- Email: [email protected]
A multi-PMT photodetector system for the Hyper-Kamiokande experiment
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
A multi-PMT photodetector system for the Hyper-Kamiokande experiment Gianfranca De Rosa, on behalf of Hyper-Kamiokande Proto-Collaboration Universitá ‘‘Federico II’’ and INFN, Naples, Italy
ARTICLE
INFO
Keywords: Neutrino Photosensors PMTs Water Cherenkov
ABSTRACT Hyper-Kamiokande (Hyper-K) is the next generation of large water Cherenkov neutrino detector with a fiducial volume which will be approximately 10 times larger than its precursor. Its broad physics program includes neutrinos from astronomical sources, nucleon decay, with the main focus the determination of leptonic CP violation. To detect the weak Cherenkov light generated by neutrino interactions or proton decay, the employment of the multi-PMT concept, first introduced in the KM3NeT detector, is considered as a possible solution. A multi-PMT Optical Module – a pressure vessel instrumented with multiple small diameter photosensors, read-out electronics and all required power supplies (incl. high voltage) – offers several advantages, such as weaker sensitivity to Earth’s magnetic field, increased granularity and directional information with an almost isotropic field of view. In this contribution the development of a multi-PMT module for Hyper-K is discussed.
1. Introduction Hyper-Kamiokande (Hyper-K) is a next generation underground water Cherenkov detector, which will be based in Japan [1]. Hyper-K is a multipurpose experiment for the observation of atmospheric, solar and accelerator neutrino oscillations, for neutrino astrophysics, proton decay and physics beyond the Standard Model. In particular, it will observe neutrinos produced at JPARC accelerator complex, about 300 km far, to investigate leptonic CP violation. Hyper-K builds on the successful experience gained in the currently operating Super-Kamiokande and T2K experiments, with a fiducial volume which will be approximately 10 times larger than its precursor for increased statistics, with improved photo-sensors for better efficiency, with higher intensity neutrino beam and updated/new near detector for accelerator neutrino study. Hyper-K consists of 2 identical detectors with staging construction. Each tank has a cylindrical shape with a fiducial mass of 187 ktons. The inner part of the tank, the inner detector, is observed with 40 000 50-cm PMTs with a photocathode coverage of 40%. The outer part, the outer detector, acts mainly as veto for entering particles and is monitored by 6700 photosensors. The construction will start in April 2020 while the data taking will start on 2026 for the first tank. A second tank has been proposed. It will be realized in Japan or in Korea. A new water Cherenkov detector of about 1 kilo-ton located at about 1 km from the accelerator neutrino source, the Intermediate Water Cherenkov Detector (IWCD), will serve to predict the expected event rates in Hyper-K as a function of the oscillation parameters and to measure the properties of neutrino interactions to reduce systematic errors related to the neutrino cross section uncertainty. A system of small photomultipliers as implemented in the KM3NeT experiment [2], the so called multi-PMT module (mPMT), is considered
as an option to improve the Hyper-K physics capability. This alternative option involves a combination between 50-cm PMTs and mPMTs, with the mPMTs supplementing the 50-cm photosensors. The IWCD baseline design is instead equipped with mPMT as photodetection system. This contribution presents a summary of ongoing R&D activities on the mPMT photodetection system for the Hyper-K project (see Fig. 1). 2. Photodetectors for Hyper-K The Hyper-K experiment will be instrumented with new technology photosensors, faster and with higher efficiency compared to the ones used in Super-Kamiokande. The requirements for photosensors are wide dynamic range, high time and charge resolutions, and high detection efficiency. 40 000 photosensors will be used to observe the Cherenkov light produced in neutrino interactions in the inner detector (ID). The baseline option foresees the use of the 50-cm Hamamatsu Photonics R12860-HQE PMT with a box-and-line dynode type, a high quantum efficiency photocathode, and with an improved design to resist the high pressure in the tank. It has been studied in great detail and shows a detection efficiency and time and charge resolution improved by a factor 2 compared with the Super-Kamiokande PMTs [3,4]. As alternative option, it is also considered to supplement it with mPMTs, each housing multiple 7.7 cm PMTs and read-out electronics in a pressure vessel. In the baseline design, the outer detector will be observed by 20-cm PMTs, but an alternative design employing 3 inch PMTs coupled with wavelength-shifting plates (WLS) is considered.
