- Email: [email protected]
Liquid xenon gamma ray detector for MEG
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 581 (2007) 522–525 www.elsevier.com/locate/nima
Liquid xenon gamma ray detector for MEG R. Sawada, on behalf of the MEG xenon detector group University of Tokyo, Tokyo, Japan Available online 8 August 2007
Abstract The MEG experiment searches for the rare muon decay mþ ! eþ g, which is forbidden in the standard model. The detection of this muon decay is a probe for new physics beyond the standard model. In order to improve the sensitivity of the experiment, a liquid xenon detector is used for measuring energy, position and time of gamma rays. The detector consists of 800 l of liquid xenon and 846 photomultiplier tubes. We have performed beam tests and evaluated resolutions by using a prototype. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 29.40.Vj; 13.35.Bv Keywords: Liquid xenon; Scintillation calorimeter; MEG experiment
1. Introduction MEG is an experiment to search for the lepton-flavor violating muon decay mþ ! eþ g at Paul Scherrer Institut (PSI) in Switzerland. The signature of this muon decay is a back-to-back positron and gamma ray, in time, with a respective energy of 52.8 MeV. In order to reduce background events, precise measurements of energy, emission angle and time of positrons and gamma rays are important. In order to measure gamma rays with such high precision, a large liquid xenon gamma-ray detector is used in this experiment. 2. Apparatus Fig. 1 is a schematic view of the MEG detector. A C-shaped gamma-ray detector is located next to the positron spectrometer. It covers 10% of the solid angle viewed from the muon stopping target. The cryostat of the gamma-ray detector is filled with over 800 l of liquid xenon surrounded by 846 photo-multiplier tubes (PMT). The PMTs are immersed in the liquid xenon to observe scintillation photons without using any transmission window. Corresponding author. Tel.: +81 3 3815 8384; fax: +81 3 3814 8806.
E-mail address: [email protected] 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.08.040
Gamma rays from the target enter the active volume of the detector through an entrance window consisting of an aluminum honeycomb and carbon fiber plates. Then they interact with the liquid xenon and lose energy to excite xenon molecules, resulting in the emission of a large amount of scintillation light. The scintillation light yield is as large as 80% of NaI. The scintillation decay time is five times faster than that of NaI. Because of these properties, the liquid xenon scintillation detector is expected to have excellent performance to measure energy, position, and time of the gamma rays. Table 1 shows the properties of xenon. In order to detect scintillation light efficiently, a new type of PMT was developed in collaboration with Hamamatsu photonics. Since the wavelength of scintillation light from liquid xenon is vacuum ultraviolet, the PMT is equipped with a quartz window which is 80% transparent to the scintillation light. It is designed to work at low temperature with minimum heat generation from its base circuit. 3. The prototype of the LXe detector We constructed a prototype liquid xenon detector in order to gain experience with the detector operation and to investigate its performance. The depth of the prototype along the incident gamma-ray direction is as long as that of the final detector in order to contain whole showers up to
ARTICLE IN PRESS R. Sawada / Nuclear Instruments and Methods in Physics Research A 581 (2007) 522–525
523
Liquid Xenon Scintillation Detector
COBRA Magnet Thin Superconducting Coil
Stopping Target Muon Beam
+
+
Timing Counter
e
e
Drift Chamber
Drift Chamber 1m
Fig. 1. MEG detector.
location of incidence. The total thickness in front of the fiducial volume corresponds to 0.24 radiation lengths.
