Hyper-Kamiokande

The red and white pole indicates the center of the dome. The hole on the left side of the photo is the tunnel excavated in 2020 for the geological survey. The tunnel in this construction connected to this survey tunnel at this dome center.

The dimensions of the tunnel in the photo are approximately 9 m high and 9 m wide. Though people look quite smaller already, the completed Hyper-Kamiokande dome will become 21 m high and 69 m in diameter, much larger than this tunnel.

Edge of the access tunnel. The approach tunnel will be excavated beyond this point.

Hyper-Kamiokande is an international research collaboration, with various countries sharing the research and development work. The Spanish group is playing a central role in the development and realization of the cover. The cover is currently being developed and is expected to be completed by 2021, with mass production beginning in 2022.

Prototype of the photosensor cover. UV transparent acrylic resin and 2.5mm thickness stainless steel are used. ©Autonomous University of Madrid

We have achieved an important milestone for the realization of Hyper-Kamiokande project. With preparations going smoothly, the construction process of the Hyper-Kamiokande is accelerating, towards the start of the experiment in 2027.

The access tunnel entrance

We have developed a new model of high-performance photosensor to improve the efficiency and the precision of the measurements. Basic properties of the new photosensors such as the detection efficiency, counting resolution, and timing resolution have been successfully improved approximately twice compared to the photosensors currently used in Super-Kamiokande.

The mass production of the photosensors for Hyper-Kamiokande has started. The first photosensors arrived at ICRR in December 2020. We will check the delivered sensors to confirm that they satisfy the performance requirements and measure their individual properties.

The mass production will be completed within six years to start the experiment in 2027.

The new photosensors arrived at ICRR

A water Cherenkov detector with the effective volume 8.4 times larger than that of Super-Kamiokande will be built using the full capacity of the cavern. By combining it with the upgrade of J-PARC high-intensity proton beam, we are aiming for the next breakthrough in the fields of particle and astroparticle physics. The objectives include shedding light on the origin of matter, deeper and more precise understanding of neutrino oscillations, searching for nucleon decay to directly probe the Grand Unified Theory, and observing cosmic neutrinos from single and cumulative supernova explosions. Hyper-Kamiokande is the project which will provide an international observatory for fundamental physics for more than 20 years.

Now, the geological survey for the excavation is in the final stages. In order to determine the position of the cavern, we have performed various geological surveys these past few months. In this fiscal year, we are performing a final large-scale survey including new adits that measures 96 m in length and borings that totals 725m.

The bedrock around the currently planned position of the cavern has been investigated in detail based on the adit and boring results. In July 2020, the excavation of the adit has been completed, and it reached the center position of the Hyper-Kamiokande dome. The dome will be on top of a huge water tank. At this special position, various in-situ experiments are in progress to identify the actual physical properties of the bedrock.

The boring which penetrates through the planned position of the cavern for the investigation of its neighborhood.

With the final geological surveys going smoothly, preparations are now underway for the excavation of an access tunnel and the subsequent excavation of one the world’s largest underground caverns.

The adit to investigate the geological survey of the Hyper-Kamiokande (HK) dome, where the “HK center” mark is found at the ceiling near the dead end (bottom center).

This mockup frame is used to determine the design of the photo sensors, the mounting procedure and the anti-chain-implosion covers. 40000 photo sensors will be mounted within Hyper Kamiokande, so it is necessary to find a quick way to install this large number of sensors efficiently and accurately in order to start operations as soon as possible.

We use the mockup structure to estimate the accuracy, rigidity and efficiency of the photo sensor installation work. We also plan to determine the installation method of the black sheets dividing the tank between inner and outer detector regions.

The preliminary black sheet design will be installed to allow the measurement of performance and the stability of the light shielding.

Currently the photo sensors and covers, developed in Japan, are ready for test measurements and several types of attachment methods have been tested. The procedures to install the photo sensors and covers developed in other counties will be tested with this mockup.

A mockup of the PMT support structure

The photo sensors and covers installed with the mockup

For Hyper-Kamiokande, which have about 60m depth, it is a important item to develop a more powerful cover than that for current Super-Kamiokande (about 40m depth). To establish the cover, we need to prove actual prevention of chain implosion in deep water. We already performed a test of a prototype in 2016, and we established an anti-chain-implosion cover for Hyper-Kamiokande.

In March 2018, we tested three type of covers, improved prototype, resin-made monolithic cover, a tube-shaped stainless steel made cover which is developed by our Spanish group (Picture 1 to 3).

(Picture 1) Test system. A photosensor with cover is located at the center of the grid, and surrounded by bare photosensors. The system is lowered down to deep water. The photosensor in the cover is crashed by a special tool (“pusher”) remotely and see the effect on the surrounding photosensors. Center is photosensor with the improved prototype. We put color tapes on glasses or cover surface to identify the surface via camera.