E-mail address: [email protected]. https://doi.org/10.1016/j.nima.2019.163033 Received 31 March 2019; Received in revised form 20 October 2019; Accepted 22 October 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
G. De Rosa and H.-K. Proto-Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 2. mPMT design for the Hyper-K project.
Fig. 1. Artistic picture of the Hyper-Kamiokande detector.
The final configuration of the photo-sensor system will be defined on the basis of the ongoing Monte-Carlo studies on the physics performance of the detector.
Fig. 3. Left: INFN Prototype: to test the acrylic vessel and electronics; Right: TRIUMF Prototype: to test new mechanics and assembling procedure.
3. Multi-PMT option for Hyper-K
thick acrylic cover. Signal and power cable are fed into the module via a feedthrough, which is placed in the cylindrical side of the mPMT module. The front-end electronics and HV supply are integrated within the pressure vessel. HV for the PMTs is provided by using active bases with Cockcroft–Walton voltage multipliers.
The mPMT features several attractive advantages compared to the conventional single-PMT concept: • Improved segmentation: With mPMTs one gets an increase in segmentation. This helps in distinguishing single-photon hits from multi-photon hits from coincident signals in neighboring PMTs. • Superior photon counting: The total number of deposited photoelectrons can be derived from the number of hit PMTs. • Extension of dynamic range: Cathode segmentation also benefits the overall dynamic range of the module, as multiple photons arriving at the same time are more likely to hit different PMTs. An additional benefit may be improved linearity for large signals. • Intrinsic directional sensitivity: As the single PMTs all have different orientations, the mPMT carries information on the direction of each detected photon which can improve signal-to-background separation for low energy signals. • Local coincidences: Uncorrelated single-hit noise, such as dark rate, can be suppressed using coincidences between individual PMT in one module.
4. Multi-PMT prototypes for Hyper-K Two initial prototypes for Hyper-K have been realized (Fig. 3) and are currently under test: • INFN Prototype: similar spherical shape as in KM3NeT, with a 17′′ vessel. This prototype has been realized to test the acrylic vessel and electronics. • TRIUMF Prototype: new design and mechanics optimized for Hyper-K. Main goal is the test of new mechanics and assembling procedure.
4.1. Acrylic vessel Several acrylics commercially available have been studied. Optical tests, radioactivity measurements, mechanical and absorption tests, and the compatibility with optical gel have been realized. The results on the optical transmittance are reported in Fig. 4 for the considered commercial acrylics. These measurements indicate the ’PLEXIGLAS GS UV transmitting’ by Evonik as the best solution for mPMT in the Hyper-K project. An accurate description of the mechanical behavior, crucial to predict the final performances of the mPMT polymeric cover in real operative conditions, has been obtained by measuring the compression under uniaxial loads. Tests on ‘PLEXIGLAS GS UV transmitting’ acrylic by Evonik were carried out on cylindrical specimens (20 mm in diameter and 18 mm in height). The measured stress–strain curves on several specimens are reported in Fig. 5. Three different zones can be clearly identified: the first zone from axis origin in which the material behavior was linearly elastic, the second zone with a small decrease in stress occurred due to strain softening, and finally the third region in which the material showed a stress increase due to the strain hardening. All the specimens satisfy the safety strength value of 50 MPa. A hydrostatic pressure test has been realized for 15 mm and 20 mmthick spherical vessels to verify the safe constraint to resist to 1.26 MPa.