Table 1 Properties of xenon Atomic number Mass number Density Boiling point Melting point
54 131.29 3:00 g=cm3 165 K 161 K
Emission peak Spectrum width Refractive index W ph for 1 MeV electrons Decay time (recombination) Decay time (fast components) Decay time (slow components) Absorption length Scattering length
178 nm [1,2] 14 nm [1,2] 1.57–1.72 [3–5] 21.6 eV [6] 45 ns [7] 4.2 ns [7] 22 ns [7] 4100 cm 29–50 cm [8–11]
the energy of the mþ ! eþ g signal. The prototype has a fiducial volume of 70 l and is surrounded by 238 PMTs. Fig. 2 shows cross-sectional views of the prototype. The cryostat consists of inner and outer vessels. The volume between the two vessels is evacuated for thermal insulation. Super-insulation layers are installed between the two vessels to suppress heat transfer due to radiation. Xenon is liquefied both by a pulse-tube refrigerator (Iwatani Co. Ltd.) and a liquid nitrogen cooling pipe at the top of the cryostat. After liquefaction, liquid xenon is kept in a stable condition by a refrigerator with temperature control. The entrance window consists of a 0.1 mm thick aluminum plate and a 20 mm thick stainless steel honeycomb. The holder for the front-side PMT is made of G10 and Acrylic plates. The gap around the PMT base circuit is filled with glass beads and epoxy resin to prevent liquid xenon from entering. This makes it possible to keep the gamma-ray detection efficiency flat, independently of the
4. Performance of the detector Two complementary beam tests were performed to study the response of the prototype to gamma rays over a wide energy range. One was carried out by using gamma rays from Laser-Compton Scattering (LCS) and the other by using gamma rays from pion charge exchange (CEX). The LCS beam test was carried out at the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. Gamma rays with a Compton-edge energy of 40 MeV are generated via backward scattering of Nd:YAG pulsed laser photons of 266 nm by 800 MeV electrons stored in a synchrotron. Gamma rays with a Compton-edge energy of 20 and 10 MeV are also available by using 532 and 1064 nm laser photons. The high-intensity gamma beam allows to investigate the position resolution by collimating it using a lead brick with a 2 mm + hole. The CEX beam test was conducted at PSI using a negative pion beam and a liquid hydrogen target. A negative pion beam stopped in the target interacts with protons and produces neutral pions and neutrons. The neutral pion produced in the target has a momentum of 28 MeV=c in the laboratory frame and decays into two ga a rays. Since this process is a two-body decay, energy and opening angle of the two gamma rays are correlated. By tagging back-to-back decays in the laboratory frame through collimation, almost monochromatic gamma rays with an energy of 55 and 83 MeV can be obtained. For selecting back-to-back events, NaI and/or LYSO crystal detectors were located opposite to the prototype with respect to the target. The LYSO crystal detector was used to measure the arrival time of the gamma rays as a reference for evaluating the absolute time resolution of the prototype.
ARTICLE IN PRESS 524
R. Sawada / Nuclear Instruments and Methods in Physics Research A 581 (2007) 522–525
Signal 1m
HV
LN2 Refrigerator Outer vessel
PMTs
241Am
Inner Vessel blue LEDs
LXe
372
y 372
x
z
y x
z
Unit: mm
Front View
Side View
Fig. 2. Liquid xenon gamma-ray detector prototype. PMTs are installed on six faces of a rectangular solid holder.
4.1. Position reconstruction The positions where the gamma rays impinge are reconstructed by calculating a weighted average of the PMT positions on the front face. The PMTs in a limited region around the impinging position are used in this calculation in order to avoid being affected by energy deposit fluctuation, which can produce a contingent tail in the light distribution. Since the size of the optimum region is different depending on the impinging position due to the PMT size, the reconstruction of the impinging position and the determination of the region are optimized in an iteration process including determination of the region’s size. The radius of the region is varied also depending on the reconstructed depth of the first interaction point. In the LCS beam test, the position resolution was investigated at ten different positions with respect to the PMT array, where it was found to be 4–5 mm, independent of the impinging position. There is no clear dependence on gamma-ray energy, indicating that the resolution is not dominated by photon statistics but by energy deposit fluctuation. The depth of the first interaction point is reconstructed from the width of the light distribution observed on the front face. When a gamma ray interacts near the front face, the scintillation photons are concentrated on a few PMTs, while in the case of deep interaction, the photons are observed by more PMTs. 4.2. Energy reconstruction The energy is reconstructed by summing the number of photons observed by all the PMTs. There is a small
Number of photons
120
Entries
100
χ / ndf
3018
Mean
9.183e+05
RMS
3.221e+05 43.33 / 19 1.141+e06 1934
Peak Transition
80
4425
Height
851.1 5.2
128.4
Sigma
1.469e+04
1150
60 40 FWHM = 4.8 %
20
x 103
0 0
200
400
600
800
1000
1200
1400
Number of photons
Fig. 3. Measured spectrum of 55 MeV peak at the CEX beam test.
