- Email: [email protected]
Upgrade of the MEG liquid xenon calorimeter with VUV-light sensitive large area SiPMs
Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Upgrade of the MEG liquid xenon calorimeter with VUV-light sensitive large area SiPMs K. Ieki The University of Tokyo, Tokyo, Japan
art ic l e i nf o
a b s t r a c t
Available online 23 November 2015
The MEG experiment searches for the muon lepton flavor violating decay, μ þ -e þ γ. An upgrade of the experiment is ongoing, aiming at reaching a sensitivity of Brðμ þ -e þ γÞ ¼ 4 10 14 , an order of magnitude better than the sensitivity of the current MEG. To achieve this goal, all of the detectors are being upgraded. In MEG, the energy, position and timing of the gamma ray were measured by a liquid Xe calorimeter, which consists of 900 l of liquid Xe and 846 2-in. round-shaped photo-multiplier tubes (PMTs). In the upgrade, the granularity at the gamma ray incident face will be improved by replacing 216 PMTs with 4092 SiPMs (MPPCs) with an active area of 12 12 mm2 each. The energy resolution for the gamma ray is expected to improve by a factor of 2, because the efficiency to collect scintillation light will become more uniform. The position resolution is also expected to improve by a factor of 2. In collaboration with Hamamatsu Photonics K.K., we have successfully developed a high performance MPPC for our detector. It has excellent photon detection efficiency for the liquid xenon scintillation light in VUV range. The size of the chips is large so that it can cover large area with a manageable number of readout channels. The characteristics of the MPPCs are being tested in liquid Xe, and also at the room temperature. The results of the tests will be presented, together with the expected performance of the upgraded detector. & 2015 Elsevier B.V. All rights reserved.
Keywords: Calorimeter Photon sensor Liquid xenon Muon physics
1. Introduction Lepton flavor violating decay μ þ -e þ γ has been sought for by a lot of experiments in the past. This decay is basically forbidden in the standard model (SM). Even if we take into account the effect of neutrino oscillations, the branching ratio is expected to be too small ( 10 54) to be observed. On the other hand, the branching ratio becomes much larger in some of the well-motivated models of new physics. For example, SUSY see–saw models predict Brðμ þ -e þ γÞ 10 14 . Observation of this decay will be, therefore, a clear evidence of physics beyond the standard model. The upper limit of this decay is Br ðμ þ -e þ γÞ o 5:7 10 13 at 90% C.L., given by the MEG experiment at the Paul Scherrer Institute [1]. MEG II is an upgrade of the MEG experiment, expected to start data taking from 2016. The target sensitivity in MEG II is 4 10 14 . In MEG II, μ þ beam stopping rate at target will increase by more than a factor of 2. The main background for the μ þ -e þ γ signal is the accidental coincidence of e þ and γ-ray, while the signal is the back-to-back decay of e þ and γ with same energy, same timing. The accidental background rate will be larger in MEG II, because of the higher μ þ stopping rate. Furthermore, the E-mail address: [email protected] http://dx.doi.org/10.1016/j.nima.2015.11.047 0168-9002/& 2015 Elsevier B.V. All rights reserved.
detection efficiency will be also improved by a factor of 2. Therefore, in order to distinguish the signal from accidental background, position, energy and timing resolution of the detectors will be improved by a factor of 2. In MEG II (and also in MEG), e þ and γ-ray are detected by a e þ spectrometer and a 900 l liquid xenon (LXe) calorimeter (Fig. 1). A LXe detector will be upgraded by replacing the photon sensors for the detection of scintillation light. A high performance MPPC (Multi-Pixel Photon Counter, a SiPM device) was newly developed for this upgrade. The overview of the LXe detector upgrade and the development of new MPPC is explained in Section 2. Performance test of the MPPC is described in Section 3. The expected performance of the upgraded detector is studied in the simulation, as described in Section 4.