An additional advantage of small PMTs is their much lower sensitivity to the Earth’s magnetic field, which makes magnetic shielding unnecessary [5]. The photo-sensors and the read-out electronics are hosted within a pressure-resistant vessel, protecting the system from the external water pressure. Detailed simulation studies are ongoing to investigate the impact of the mPMT system in Hyper-K physics in both the high and low energy sectors. Preliminary results show improved vertex resolution as well as an increased PID separation using mPMTs compared to 50-cm PMTs only [6]. A new design for mPMT, including the PMT read-out, the cooling and closure systems, the vessel material with reduced radioactive contaminations, and the different configuration of the small PMT, is under study for the Hyper-K project. The baseline design of a Hyper-K mPMT is based on the use of 19 7.7 cm PMTs in a cylindrical vessel with a radius of about 26 cm with an acrylic cover and a metallic body, as illustrated in Fig. 2. Reflector cones are added to each 7.7 cm PMT to increase the effective photocathode area. The 7.7 cm PMTs are supported by a 3D printed structure and optically coupled by Silicon Gel to the 15 mm 2
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
G. De Rosa and H.-K. Proto-Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 4. Transmittance of acrylic samples.
Fig. 7. Block diagram for the Q/T digitization for mPMT.
No damage has been registered up to 1.8 MPa for both the vessels. The 20 mm-thick vessel has been pressurized for a crash test until the breakdown, which happened at 8.6 MPa, much higher than the Hyper-K Fig. 5. Stress–strain curves acquired during the compression tests for ’PLEXIGLAS GS UV transmitting’ by Evonik compared with the one for Kuraray acrylic used in Super-Kamiokande.
constraints. Nuclear tests at the National Gran Sasso Laboratories (LNGS) have been realized to investigate radioactive contaminations by measuring gamma emissions. The measurement results are reported in Table 1.
Fig. 6. Block diagram for the FADC digitization for mPMT.
3
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
G. De Rosa and H.-K. Proto-Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 8. Q/T digitization: ADC channel as a function of the signal amplitude for the Front End Board tested with a pulser and a passive attenuator.
Table 1 Results of nuclear contamination measurements of Evonik samples. Isotope
Activity
Contamination
<0.11 mBq/kg <93 μBq/kg
<0.027 ppb <0.023 ppb
U: Uranium series Ra-226 Th-234 Pa-234 m
<65 μBq/kg <4.6 mBq/kg <2.5 mBq/kg
<0.0052 ppb <0.38 ppb <0.20 ppb
U-235 K-40 Cs-137
(0.15 ± 0.07) mBq/kg <0.69 mBq/kg <25 μBq/kg
(3 ± 1)⋅10−1 ppb <0.022 ppm –
232
Th: Thorium series Ra-228 Th-228
238
only one connection. The output of the single channel is merged to the main board where we process the information. The acquired data are transmitted out of the mPMT with a single Ethernet cable, serving also for power supply, clock and trigger. The mPMT prototype based on the Q/T digitization solution is currently under test at INFN laboratories. The block diagram for the Q/T digitization is described in Fig. 7. Current prototypes of both the PMT base and the digitizer unit based on Q/T digitization with a TDC and a 2-step integrator coupled to a sample-and-hold ADC showed good performance. The achieved timing resolution is ∼100 ps, the charge resolution is 0.1% and good linearity of the circuit response to charge was observed (see Fig. 8). The system has very low power consumption, reaching 12.5 mW for the HV circuit and 40.5 mW for the digitizer per-channel, with a total power consumption of ∼4 W per mPMT module.