correlation between the total number of photons and the first interaction depth because the PMT coverage viewed from the gamma-ray interaction point is slightly larger at the center of the detector than that around the edges. This dependence is corrected by a linear function obtained by fitting the data. Fig. 3 shows an energy spectrum obtained at the CEX beam test for 55 MeV gamma-ray events selected by tagging 83 MeV photons in the NaI detector. The low energy tail comes from energy leakage and gamma-ray interactions outside the fiducial volume. Since the energy of the background gamma rays in the mþ ! eþ g search is always lower than that of the signal gamma ray, the energy resolution at the right side of the peak is more important than the tail shape.
ARTICLE IN PRESS R. Sawada / Nuclear Instruments and Methods in Physics Research A 581 (2007) 522–525
Energy resolution (σ) [%]
10
1
102
10 Energy [MeV]
525
The intrinsic time resolution was estimated by taking a difference tleft tright , where tleft ðtright Þ is the evaluated interaction time by using only the left(right) half of the detector. The estimated resolution in this analysis is 56 ps in sigma, which is sufficiently good although this does not include the ambiguity of depth reconstruction. The absolute time resolution was estimated by using txe tLYSO , where txe and tLYSO are the reconstructed interaction time of gamma rays from pion decays at the xenon detector and LYSO crystal detector, respectively. The sigma of the txe tLYSO distribution is 110 ps. By subtracting the resolution of the LYSO detector (64 ps), which was evaluated by an internal calibration of the LYSO detector, the absolute time resolution of the prototype is estimated to be 65 ps.
Fig. 4. Energy resolutions as a function of energies.
5. Summary Fig. 4 shows the energy resolution of the prototype as a function of gamma-ray energy, where the resolution is shown in sigmas of the high edge of the spectrum. 4.3. Time reconstruction The time information from each PMT is measured by leading edge discriminators and TDCs. The time-walk effect is corrected before taking an average over the PMTs. The gamma-ray interaction time is evaluated by taking a weighted average of PMTs on all faces as described in Eq. (1). Since the time resolution of a single PMT is poor when its pulse height is small, only PMTs with large pulse amplitude were used in this evaluation: X pffiffiffiffi t¼ qi t i . (1) There is a correlation between the first interaction depth and the evaluated time because a gamma ray and the scintillation light travel a longer path when the gamma ray interacts at a deeper position in the fiducial volume. Due to this, the reconstructed time is corrected by a linear function of the reconstructed depth. In order to estimate the intrinsic and absolute time resolutions, two kinds of analysis have been performed.
A liquid xenon gamma-ray detector is used in the MEG experiment. Studies of the detector were done by using a prototype detector. The prototype was operated stably and safely. Two kinds of beam tests were carried out to evaluate resolutions. Measured resolutions satisfy the requirements for the MEG experiment to improve the experimental upper limit of the branching ratio of mþ ! eþ g by the two orders of magnitude. References [1] J. Jortner, et al., J. Chem. Phys. 42 (1965) 4250. [2] N. Schwenter, E.-E. Koch, J. Jortner, Electronic Excitations in Condensed Rare Gases, Springer, Berlin, 1985. [3] L.M. Barkov, et al., Nucl. Instr. and Meth. A 379 (1996) 482. [4] V.N. Solovov, V. Chepel, et al., Nucl. Instr. and Meth. A 516 (2004) 462. [5] G.M. Seidel, R.E. Lanou, W. Yao, Nucl. Instr. and Meth. A 489 (2002) 189. [6] T. Doke, et al., Jpn. J. Appl. Phys. 41 (2002) 1538. [7] T. Doke, Portugal Phys. 12 (1981) 9. [8] A. Braem, et al., Nucl. Instr. and Meth. A 320 (1992) 228. [9] V.Y. Chepel, et al., Nucl. Instr. and Meth. A 349 (1994) 500. [10] N. Ishida, et al., Nucl. Instr. and Meth. A 384 (1997) 380. [11] G.M. Seidel, et al., Nucl. Instr. and Meth. A 489 (2002) 189.