2. Upgrade of the LXe calorimeter In MEG, the scintillation light from LXe was detected by 2 in. round-shaped photo-multipliers (PMTs). In total, 846 PMTs were installed surrounding the LXe. The resolution of the MEG LXe detector was limited by the non-uniform coverage of the photon sensors. Photon collection efficiency was different depending on the relative position of the γ-ray conversion point and the PMTs
K. Ieki / Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
(Fig. 2), which cannot be perfectly corrected in analysis because of the limited position resolution and the shower fluctuations. Therefore, in the upgrade 216 PMTs at the γ-ray incident wall will be replaced by 12 12 mm2 MPPCs (Fig. 3). The total number of MPPCs will be 4092. Thanks to the higher granularity and uniformity of the sensor coverage, energy and position resolution of the γ-ray are expected to improve significantly, as we describe in Section 4. In addition, the layout of the PMTs will be slightly improved (Fig. 4). The γ-ray entrance face will be extended along the beam direction by 10% at each side, to reduce the energy leakage near the walls. The orientation of the PMTs in the upstream and downstream sides of
687
the beam will also change, in order to minimize the effect of shower fluctuation near the walls. Development of a MPPC is a key point of this upgrade. We have successfully developed high performance MPPC for our detector, in collaboration with Hamamatsu Photonics K.K. Basic properties of the MPPC and the results of the performance tests are described in the following sections. 2.1. Development of VUV-sensitive MPPC Requirements for the MPPC are as follows: (1) It must be sensitive to the vacuum ultraviolet (VUV) scintillation light from LXe (wavelength 178 nm). (2) Active area must be large enough, in order to cover the inner face with reasonable number of channels. (3) Single photon resolution must be good. Fig. 5 shows a schematic view of the new MPPC. Standard type of MPPCs are not sensitive to VUV-light, because the VUV-light is absorbed on surface materials before reaching the sensitive layer, due to very short attenuation length of VUV-light in silicon ( 5 nm). In order to improve the photon detection efficiency (PDE) for the VUV-light, the protection coating is removed. A VUVtransparent quartz window is used for protection. Optical matching between LXe and sensor surface is optimized, and the top contact layer is thinned down.
Fig. 1. Overview of the MEG detectors. C-shaped object in the middle is the LXe γ-ray detector.
Fig. 4. Layout of the PMTs in MEG (left) and MEG II (right).
Fig. 2. Photon collection efficiency vs. γ-ray conversion depth in MEG LXe detector.
Fig. 5. VUV-sensitive MPPC S10943-3186(X), developed in collaboration with Hamamatsu Photonics K.K.
Fig. 3. Replacement of the photon sensors at the γ-ray incident wall, from 2 in. PMTs (left) to 12 12 mm2 MPPCs (right).
688
K. Ieki / Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
220 200 180 160 140 120 100 80
Fig. 6. PCB and PCB-based feed-through for the readout of MPPC signal. “Co-axial like” structure is implemented as shown in the right image.
60 40 20 0
−
Charge Fig. 8. Example of the integrated charge distribution for LED light.
Fig. 9. Setup for the mass test at room temperature. Fig. 7. Setup for the small sample test.
As the active area become large, capacitance of the chips become large. This corresponds to a long tail in the signal, which could be a problem in case of a high rate environment such as that expected for MEG II. This problem is solved by having four small individual chips instead of single large chip, and connecting them in series. The total capacitance is kept small by the series connection. Each of the chips has an active area of 6 6 mm2, with a 50 μm pixel pitch. Metal quench resistors are employed because of their smaller temperature dependence. 2.2. Signal transmission Signal from 4092 MPPCs must be transmitted outside of the LXe cryostat without being affected by noise or distortion. Fig. 6 shows the PCB (Printed Circuit Board) and feed-through for the MPPC signal transmission. The MPPCs will be mounted on PCBs, where 4 chips of each MPPC are connected in series. Then, the signal from the PCBs are transmitted by co-axial cables to the feedthrough. A PCB-based feed-through transmits the signal from inner side of the cryostat to the outside readout electronics. PCBs and PCB-based feed-through have “co-axial like” layer structure, to realize good shielding, high bandwidth and small crosstalk ( o 0:3%).