These results show radioactivity levels comparable to the ’KURARAY PARAGLAS-UV00’ acrylic currently used in Super-Kamiokande. 4.2. Mechanics The mPMT module is designed to occupy the same 70 × 70 cm2 footprint as the 50-cm PMT to aid integration within the Hyper-K photosensor support structure. The proposed mechanical design reduces the mass of the modules, making production and installation easier. Pressure tests at 0.7 MPa with the new mPMT design and mechanics were achieved with no problem. 4.3. mPMT electronics To match the experimental constraints, we need to provide a system that has low power consumption (<3–4 W for the Hyper-K far detector), good charge resolution and timing accuracy better than 100 ps, so that the good timing performance of the small PMTs can be maintained. A basic Cockcroft–Walton voltage multiplier circuit is used to generate multiple voltages for the PMT. The system draws less than 1.5 mA of supply current at 3.3 V and can provide up to −1500 V cathode voltage. To match the experimental constraints, two different designs for the mPMT digitization are currently under development: one is a Q/T digitization based on discrete components, the other is based on FADC digitization, with on-board signal processing (see the block diagram in Fig. 6). In the Q/T digitization option, the single channel electronics is an integrated HV and FEB system, using the same MCU for both boards and
5. Summary The use of a system of small PMTs, the mPMT, is considered by the Hyper-K Proto-Collaboration as the photodetection system to be used in the IWCD and as option to supplement 50-cm PMTs in the far detector. The mPMT features several advantages, as improved segmentation, superior photon counting, and intrinsic directional sensitivity, improving the reconstruction performance, especially close to the inner detector wall, as shown by preliminary results on simulation studies [6]. Two mPMT prototypes have been realized and are currently under test. Optimization studies are ongoing to define the final design for Hyper-K mPMT. New prototypes are planned by the end of 2019. 4
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
G. De Rosa and H.-K. Proto-Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
References
[4] Y. Nishimura, Large aperture photodetectors for water Cherenkov detectors, in: TMEX 2018 WCP: European Workshop on Water Cherenkov Precision Detectors for Neutrino and Nucleon Decay Physics, Sep. 2018. URL https://events.ncbj.gov. pl/event/2/contributions/257/. [5] V. Giordano, S. Aiello, E. Leonora, Study on 3-inch Hamamatsu photomultipliers, in: RICAP-14 The Roma International Conference on Astroparticle Physics in Noto, Sicily, Italy, Sep. 2014. URL https://agenda.infn.it/event/7620/. [6] B. Quilain, Multi-PMT modules for Hyper-Kamiokande, in: Proceedings. URL https://www-conf.kek.jp/PD18. in preparation.
[1] Hyper-Kamiokande Proto-Collaboration, Hyper-Kamiokande design report, arXiv: 1805.04163. [physics.ins-det]. [2] S. Adrian-Martinez, et al., KM3NeT Collaboration, Deep sea tests of a prototype of the KM3NeT digital optical module, Eur. Phys. J. C 74 (9) (2014) 3056, Preprint arXiv:1405.0839. [3] Y. Nishimura, O. behalf of the Hyper-Kamiokande Proto-collaboration, New 50cm photo-detectors for Hyper-Kamiokande, in: Proceedings of 38th International Conference on High Energy Physics PoS, Vol. 282, ICHEP2016, SISSA Medialab, 2017, p. 303, http://dx.doi.org/10.22323/1.282.0303.
5
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
A multi-PMT photodetector system for the Hyper-Kamiokande experiment Gianfranca De Rosa, on behalf of Hyper-Kamiokande Proto-Collaboration Universitá ‘‘Federico II’’ and INFN, Naples, Italy
ARTICLE
INFO
Keywords: Neutrino Photosensors PMTs Water Cherenkov
ABSTRACT Hyper-Kamiokande (Hyper-K) is the next generation of large water Cherenkov neutrino detector with a fiducial volume which will be approximately 10 times larger than its precursor. Its broad physics program includes neutrinos from astronomical sources, nucleon decay, with the main focus the determination of leptonic CP violation. To detect the weak Cherenkov light generated by neutrino interactions or proton decay, the employment of the multi-PMT concept, first introduced in the KM3NeT detector, is considered as a possible solution. A multi-PMT Optical Module – a pressure vessel instrumented with multiple small diameter photosensors, read-out electronics and all required power supplies (incl. high voltage) – offers several advantages, such as weaker sensitivity to Earth’s magnetic field, increased granularity and directional information with an almost isotropic field of view. In this contribution the development of a multi-PMT module for Hyper-K is discussed.