Nuclear Instruments and Methods in Physics Research A 581 (2007) 522–525 www.elsevier.com/locate/nima
Liquid xenon gamma ray detector for MEG R. Sawada, on behalf of the MEG xenon detector group University of Tokyo, Tokyo, Japan Available online 8 August 2007
Abstract The MEG experiment searches for the rare muon decay mþ ! eþ g, which is forbidden in the standard model. The detection of this muon decay is a probe for new physics beyond the standard model. In order to improve the sensitivity of the experiment, a liquid xenon detector is used for measuring energy, position and time of gamma rays. The detector consists of 800 l of liquid xenon and 846 photomultiplier tubes. We have performed beam tests and evaluated resolutions by using a prototype. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Mc; 29.40.Vj; 13.35.Bv Keywords: Liquid xenon; Scintillation calorimeter; MEG experiment
1. Introduction MEG is an experiment to search for the lepton-flavor violating muon decay mþ ! eþ g at Paul Scherrer Institut (PSI) in Switzerland. The signature of this muon decay is a back-to-back positron and gamma ray, in time, with a respective energy of 52.8 MeV. In order to reduce background events, precise measurements of energy, emission angle and time of positrons and gamma rays are important. In order to measure gamma rays with such high precision, a large liquid xenon gamma-ray detector is used in this experiment. 2. Apparatus Fig. 1 is a schematic view of the MEG detector. A C-shaped gamma-ray detector is located next to the positron spectrometer. It covers 10% of the solid angle viewed from the muon stopping target. The cryostat of the gamma-ray detector is filled with over 800 l of liquid xenon surrounded by 846 photo-multiplier tubes (PMT). The PMTs are immersed in the liquid xenon to observe scintillation photons without using any transmission window. Corresponding author. Tel.: +81 3 3815 8384; fax: +81 3 3814 8806.
E-mail address: [email protected] 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.08.040
Gamma rays from the target enter the active volume of the detector through an entrance window consisting of an aluminum honeycomb and carbon fiber plates. Then they interact with the liquid xenon and lose energy to excite xenon molecules, resulting in the emission of a large amount of scintillation light. The scintillation light yield is as large as 80% of NaI. The scintillation decay time is five times faster than that of NaI. Because of these properties, the liquid xenon scintillation detector is expected to have excellent performance to measure energy, position, and time of the gamma rays. Table 1 shows the properties of xenon. In order to detect scintillation light efficiently, a new type of PMT was developed in collaboration with Hamamatsu photonics. Since the wavelength of scintillation light from liquid xenon is vacuum ultraviolet, the PMT is equipped with a quartz window which is 80% transparent to the scintillation light. It is designed to work at low temperature with minimum heat generation from its base circuit. 3. The prototype of the LXe detector We constructed a prototype liquid xenon detector in order to gain experience with the detector operation and to investigate its performance. The depth of the prototype along the incident gamma-ray direction is as long as that of the final detector in order to contain whole showers up to
ARTICLE IN PRESS R. Sawada / Nuclear Instruments and Methods in Physics Research A 581 (2007) 522–525
523
Liquid Xenon Scintillation Detector
COBRA Magnet Thin Superconducting Coil
Stopping Target Muon Beam
+
+
Timing Counter
e
e
Drift Chamber
Drift Chamber 1m
Fig. 1. MEG detector.
location of incidence. The total thickness in front of the fiducial volume corresponds to 0.24 radiation lengths.