3.1. Small sample tests Detailed performance tests were done with a small number of MPPCs, using a 2 l LXe cryostat. Schematic view of the setup is shown in Fig. 7. This setup was submerged in LXe. Basic properties of the MPPCs, such as gain, breakdown voltage (Vbd), crosstalk þ afterpulse (CTAP) probability and PDE were measured in this setup. Gain is calculated from the integrated charge of a single photoelectron (p.e.) signal using LED. Very high gain of 8 105 is obtained at over-voltage ðΔVÞ7 V. Dark noise rate is very low ( 1 Hz/mm3) at LXe temperature. Thanks to good cell-to-cell gain uniformity, integrated charge of 1 p.e., 2 p.e., … signals were clearly distinguishable from each other, even after connecting four chips in series (Fig. 8). CTAP probability at ΔV ¼ 7 V was measured to be 25%. Signal decay time was measured to be short ( 30 ns), thanks to the series connection of chips. PDE for the VUV-light is measured by comparing the ratio between observed light yield and expected light yield for the scintillation signal from an 241Am α source. PDE at ΔV ¼ 7 V was measured to be 16 27% (large uncertainty comes from the geometry of the setup), assuming that averaged energy expended per scintillation photon is 19.6 eV [2]. The important features such as VUV sensitivity and short signal decay time are confirmed in the tests. The energy resolution was also measured with this setup using the α source. Even though the number of scintillation photons for α signal is much smaller than that of the signal γ-ray in μ þ -e þ γ decay, 1% level energy resolution was already obtained with this setup.
3. Performance tests 3.2. Mass test at room temperature Several different types of performance tests have been done for the MPPCs and readout system. Detailed performance of the MPPCs are measured in a small setup. Mass tests have been done at room temperature, and also in LXe.
Mass test of 600 prototype MPPCs was performed at room temperature. Prototype MPPCs are basically same as the final version, except that the feature of crosstalk suppression is not
K. Ieki / Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
implemented. Gain, Vbd, CTAP probability and noise rate were measured for the four individual chips of all MPPCs. Temperature was kept at 20 °C using a temperature controlled chamber. A special PCB was used for this measurement, which contains relay switches to change readout chips on MPPC (Fig. 9). With this PCB, 16 MPPCs are measured at once. Measurement of all chips finished in 2 weeks. None of the chips were found to be dead. The setup is being reused for the mass test of final 4000 MPPCs. 3.3. Mass test in liquid xenon Mass test was also done in LXe for the prototype MPPCs which were tested at room temperature. Fig. 10 shows the setup for this test. MPPCs were mounted on PCBs, and the signals were transmitted outside of the cryostat using the PCB-based feed-through. The goal of this test was to test all of the readout system in LXe. Response to the α scintillation signal and LED light was measured for all of the MPPCs, to measure their basic properties as we did in the small sample tests. No problem was found in the test, except for a small number of bad channels ( 5%). Most of them were caused by bad connections of cables or MPPCs at the PCBs and feed-through. This is due to problems and difficulties in the assembly process, which required to connect the cables and MPPCs in a limited space. The design of the cable connectors, PCB connectors and feed-through connectors will be improved for the final detector. For example, the spacing between the connectors will be larger on PCBs, and the
Fig. 10. Setup for the mass test in liquid Xe.
689
cables will be directly soldered on the feed-through boards, instead of using connectors. Thanks to these improvements, the problems are not expected to happen again for the final detector.
4. Expected improvement of the upgrade A Monte-Carlo simulation study was performed to estimate the performance of the upgraded LXe detector. Measured properties of MPPC, such as gain, PDE, CTAP and waveform of 1 p.e. signal were implemented in the simulation. Reconstruction algorithms were unchanged from MEG ones, but some parameters were optimized to exploit the advantages of MPPC. Fig. 11 shows the reconstructed energy distribution for shallow events (γ-ray conversion depth o 2 cm) and deep events (depth Z 2 cm). The broken lines (red) are the result of PMT case, while the solid lines (blue) are the result of MPPC þPMT case. The energy resolution is improved from 2.4% to 1.1% for shallow events, and from 1.7% to 1.0% for deep events. Fig. 12 shows the position resolution vs.γ-ray conversion depth, again for PMT case and MPPC þPMT case. There is a significant improvement in the resolution for shallow events. Overall, the resolution is improved by a factor of 2. Timing resolution is estimated to be 67 ps for PMT case and 60 ps for MPPC þPMT case. The detection efficiency of the γ-ray is
Fig. 12. Expected position resolution (beam direction) vs. γ-ray conversion depth before (red, open circle) and after (blue, filled circle) the upgrade. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
Fig. 11. Reconstructed energy distribution in the simulation, before (red, broken line) and after (blue, solid line) the upgrade, for γ-ray conversion depth o 2 cm (left figure) and depth Z 2 cm (right figure). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
690
K. Ieki / Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
also expected to improve from 63% to 69%, because the amount of materials between μ stopping target and LXe will be reduced by replacing PMTs to MPPCs.