1. Introduction Hyper-Kamiokande (Hyper-K) is a next generation underground water Cherenkov detector, which will be based in Japan [1]. Hyper-K is a multipurpose experiment for the observation of atmospheric, solar and accelerator neutrino oscillations, for neutrino astrophysics, proton decay and physics beyond the Standard Model. In particular, it will observe neutrinos produced at JPARC accelerator complex, about 300 km far, to investigate leptonic CP violation. Hyper-K builds on the successful experience gained in the currently operating Super-Kamiokande and T2K experiments, with a fiducial volume which will be approximately 10 times larger than its precursor for increased statistics, with improved photo-sensors for better efficiency, with higher intensity neutrino beam and updated/new near detector for accelerator neutrino study. Hyper-K consists of 2 identical detectors with staging construction. Each tank has a cylindrical shape with a fiducial mass of 187 ktons. The inner part of the tank, the inner detector, is observed with 40 000 50-cm PMTs with a photocathode coverage of 40%. The outer part, the outer detector, acts mainly as veto for entering particles and is monitored by 6700 photosensors. The construction will start in April 2020 while the data taking will start on 2026 for the first tank. A second tank has been proposed. It will be realized in Japan or in Korea. A new water Cherenkov detector of about 1 kilo-ton located at about 1 km from the accelerator neutrino source, the Intermediate Water Cherenkov Detector (IWCD), will serve to predict the expected event rates in Hyper-K as a function of the oscillation parameters and to measure the properties of neutrino interactions to reduce systematic errors related to the neutrino cross section uncertainty. A system of small photomultipliers as implemented in the KM3NeT experiment [2], the so called multi-PMT module (mPMT), is considered
as an option to improve the Hyper-K physics capability. This alternative option involves a combination between 50-cm PMTs and mPMTs, with the mPMTs supplementing the 50-cm photosensors. The IWCD baseline design is instead equipped with mPMT as photodetection system. This contribution presents a summary of ongoing R&D activities on the mPMT photodetection system for the Hyper-K project (see Fig. 1). 2. Photodetectors for Hyper-K The Hyper-K experiment will be instrumented with new technology photosensors, faster and with higher efficiency compared to the ones used in Super-Kamiokande. The requirements for photosensors are wide dynamic range, high time and charge resolutions, and high detection efficiency. 40 000 photosensors will be used to observe the Cherenkov light produced in neutrino interactions in the inner detector (ID). The baseline option foresees the use of the 50-cm Hamamatsu Photonics R12860-HQE PMT with a box-and-line dynode type, a high quantum efficiency photocathode, and with an improved design to resist the high pressure in the tank. It has been studied in great detail and shows a detection efficiency and time and charge resolution improved by a factor 2 compared with the Super-Kamiokande PMTs [3,4]. As alternative option, it is also considered to supplement it with mPMTs, each housing multiple 7.7 cm PMTs and read-out electronics in a pressure vessel. In the baseline design, the outer detector will be observed by 20-cm PMTs, but an alternative design employing 3 inch PMTs coupled with wavelength-shifting plates (WLS) is considered.
E-mail address: [email protected]. https://doi.org/10.1016/j.nima.2019.163033 Received 31 March 2019; Received in revised form 20 October 2019; Accepted 22 October 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
G. De Rosa and H.-K. Proto-Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 2. mPMT design for the Hyper-K project.
Fig. 1. Artistic picture of the Hyper-Kamiokande detector.
The final configuration of the photo-sensor system will be defined on the basis of the ongoing Monte-Carlo studies on the physics performance of the detector.
Fig. 3. Left: INFN Prototype: to test the acrylic vessel and electronics; Right: TRIUMF Prototype: to test new mechanics and assembling procedure.
3. Multi-PMT option for Hyper-K
thick acrylic cover. Signal and power cable are fed into the module via a feedthrough, which is placed in the cylindrical side of the mPMT module. The front-end electronics and HV supply are integrated within the pressure vessel. HV for the PMTs is provided by using active bases with Cockcroft–Walton voltage multipliers.