(Picture 2) A resin-made monolithic cover. It is much lighter than current prototype made by stainless steel.

(Picture 3) A tube-shaped cover developed by our Spain group. Structure is relatively simple and cheaper production cost is expected.

We carried out the test from March 6th to March 10th in old JAMIC (Japan Microgravity Center) building in Kami-Sunagawa city, Hokkaido, Japan (Picture 4).

We proved that the covers prevents chain implosion in 80m, 40m, and 60m, respectively. We also found differences of the shockwave generation depending on the covers and different deformation of covers, which are important to understand the process of implosion of large photo-sensor in deep water. In addition to that, we tested an implosion of smaller (8 inch diameter) photosensor in deep water, and we tested a new type of test apparatus to crash photosensors.

We successfully took such valuable data in short time. This is fully supported by Kami-Sunagawa town.

(Picture 4) Old JAMIC (Japan Microgravity Center) building in Kami-Sunagawa city, Hokkaido, Japan

(Picture 5) During the test, lowest temperature in the laboratory was below minus 10 degree. This is icicles on a rope in laboratory.

In this virtual experiment, there are two simulations: simulation for physics process, like how neutrino interacts with matter and produces particles, and simulation of detector which denotes how particles are detected and transformed to signal. As for physics simulation, we can use programs used in SK or T2K because physics phenomena are universal. We have to establish the detector simulation by our own including tank size and shape, optical property of water, performance of photo sensor, readout of signal, and so on. We (core members from Duke university, US) are developing water Cherenkov detector simulator for wide use based on GEANT4, a software package of CERN. This simulator can easily change tank size, number of photo sensor, and type of photo sensor and read out by modularize them. Figure 1 shows an event sample of HK simulation.

Figure 1; Event display of HK simulation in which muon is emitted from center of the tank toward side wall. Yellow dots correspond to photo sensor which detect Cherenkov light.

To understand detector performance, software which reconstructs physics variables from information of photo sensors is also needed. The conventional method used in SK determines physics variables one by one sequentially. We are developing a new method which determines all physics variables at once by fitting expected distributions to photo sensor information. The new method uses information of un-hitted photo sensors which are not used in the conventional method, and we can expect to reconstruct physics variables more precisely.

Figure 2 show momentum resolution calculated by the new method for electron and muon which are generated by HK simulation with various momentum. The new photo sensor for HK has two times better efficiency for detecting one photon than SK and if the photo sensors cover 40% of the detector wall as SK, this figure shows the momentum resolution improves much better than SK.

Figure 2; Momentum (horizontal axis) vs momentum resolution (vertical) for electron (left) and muon (right) reconstructed by the new method. Green corresponds to SK performance, red is HK tank which new photo sensors cover 14 % of detector wall, blue is 40 % coverage case.

We have developed a custom charge-to-time converter (QTC) ASIC for Super-Kamiokande. This ASIC converts the amount of the input charge to a digital pulse, whose width is proportional to the integrated charge collected by the photo-sensor, and whose leading edge may be digitized by a TDC to determine the time the photo-sensor was hit.

LSI developed for Super-Kamiokande; CLC101(ICRR/IWATSU Electric Co. Ltd.)

In Hyper-Kamiokande, the QTC is considered as one of the primary options for instrumenting the detector. We are designing proto-type electronics board using the QTC ASIC for Super-Kamiokande. In order to measure timing, it is planned to implement the TDC function in an FPGA by programmable logic. The FPGA is commonly used in consumer products and recent development makes it possible to measure the timing very precisely. With the latest technology, the measurement accuracy is better than 10^-10 seconds.

We are developing a proto-type module, which composed of these two elements with university and laboratories in the United States.

Evaluation board developed with the group in US.

We are also evaluating electronic readout based on an FADC (Flash Analog-to-Digital Converter), as another promising candidate for the signal processing. The FADC records the waveform of the input charge signal, sampling the waveform of the signal from the photo-sensor several million times per second. The recorded waveform is further processed digitally to precisely measure the timing and the charge of the signal. This system has been developed by the groups in Canada and Poland.

Through these developments, we are aiming to develop a signal processing system capable of handling signals from 50,000 photo sensors per Hyper-Kamiokande detector module, with goals of high performance, low power consumption and low cost per channel.

Evaluation board developed by the groups in Canada and Poland.

The new photosensors being developed for the Hyper-Kamiokande project have been greatly improved in terms of not only photodetection but also strength under a high water pressure. A strength test of these new photosensors has been commenced.

A new photosensor made of 50-cm-diameter glass has been placed into a metal cylinder, and the metal cylinder has been placed into a blue tank for a test under a high water pressure.

Thus far, we have tested a few photosensors and confirmed that they can withstand a water depth of over 100 m, which is over two times the depth of the Hyper-Kamiokande detector.

We aim to improve the capacity of the photosensors to resist a high water pressure based on the test results.