5. Conclusion MEG II is the upgrade of the MEG experiment aiming at searching for the μ þ -e þ γ decay with a target sensitivity of Br ðμ þ -e þ γÞ ¼ 4 10 14 . All of the detectors will be upgraded in MEG II. The LXe γ-ray detector is going to be upgraded by replacing 216 PMTs in the inner face to 4092 MPPCs. VUV-light sensitive 12 12 mm2 large area MPPCs were newly developed in collaboration with Hamamatsu Photonics K.K. Performance tests in LXe confirm high sensitivity to VUV scintillation light (PDE 16 27%), and short signal tail ( 30 ns) thanks to a series connection of chips. Mass test of 600 prototype MPPCs was successfully performed at room and LXe temperature. The PCB and PCB-based feed-through which contain “co-axial like” structure were newly developed for the signal transmission, and they also were tested in the mass test in LXe. They worked fine in LXe, and
their design was improved so that the misassembly problem will not happen for the final detector. Simulation studies were performed using the measured properties of MPPCs. Energy and position resolutions are expected to improve by a factor of 2 with this upgrade and γ-ray detection efficiency is also expected to improve by 10%.
Acknowledgments This work was supported by Grant-in-Aid for JSPS Fellows, and JSPS KAKENHI Grant numbers 22000004 and 26000004.
References [1] J. Adam, et al., Physical Review Letters 110 (2013) 201801. [2] T. Doke, K. Masuda, Nuclear Instruments and Methods in Physics Research Section A 420 (1999) 62.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Upgrade of the MEG liquid xenon calorimeter with VUV-light sensitive large area SiPMs K. Ieki The University of Tokyo, Tokyo, Japan
art ic l e i nf o
a b s t r a c t
Available online 23 November 2015
The MEG experiment searches for the muon lepton flavor violating decay, μ þ -e þ γ. An upgrade of the experiment is ongoing, aiming at reaching a sensitivity of Brðμ þ -e þ γÞ ¼ 4 10 14 , an order of magnitude better than the sensitivity of the current MEG. To achieve this goal, all of the detectors are being upgraded. In MEG, the energy, position and timing of the gamma ray were measured by a liquid Xe calorimeter, which consists of 900 l of liquid Xe and 846 2-in. round-shaped photo-multiplier tubes (PMTs). In the upgrade, the granularity at the gamma ray incident face will be improved by replacing 216 PMTs with 4092 SiPMs (MPPCs) with an active area of 12 12 mm2 each. The energy resolution for the gamma ray is expected to improve by a factor of 2, because the efficiency to collect scintillation light will become more uniform. The position resolution is also expected to improve by a factor of 2. In collaboration with Hamamatsu Photonics K.K., we have successfully developed a high performance MPPC for our detector. It has excellent photon detection efficiency for the liquid xenon scintillation light in VUV range. The size of the chips is large so that it can cover large area with a manageable number of readout channels. The characteristics of the MPPCs are being tested in liquid Xe, and also at the room temperature. The results of the tests will be presented, together with the expected performance of the upgraded detector. & 2015 Elsevier B.V. All rights reserved.