Table 1 Properties of xenon Atomic number Mass number Density Boiling point Melting point
54 131.29 3:00 g=cm3 165 K 161 K
Emission peak Spectrum width Refractive index W ph for 1 MeV electrons Decay time (recombination) Decay time (fast components) Decay time (slow components) Absorption length Scattering length
178 nm [1,2] 14 nm [1,2] 1.57–1.72 [3–5] 21.6 eV [6] 45 ns [7] 4.2 ns [7] 22 ns [7] 4100 cm 29–50 cm [8–11]
the energy of the mþ ! eþ g signal. The prototype has a fiducial volume of 70 l and is surrounded by 238 PMTs. Fig. 2 shows cross-sectional views of the prototype. The cryostat consists of inner and outer vessels. The volume between the two vessels is evacuated for thermal insulation. Super-insulation layers are installed between the two vessels to suppress heat transfer due to radiation. Xenon is liquefied both by a pulse-tube refrigerator (Iwatani Co. Ltd.) and a liquid nitrogen cooling pipe at the top of the cryostat. After liquefaction, liquid xenon is kept in a stable condition by a refrigerator with temperature control. The entrance window consists of a 0.1 mm thick aluminum plate and a 20 mm thick stainless steel honeycomb. The holder for the front-side PMT is made of G10 and Acrylic plates. The gap around the PMT base circuit is filled with glass beads and epoxy resin to prevent liquid xenon from entering. This makes it possible to keep the gamma-ray detection efficiency flat, independently of the
4. Performance of the detector Two complementary beam tests were performed to study the response of the prototype to gamma rays over a wide energy range. One was carried out by using gamma rays from Laser-Compton Scattering (LCS) and the other by using gamma rays from pion charge exchange (CEX). The LCS beam test was carried out at the National Institute of Advanced Industrial Science and Technology (AIST) in Japan. Gamma rays with a Compton-edge energy of 40 MeV are generated via backward scattering of Nd:YAG pulsed laser photons of 266 nm by 800 MeV electrons stored in a synchrotron. Gamma rays with a Compton-edge energy of 20 and 10 MeV are also available by using 532 and 1064 nm laser photons. The high-intensity gamma beam allows to investigate the position resolution by collimating it using a lead brick with a 2 mm + hole. The CEX beam test was conducted at PSI using a negative pion beam and a liquid hydrogen target. A negative pion beam stopped in the target interacts with protons and produces neutral pions and neutrons. The neutral pion produced in the target has a momentum of 28 MeV=c in the laboratory frame and decays into two ga a rays. Since this process is a two-body decay, energy and opening angle of the two gamma rays are correlated. By tagging back-to-back decays in the laboratory frame through collimation, almost monochromatic gamma rays with an energy of 55 and 83 MeV can be obtained. For selecting back-to-back events, NaI and/or LYSO crystal detectors were located opposite to the prototype with respect to the target. The LYSO crystal detector was used to measure the arrival time of the gamma rays as a reference for evaluating the absolute time resolution of the prototype.
ARTICLE IN PRESS 524
R. Sawada / Nuclear Instruments and Methods in Physics Research A 581 (2007) 522–525
Signal 1m
HV
LN2 Refrigerator Outer vessel
PMTs
241Am
Inner Vessel blue LEDs
LXe
372
y 372
x
z
y x
z
Unit: mm
Front View
Side View
Fig. 2. Liquid xenon gamma-ray detector prototype. PMTs are installed on six faces of a rectangular solid holder.
4.1. Position reconstruction The positions where the gamma rays impinge are reconstructed by calculating a weighted average of the PMT positions on the front face. The PMTs in a limited region around the impinging position are used in this calculation in order to avoid being affected by energy deposit fluctuation, which can produce a contingent tail in the light distribution. Since the size of the optimum region is different depending on the impinging position due to the PMT size, the reconstruction of the impinging position and the determination of the region are optimized in an iteration process including determination of the region’s size. The radius of the region is varied also depending on the reconstructed depth of the first interaction point. In the LCS beam test, the position resolution was investigated at ten different positions with respect to the PMT array, where it was found to be 4–5 mm, independent of the impinging position. There is no clear dependence on gamma-ray energy, indicating that the resolution is not dominated by photon statistics but by energy deposit fluctuation. The depth of the first interaction point is reconstructed from the width of the light distribution observed on the front face. When a gamma ray interacts near the front face, the scintillation photons are concentrated on a few PMTs, while in the case of deep interaction, the photons are observed by more PMTs. 4.2. Energy reconstruction The energy is reconstructed by summing the number of photons observed by all the PMTs. There is a small
Number of photons
120
Entries
100
χ / ndf
3018
Mean
9.183e+05
RMS
3.221e+05 43.33 / 19 1.141+e06 1934
Peak Transition
80
4425
Height
851.1 5.2
128.4
Sigma
1.469e+04
1150
60 40 FWHM = 4.8 %
20
x 103
0 0
200
400
600
800
1000
1200
1400
Number of photons
Fig. 3. Measured spectrum of 55 MeV peak at the CEX beam test.
