuable comments. We thank the counter-experimental-hall and cryogenic groups in IPNS, KEK for indispensable support to perform our tests. This work has been supported by a Grant-in aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. References [1] Mihara S et al. IEEE TNS 2002;49:588. [2] Barbieri R, Hall LJ. Phys Lett B 1994;338:212. [3] Barbieri R et al. Nucl Phys B 1995;445:219. [4] Hisano J, Nomura D. Phys Rev D 1999;59:116005, and references therein.
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