Keywords: Calorimeter Photon sensor Liquid xenon Muon physics
1. Introduction Lepton flavor violating decay μ þ -e þ γ has been sought for by a lot of experiments in the past. This decay is basically forbidden in the standard model (SM). Even if we take into account the effect of neutrino oscillations, the branching ratio is expected to be too small ( 10 54) to be observed. On the other hand, the branching ratio becomes much larger in some of the well-motivated models of new physics. For example, SUSY see–saw models predict Brðμ þ -e þ γÞ 10 14 . Observation of this decay will be, therefore, a clear evidence of physics beyond the standard model. The upper limit of this decay is Br ðμ þ -e þ γÞ o 5:7 10 13 at 90% C.L., given by the MEG experiment at the Paul Scherrer Institute [1]. MEG II is an upgrade of the MEG experiment, expected to start data taking from 2016. The target sensitivity in MEG II is 4 10 14 . In MEG II, μ þ beam stopping rate at target will increase by more than a factor of 2. The main background for the μ þ -e þ γ signal is the accidental coincidence of e þ and γ-ray, while the signal is the back-to-back decay of e þ and γ with same energy, same timing. The accidental background rate will be larger in MEG II, because of the higher μ þ stopping rate. Furthermore, the E-mail address: [email protected] http://dx.doi.org/10.1016/j.nima.2015.11.047 0168-9002/& 2015 Elsevier B.V. All rights reserved.
detection efficiency will be also improved by a factor of 2. Therefore, in order to distinguish the signal from accidental background, position, energy and timing resolution of the detectors will be improved by a factor of 2. In MEG II (and also in MEG), e þ and γ-ray are detected by a e þ spectrometer and a 900 l liquid xenon (LXe) calorimeter (Fig. 1). A LXe detector will be upgraded by replacing the photon sensors for the detection of scintillation light. A high performance MPPC (Multi-Pixel Photon Counter, a SiPM device) was newly developed for this upgrade. The overview of the LXe detector upgrade and the development of new MPPC is explained in Section 2. Performance test of the MPPC is described in Section 3. The expected performance of the upgraded detector is studied in the simulation, as described in Section 4.
2. Upgrade of the LXe calorimeter In MEG, the scintillation light from LXe was detected by 2 in. round-shaped photo-multipliers (PMTs). In total, 846 PMTs were installed surrounding the LXe. The resolution of the MEG LXe detector was limited by the non-uniform coverage of the photon sensors. Photon collection efficiency was different depending on the relative position of the γ-ray conversion point and the PMTs
K. Ieki / Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
(Fig. 2), which cannot be perfectly corrected in analysis because of the limited position resolution and the shower fluctuations. Therefore, in the upgrade 216 PMTs at the γ-ray incident wall will be replaced by 12 12 mm2 MPPCs (Fig. 3). The total number of MPPCs will be 4092. Thanks to the higher granularity and uniformity of the sensor coverage, energy and position resolution of the γ-ray are expected to improve significantly, as we describe in Section 4. In addition, the layout of the PMTs will be slightly improved (Fig. 4). The γ-ray entrance face will be extended along the beam direction by 10% at each side, to reduce the energy leakage near the walls. The orientation of the PMTs in the upstream and downstream sides of
687
the beam will also change, in order to minimize the effect of shower fluctuation near the walls. Development of a MPPC is a key point of this upgrade. We have successfully developed high performance MPPC for our detector, in collaboration with Hamamatsu Photonics K.K. Basic properties of the MPPC and the results of the performance tests are described in the following sections. 2.1. Development of VUV-sensitive MPPC Requirements for the MPPC are as follows: (1) It must be sensitive to the vacuum ultraviolet (VUV) scintillation light from LXe (wavelength 178 nm). (2) Active area must be large enough, in order to cover the inner face with reasonable number of channels. (3) Single photon resolution must be good. Fig. 5 shows a schematic view of the new MPPC. Standard type of MPPCs are not sensitive to VUV-light, because the VUV-light is absorbed on surface materials before reaching the sensitive layer, due to very short attenuation length of VUV-light in silicon ( 5 nm). In order to improve the photon detection efficiency (PDE) for the VUV-light, the protection coating is removed. A VUVtransparent quartz window is used for protection. Optical matching between LXe and sensor surface is optimized, and the top contact layer is thinned down.
Fig. 1. Overview of the MEG detectors. C-shaped object in the middle is the LXe γ-ray detector.
Fig. 4. Layout of the PMTs in MEG (left) and MEG II (right).
Fig. 2. Photon collection efficiency vs. γ-ray conversion depth in MEG LXe detector.