The mPMT features several attractive advantages compared to the conventional single-PMT concept: • Improved segmentation: With mPMTs one gets an increase in segmentation. This helps in distinguishing single-photon hits from multi-photon hits from coincident signals in neighboring PMTs. • Superior photon counting: The total number of deposited photoelectrons can be derived from the number of hit PMTs. • Extension of dynamic range: Cathode segmentation also benefits the overall dynamic range of the module, as multiple photons arriving at the same time are more likely to hit different PMTs. An additional benefit may be improved linearity for large signals. • Intrinsic directional sensitivity: As the single PMTs all have different orientations, the mPMT carries information on the direction of each detected photon which can improve signal-to-background separation for low energy signals. • Local coincidences: Uncorrelated single-hit noise, such as dark rate, can be suppressed using coincidences between individual PMT in one module.
4. Multi-PMT prototypes for Hyper-K Two initial prototypes for Hyper-K have been realized (Fig. 3) and are currently under test: • INFN Prototype: similar spherical shape as in KM3NeT, with a 17′′ vessel. This prototype has been realized to test the acrylic vessel and electronics. • TRIUMF Prototype: new design and mechanics optimized for Hyper-K. Main goal is the test of new mechanics and assembling procedure.
4.1. Acrylic vessel Several acrylics commercially available have been studied. Optical tests, radioactivity measurements, mechanical and absorption tests, and the compatibility with optical gel have been realized. The results on the optical transmittance are reported in Fig. 4 for the considered commercial acrylics. These measurements indicate the ’PLEXIGLAS GS UV transmitting’ by Evonik as the best solution for mPMT in the Hyper-K project. An accurate description of the mechanical behavior, crucial to predict the final performances of the mPMT polymeric cover in real operative conditions, has been obtained by measuring the compression under uniaxial loads. Tests on ‘PLEXIGLAS GS UV transmitting’ acrylic by Evonik were carried out on cylindrical specimens (20 mm in diameter and 18 mm in height). The measured stress–strain curves on several specimens are reported in Fig. 5. Three different zones can be clearly identified: the first zone from axis origin in which the material behavior was linearly elastic, the second zone with a small decrease in stress occurred due to strain softening, and finally the third region in which the material showed a stress increase due to the strain hardening. All the specimens satisfy the safety strength value of 50 MPa. A hydrostatic pressure test has been realized for 15 mm and 20 mmthick spherical vessels to verify the safe constraint to resist to 1.26 MPa.
An additional advantage of small PMTs is their much lower sensitivity to the Earth’s magnetic field, which makes magnetic shielding unnecessary [5]. The photo-sensors and the read-out electronics are hosted within a pressure-resistant vessel, protecting the system from the external water pressure. Detailed simulation studies are ongoing to investigate the impact of the mPMT system in Hyper-K physics in both the high and low energy sectors. Preliminary results show improved vertex resolution as well as an increased PID separation using mPMTs compared to 50-cm PMTs only [6]. A new design for mPMT, including the PMT read-out, the cooling and closure systems, the vessel material with reduced radioactive contaminations, and the different configuration of the small PMT, is under study for the Hyper-K project. The baseline design of a Hyper-K mPMT is based on the use of 19 7.7 cm PMTs in a cylindrical vessel with a radius of about 26 cm with an acrylic cover and a metallic body, as illustrated in Fig. 2. Reflector cones are added to each 7.7 cm PMT to increase the effective photocathode area. The 7.7 cm PMTs are supported by a 3D printed structure and optically coupled by Silicon Gel to the 15 mm 2
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
G. De Rosa and H.-K. Proto-Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 4. Transmittance of acrylic samples.
Fig. 7. Block diagram for the Q/T digitization for mPMT.
No damage has been registered up to 1.8 MPa for both the vessels. The 20 mm-thick vessel has been pressurized for a crash test until the breakdown, which happened at 8.6 MPa, much higher than the Hyper-K Fig. 5. Stress–strain curves acquired during the compression tests for ’PLEXIGLAS GS UV transmitting’ by Evonik compared with the one for Kuraray acrylic used in Super-Kamiokande.
constraints. Nuclear tests at the National Gran Sasso Laboratories (LNGS) have been realized to investigate radioactive contaminations by measuring gamma emissions. The measurement results are reported in Table 1.