correlation between the total number of photons and the first interaction depth because the PMT coverage viewed from the gamma-ray interaction point is slightly larger at the center of the detector than that around the edges. This dependence is corrected by a linear function obtained by fitting the data. Fig. 3 shows an energy spectrum obtained at the CEX beam test for 55 MeV gamma-ray events selected by tagging 83 MeV photons in the NaI detector. The low energy tail comes from energy leakage and gamma-ray interactions outside the fiducial volume. Since the energy of the background gamma rays in the mþ ! eþ g search is always lower than that of the signal gamma ray, the energy resolution at the right side of the peak is more important than the tail shape.
ARTICLE IN PRESS R. Sawada / Nuclear Instruments and Methods in Physics Research A 581 (2007) 522–525
Energy resolution (σ) [%]
10
1
102
10 Energy [MeV]
525
The intrinsic time resolution was estimated by taking a difference tleft tright , where tleft ðtright Þ is the evaluated interaction time by using only the left(right) half of the detector. The estimated resolution in this analysis is 56 ps in sigma, which is sufficiently good although this does not include the ambiguity of depth reconstruction. The absolute time resolution was estimated by using txe tLYSO , where txe and tLYSO are the reconstructed interaction time of gamma rays from pion decays at the xenon detector and LYSO crystal detector, respectively. The sigma of the txe tLYSO distribution is 110 ps. By subtracting the resolution of the LYSO detector (64 ps), which was evaluated by an internal calibration of the LYSO detector, the absolute time resolution of the prototype is estimated to be 65 ps.
Fig. 4. Energy resolutions as a function of energies.
5. Summary Fig. 4 shows the energy resolution of the prototype as a function of gamma-ray energy, where the resolution is shown in sigmas of the high edge of the spectrum. 4.3. Time reconstruction The time information from each PMT is measured by leading edge discriminators and TDCs. The time-walk effect is corrected before taking an average over the PMTs. The gamma-ray interaction time is evaluated by taking a weighted average of PMTs on all faces as described in Eq. (1). Since the time resolution of a single PMT is poor when its pulse height is small, only PMTs with large pulse amplitude were used in this evaluation: X pffiffiffiffi t¼ qi t i . (1) There is a correlation between the first interaction depth and the evaluated time because a gamma ray and the scintillation light travel a longer path when the gamma ray interacts at a deeper position in the fiducial volume. Due to this, the reconstructed time is corrected by a linear function of the reconstructed depth. In order to estimate the intrinsic and absolute time resolutions, two kinds of analysis have been performed.
A liquid xenon gamma-ray detector is used in the MEG experiment. Studies of the detector were done by using a prototype detector. The prototype was operated stably and safely. Two kinds of beam tests were carried out to evaluate resolutions. Measured resolutions satisfy the requirements for the MEG experiment to improve the experimental upper limit of the branching ratio of mþ ! eþ g by the two orders of magnitude. References [1] J. Jortner, et al., J. Chem. Phys. 42 (1965) 4250. [2] N. Schwenter, E.-E. Koch, J. Jortner, Electronic Excitations in Condensed Rare Gases, Springer, Berlin, 1985. [3] L.M. Barkov, et al., Nucl. Instr. and Meth. A 379 (1996) 482. [4] V.N. Solovov, V. Chepel, et al., Nucl. Instr. and Meth. A 516 (2004) 462. [5] G.M. Seidel, R.E. Lanou, W. Yao, Nucl. Instr. and Meth. A 489 (2002) 189. [6] T. Doke, et al., Jpn. J. Appl. Phys. 41 (2002) 1538. [7] T. Doke, Portugal Phys. 12 (1981) 9. [8] A. Braem, et al., Nucl. Instr. and Meth. A 320 (1992) 228. [9] V.Y. Chepel, et al., Nucl. Instr. and Meth. A 349 (1994) 500. [10] N. Ishida, et al., Nucl. Instr. and Meth. A 384 (1997) 380. [11] G.M. Seidel, et al., Nucl. Instr. and Meth. A 489 (2002) 189.