R. Dolenec

Studies of a proximity focusing RICH with aerogel radiator for Belle II experiment

For the Belle II experiment, we are developing a proximity focusing ring imaging Cherenkov counter using a silica aerogel as a radiator. Recently, a new aerogel fabrication method, called “pin-hole drying”, was invented and remarkably improved optical transparency has been realized in the region of refractive index larger than 1.05. For the photo-detector, we have developed a 144-channel Hybrid Avalanche Photo-Detector with Hamamatsu Photonics K. K. The HAPD has excellent single photon detection performance, 5 × 5 mm2 position resolution and immunity to magnetic field. The tolerance to the neutron irradiation up to 5 × 1011 neutron/cm2 has been confirmed. In November 2009, we have performed a test beam experiment at KEK for a prototype A-RICH counter with 6 HAPDs and aerogel tiles produced by the “pin-hole drying” method. We successfully observed a clear Cherenkov ring image and obtained the single photon Cherenkov angle resolution of 13.5 mrad and 15.3 detected photoelectrons. This result corresponds to more than 6σ K/π separation at 4 GeV/c and fulfills the requirement for the Belle II.

Time-of-flight measurements with Cherenkov photons produced by 511 keV photons in lead crystals

The resolution of time-of-flight measurements is usually limited by the decay time constant of the scintillating material used. If Cherenkov photons, which are produced promptly, could be used to measure the time-of-flight, the limiting factor would become the photo detector. The excellent time resolution of the microchannel plate photomultiplier (MCP PMT) lead us to consider the possibility of performing such time-of-flight measurements for pairs of annihilation 511 keV photons, which are used in positron emission tomography (PET) for source distribution reconstruction. In this work we present simulation and experimental results, obtained with a simple back-to-back detector, using PbF2 or PWO crystal as Cherenkov radiator and Hamamatsu MCP PMT as a photo detector.

Measurements of Cherenkov Photons with Silicon Photomultipliers

A novel photon detector, the Silicon Photomultiplier (SiPM), has been tested in proximity focusing Ring Imaging Cherenkov (RICH) counters that were exposed to cosmic-ray particles in Ljubljana, and a 2 GeV electron beam at the KEK research facility. This type of RICH detector is a candidate for the particle identification detector upgrade of the BELLE detector at the KEK B-factory, for which the use of SiPMs, microchannel plate photomultiplier tubes or hybrid avalanche photodetectors, rather than traditional Photomultiplier Tubes, is essential due to the presence of high magnetic fields. In both experiments, SiPMs are found to compare favourably with PMTs, with higher photon detection rates per unit area. Through the use of hemispherical and truncated pyramid light guides to concentrate photons onto the active surface area, the light yield increases significantly. An estimate of the contribution to dark noise from false coincidences between SiPMs in an array is also presented.

Tests of silicon photomultiplier PET modules

The use of Silicon Photomultipliers (SiPMs) as photon detectors in Positron Emission Tomography (PET) modules offers significant advantages over conventional light sensors, including application in a magnetic field, better resolution and easier operation. Two PET modules have been constructed by coupling 4 × 4 arrays of LYSO sctintillation crystals of area 4.2 × 4.2 mm2 and length 20 mm to SiPMs. Two types of SiPM have been tested: the Hamamatsu S10931-100P and Photonique PCB-PET07 of active surface areas 3 × 3 mm2 and 2.1 × 2.1 mm2 respectively. The energy resolutions, signal pulse properties and energy response of the arrays are presented in view of arrangement into a larger module. Additionally, the surface sensitivity and time resolutions of smaller 1 × 1 mm2 SiPMs from the same manufacturers have been analysed.

Development of a RICH counter with 144-ch Hybrid Avalanche Photo-Detectors for the Belle II experiment

For the Belle II experiment at the superKEKB accelerator, we have been developing a proximity focusing ring imaging Cherenkov counter using silica aerogel as a radiator (ARICH). 144-channel Hybrid Avalanche Photo-Detectors (HAPD) developed with Hamamatsu Photonics K.K. were adopted as the Cherenkov photon detectors for the ARICH. We confirmed that the HAPD has sufficient performance for the ARICH photon detector by the bench test and carried out a test beam experiment with a prototype ARICH counter with six HAPDs, arranged as the actual ARICH configuration, at CERN in September 2011. As the result of the experiment, we succeeded in observing distinct Cherenkov images and confirmed that prototype ARICH had potentially 5.5σ K/π separation ability. One of our remaining concerns is radiation hardness of the HAPDs especially for γ-rays and neutrons. The HAPD is required the radiation hardness of the 10-year Belle II operation. From past studies of APDs for γ-ray and neutrons, we found the major cause of the radiation damage to the APD, and improved the structures of the APD. In order to confirm the improvement of HAPD for radiation hardness, we carried out a series of irradiation tests with various types of HAPDs using a 60Co facility and also a neutron beam facility. By understanding the reasons of the radiation damages in the APD, we have successfully improved radiation hardness of the APD by modification of electronic structure on it. In this paper, the latest status of HAPD development and the study of the radiation hardness for the HAPD are described.