Fig. 6. Block diagram for the FADC digitization for mPMT.
3
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
G. De Rosa and H.-K. Proto-Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 8. Q/T digitization: ADC channel as a function of the signal amplitude for the Front End Board tested with a pulser and a passive attenuator.
Table 1 Results of nuclear contamination measurements of Evonik samples. Isotope
Activity
Contamination
<0.11 mBq/kg <93 μBq/kg
<0.027 ppb <0.023 ppb
U: Uranium series Ra-226 Th-234 Pa-234 m
<65 μBq/kg <4.6 mBq/kg <2.5 mBq/kg
<0.0052 ppb <0.38 ppb <0.20 ppb
U-235 K-40 Cs-137
(0.15 ± 0.07) mBq/kg <0.69 mBq/kg <25 μBq/kg
(3 ± 1)⋅10−1 ppb <0.022 ppm –
232
Th: Thorium series Ra-228 Th-228
238
only one connection. The output of the single channel is merged to the main board where we process the information. The acquired data are transmitted out of the mPMT with a single Ethernet cable, serving also for power supply, clock and trigger. The mPMT prototype based on the Q/T digitization solution is currently under test at INFN laboratories. The block diagram for the Q/T digitization is described in Fig. 7. Current prototypes of both the PMT base and the digitizer unit based on Q/T digitization with a TDC and a 2-step integrator coupled to a sample-and-hold ADC showed good performance. The achieved timing resolution is ∼100 ps, the charge resolution is 0.1% and good linearity of the circuit response to charge was observed (see Fig. 8). The system has very low power consumption, reaching 12.5 mW for the HV circuit and 40.5 mW for the digitizer per-channel, with a total power consumption of ∼4 W per mPMT module.
These results show radioactivity levels comparable to the ’KURARAY PARAGLAS-UV00’ acrylic currently used in Super-Kamiokande. 4.2. Mechanics The mPMT module is designed to occupy the same 70 × 70 cm2 footprint as the 50-cm PMT to aid integration within the Hyper-K photosensor support structure. The proposed mechanical design reduces the mass of the modules, making production and installation easier. Pressure tests at 0.7 MPa with the new mPMT design and mechanics were achieved with no problem. 4.3. mPMT electronics To match the experimental constraints, we need to provide a system that has low power consumption (<3–4 W for the Hyper-K far detector), good charge resolution and timing accuracy better than 100 ps, so that the good timing performance of the small PMTs can be maintained. A basic Cockcroft–Walton voltage multiplier circuit is used to generate multiple voltages for the PMT. The system draws less than 1.5 mA of supply current at 3.3 V and can provide up to −1500 V cathode voltage. To match the experimental constraints, two different designs for the mPMT digitization are currently under development: one is a Q/T digitization based on discrete components, the other is based on FADC digitization, with on-board signal processing (see the block diagram in Fig. 6). In the Q/T digitization option, the single channel electronics is an integrated HV and FEB system, using the same MCU for both boards and
5. Summary The use of a system of small PMTs, the mPMT, is considered by the Hyper-K Proto-Collaboration as the photodetection system to be used in the IWCD and as option to supplement 50-cm PMTs in the far detector. The mPMT features several advantages, as improved segmentation, superior photon counting, and intrinsic directional sensitivity, improving the reconstruction performance, especially close to the inner detector wall, as shown by preliminary results on simulation studies [6]. Two mPMT prototypes have been realized and are currently under test. Optimization studies are ongoing to define the final design for Hyper-K mPMT. New prototypes are planned by the end of 2019. 4
Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.
G. De Rosa and H.-K. Proto-Collaboration
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
References
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Please cite this article as: G. De Rosa, A multi-PMT photodetector system for the Hyper-Kamiokande experiment, Nuclear Inst. and Methods in Physics Research, A (2019) 163033, https://doi.org/10.1016/j.nima.2019.163033.