Fig. 5. VUV-sensitive MPPC S10943-3186(X), developed in collaboration with Hamamatsu Photonics K.K.
Fig. 3. Replacement of the photon sensors at the γ-ray incident wall, from 2 in. PMTs (left) to 12 12 mm2 MPPCs (right).
688
K. Ieki / Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
220 200 180 160 140 120 100 80
Fig. 6. PCB and PCB-based feed-through for the readout of MPPC signal. “Co-axial like” structure is implemented as shown in the right image.
60 40 20 0
−
Charge Fig. 8. Example of the integrated charge distribution for LED light.
Fig. 9. Setup for the mass test at room temperature. Fig. 7. Setup for the small sample test.
As the active area become large, capacitance of the chips become large. This corresponds to a long tail in the signal, which could be a problem in case of a high rate environment such as that expected for MEG II. This problem is solved by having four small individual chips instead of single large chip, and connecting them in series. The total capacitance is kept small by the series connection. Each of the chips has an active area of 6 6 mm2, with a 50 μm pixel pitch. Metal quench resistors are employed because of their smaller temperature dependence. 2.2. Signal transmission Signal from 4092 MPPCs must be transmitted outside of the LXe cryostat without being affected by noise or distortion. Fig. 6 shows the PCB (Printed Circuit Board) and feed-through for the MPPC signal transmission. The MPPCs will be mounted on PCBs, where 4 chips of each MPPC are connected in series. Then, the signal from the PCBs are transmitted by co-axial cables to the feedthrough. A PCB-based feed-through transmits the signal from inner side of the cryostat to the outside readout electronics. PCBs and PCB-based feed-through have “co-axial like” layer structure, to realize good shielding, high bandwidth and small crosstalk ( o 0:3%).
3.1. Small sample tests Detailed performance tests were done with a small number of MPPCs, using a 2 l LXe cryostat. Schematic view of the setup is shown in Fig. 7. This setup was submerged in LXe. Basic properties of the MPPCs, such as gain, breakdown voltage (Vbd), crosstalk þ afterpulse (CTAP) probability and PDE were measured in this setup. Gain is calculated from the integrated charge of a single photoelectron (p.e.) signal using LED. Very high gain of 8 105 is obtained at over-voltage ðΔVÞ7 V. Dark noise rate is very low ( 1 Hz/mm3) at LXe temperature. Thanks to good cell-to-cell gain uniformity, integrated charge of 1 p.e., 2 p.e., … signals were clearly distinguishable from each other, even after connecting four chips in series (Fig. 8). CTAP probability at ΔV ¼ 7 V was measured to be 25%. Signal decay time was measured to be short ( 30 ns), thanks to the series connection of chips. PDE for the VUV-light is measured by comparing the ratio between observed light yield and expected light yield for the scintillation signal from an 241Am α source. PDE at ΔV ¼ 7 V was measured to be 16 27% (large uncertainty comes from the geometry of the setup), assuming that averaged energy expended per scintillation photon is 19.6 eV [2]. The important features such as VUV sensitivity and short signal decay time are confirmed in the tests. The energy resolution was also measured with this setup using the α source. Even though the number of scintillation photons for α signal is much smaller than that of the signal γ-ray in μ þ -e þ γ decay, 1% level energy resolution was already obtained with this setup.
3. Performance tests 3.2. Mass test at room temperature Several different types of performance tests have been done for the MPPCs and readout system. Detailed performance of the MPPCs are measured in a small setup. Mass tests have been done at room temperature, and also in LXe.