Silicon Photo-multipliers as photon detectors for PET

A novel photon detector, the Silicon Photo-multiplier (SiPM), has been tested as a photosensitive device for Positron Emission Tomography (PET). PET is one of the most important non-invasive methods for in-depth and in-vivo imaging of tissue. The SiPM has advantages over existing photon detectors as it can be used in a magnetic field, has a better resolution and is easier to operate. The results of measurements of spatial dependence variation in response for individual SiPMs are presented. A PET module has been constructed by coupling an array of Photonique 2.1mm × 2.1mm SiPMs to LYSO scintillation crystals. The properties of the module prototype are presented.

Tests of a silicon photo-multiplier module for detection of Cherenkov photons

The present work investigates the potential of silicon photomultipliers as position sensitive detectors of single photons in a ring imaging Cherenkov counter. The main advantage of silicon photomultipliers is their insensitivity to high magnetic fields, so they can be used inside large magnetic spectrometers. A module, consisting of 64 (=8x8) Hamamatsu MPPC S10362-11-100P silicon photomultipliers, has been constructed and tested with Cherenkov photons emitted in an aerogel radiator by 120 GeV/c pions from the CERN T4-H6 beam. In order to increase the efficiency, i.e. the effective surface on which light is detected, we have made and tested pyramidal light concentrators. The results of these tests indicate that a RICH counter based on silicon photomultipliers is feasible.

Selection of optimal multispectral imaging system parameters for small joint arthritis detection

Early detection and treatment of arthritis is essential for a successful outcome of the treatment, but it has proven to be very challenging with existing diagnostic methods. Novel methods based on the optical imaging of the affected joints are becoming an attractive alternative. A non-contact multispectral imaging (MSI) system for imaging of small joints of human hands and feet is being developed. In this work, a numerical simulation of the MSI system is presented. The purpose of the simulation is to determine the optimal design parameters. Inflamed and unaffected human joint models were constructed with a realistic geometry and tissue distributions, based on a MRI scan of a human finger with a spatial resolution of 0.2 mm. The light transport simulation is based on a weighted-photon 3D Monte Carlo method utilizing CUDA GPU acceleration. An uniform illumination of the finger within the 400-1100 nm spectral range was simulated and the photons exiting the joint were recorded using different acceptance angles. From the obtained reflectance and transmittance images the spectral and spatial features most indicative of inflammation were identified. Optimal acceptance angle and spectral bands were determined. This study demonstrates that proper selection of MSI system parameters critically affects ability of a MSI system to discriminate the unaffected and inflamed joints. The presented system design optimization approach could be applied to other pathologies.

Behavior of 144ch HAPDs for the Belle II Aerogel RICH in the Magnetic Field

We report behavior of Hybrid Avalanche Photon Detectors (HAPDs), which were newly developed for Aerogel Ring Imaging Cherenkov detector (ARICH) counter that will be installed in the endcap region of the Belle II detector, in the magnetic field. Since HAPDs are used in a 1.5 T magnetic field in the Belle II detector, we have tested 520 HAPDs from the mass production in the magnetic field. We observed anomalously large pulses in the magnetic field in many HAPDs. The main adverse effect of these large pulses is the induction of a short dead time of the readout electronics after each pulse. We performed several studies to understand the mechanism generating large pulses.

The Aerogel Ring Image Cherenkov Counter for Particle Identification in the Belle II Experiment

The Aerogel Ring Imaging Cherenkov (ARICH) counter is an upgrade of the forward endcap particle identification device in the upcoming Belle II experiment to secure \(4\sigma \) separation of charged kaons and pions up to momenta of 3.5 GeV. Developments and productions of Silica aerogel radiator, Hybrid Avalanche Photo Detector (HAPD) and its readout electronics, key components of the ARICH counter, are done while surrounding subsystems; a power supply, a readout slow control and a LED light injection system are in operation. And then construction of the ARICH counter is ongoing in parallel to cosmic ray test. Details of the detector components with construction status are described and the test operation are discussed in the letter.