Mass test of 600 prototype MPPCs was performed at room temperature. Prototype MPPCs are basically same as the final version, except that the feature of crosstalk suppression is not
K. Ieki / Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
implemented. Gain, Vbd, CTAP probability and noise rate were measured for the four individual chips of all MPPCs. Temperature was kept at 20 °C using a temperature controlled chamber. A special PCB was used for this measurement, which contains relay switches to change readout chips on MPPC (Fig. 9). With this PCB, 16 MPPCs are measured at once. Measurement of all chips finished in 2 weeks. None of the chips were found to be dead. The setup is being reused for the mass test of final 4000 MPPCs. 3.3. Mass test in liquid xenon Mass test was also done in LXe for the prototype MPPCs which were tested at room temperature. Fig. 10 shows the setup for this test. MPPCs were mounted on PCBs, and the signals were transmitted outside of the cryostat using the PCB-based feed-through. The goal of this test was to test all of the readout system in LXe. Response to the α scintillation signal and LED light was measured for all of the MPPCs, to measure their basic properties as we did in the small sample tests. No problem was found in the test, except for a small number of bad channels ( 5%). Most of them were caused by bad connections of cables or MPPCs at the PCBs and feed-through. This is due to problems and difficulties in the assembly process, which required to connect the cables and MPPCs in a limited space. The design of the cable connectors, PCB connectors and feed-through connectors will be improved for the final detector. For example, the spacing between the connectors will be larger on PCBs, and the
Fig. 10. Setup for the mass test in liquid Xe.
689
cables will be directly soldered on the feed-through boards, instead of using connectors. Thanks to these improvements, the problems are not expected to happen again for the final detector.
4. Expected improvement of the upgrade A Monte-Carlo simulation study was performed to estimate the performance of the upgraded LXe detector. Measured properties of MPPC, such as gain, PDE, CTAP and waveform of 1 p.e. signal were implemented in the simulation. Reconstruction algorithms were unchanged from MEG ones, but some parameters were optimized to exploit the advantages of MPPC. Fig. 11 shows the reconstructed energy distribution for shallow events (γ-ray conversion depth o 2 cm) and deep events (depth Z 2 cm). The broken lines (red) are the result of PMT case, while the solid lines (blue) are the result of MPPC þPMT case. The energy resolution is improved from 2.4% to 1.1% for shallow events, and from 1.7% to 1.0% for deep events. Fig. 12 shows the position resolution vs.γ-ray conversion depth, again for PMT case and MPPC þPMT case. There is a significant improvement in the resolution for shallow events. Overall, the resolution is improved by a factor of 2. Timing resolution is estimated to be 67 ps for PMT case and 60 ps for MPPC þPMT case. The detection efficiency of the γ-ray is
Fig. 12. Expected position resolution (beam direction) vs. γ-ray conversion depth before (red, open circle) and after (blue, filled circle) the upgrade. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
Fig. 11. Reconstructed energy distribution in the simulation, before (red, broken line) and after (blue, solid line) the upgrade, for γ-ray conversion depth o 2 cm (left figure) and depth Z 2 cm (right figure). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)
690
K. Ieki / Nuclear Instruments and Methods in Physics Research A 824 (2016) 686–690
also expected to improve from 63% to 69%, because the amount of materials between μ stopping target and LXe will be reduced by replacing PMTs to MPPCs.
5. Conclusion MEG II is the upgrade of the MEG experiment aiming at searching for the μ þ -e þ γ decay with a target sensitivity of Br ðμ þ -e þ γÞ ¼ 4 10 14 . All of the detectors will be upgraded in MEG II. The LXe γ-ray detector is going to be upgraded by replacing 216 PMTs in the inner face to 4092 MPPCs. VUV-light sensitive 12 12 mm2 large area MPPCs were newly developed in collaboration with Hamamatsu Photonics K.K. Performance tests in LXe confirm high sensitivity to VUV scintillation light (PDE 16 27%), and short signal tail ( 30 ns) thanks to a series connection of chips. Mass test of 600 prototype MPPCs was successfully performed at room and LXe temperature. The PCB and PCB-based feed-through which contain “co-axial like” structure were newly developed for the signal transmission, and they also were tested in the mass test in LXe. They worked fine in LXe, and
their design was improved so that the misassembly problem will not happen for the final detector. Simulation studies were performed using the measured properties of MPPCs. Energy and position resolutions are expected to improve by a factor of 2 with this upgrade and γ-ray detection efficiency is also expected to improve by 10%.
Acknowledgments This work was supported by Grant-in-Aid for JSPS Fellows, and JSPS KAKENHI Grant numbers 22000004 and 26000004.
References [1] J. Adam, et al., Physical Review Letters 110 (2013) 201801. [2] T. Doke, K. Masuda, Nuclear Instruments and Methods in Physics Research Section A 420 (1999) 62.