Michito Hirayanagi

Performance and field tests of a handheld Compton camera using 3-D position-sensitive scintillators coupled to multi-pixel photon counter arrays

After the nuclear disaster in Fukushima, radiation decontamination has become particularly urgent. To help identify radiation hotspots and ensure effective decontamination operation, we have developed a novel Compton camera based on Ce-doped Gd3Al2Ga3O12 scintillators and multi-pixel photon counter (MPPC) arrays. Even though its sensitivity is several times better than that of other cameras being tested in Fukushima, we introduce a depth-of-interaction (DOI) method to further improve the angular resolution. For gamma rays, the DOI information, in addition to 2-D position, is obtained by measuring the pulse-height ratio of the MPPC arrays coupled to ends of the scintillator. We present the detailed performance and results of various field tests conducted in Fukushima with the prototype 2-D and DOI Compton cameras. Moreover, we demonstrate stereo measurement of gamma rays that enables measurement of not only direction but also approximate distance to radioactive hotspots.

Gamma-ray visualization module

Gamma ray cameras can easily locate radiation hotspots where decontamination is required. Among them, the Compton camera that utilizes the Compton scattering is compact and lightweight because no radiation shielding is required. We have developed a Compton camera for quick visualization of the radioactive contamination. It features high detection efficiency by utilizing gamma ray detectors which is a combination of Multi-Pixel Photon Counter (MPPC) array and Gadolinium Aluminum Gallium Garnet (GAGG) scintillator arrays.

Development of MPPC array module

Multi-Pixel Photon Counter (MPPC) is a solid state photon counting device consisting of Geiger-mode APD pixels and self-quenching resistors. The MPPC has attractive features, such as low bias voltage operation, high gain, compact size, robustness and insensitivity to magnetic fields. Therefore, MPPC applications are now being expanded, especially for very low light level measurements. We have developed a new MPPC based PET detector module dedicated to TOF-PET systems. The module consists of a 16 × 16 Lutetium Fine Silicate (LFS) scintillator array with 3.2 mm pitch and a 16 × 16 Through-Silicone-Via (TSV) MPPC array, included analyzing circuits for gamma-ray detection, The performance of the module has been evaluated. The coincidence timing resolution was obtained 440 ps FWHM with a pair of the modules. The energy resolution value was 12.3% at 511 keV. All crystal segments were also clearly separated with uniform irradiation of 511 keV gamma-rays.

Multi-pixel photon counter module for MRI compatible application

A new detector for PET using multi-pixel photon counters (MPPCs) was developed for MRI compatible applications. This module has an 8 × 8 MPPC array, each segment has a 3 mm × 3 mm active area, and the pitch of the array is 4.1 mm in both directions. A temperature sensor is attached to the back of the array for temperature compensation. The MPPC array is connected to the front-end circuit with a detachable flexible printed circuit cable (FPC), which provides flexibility for detector arrangement. The front-end circuit consists of preamplifiers, a register network, buffer amplifiers, a built-in high voltage (HV) unit, and an embedded microprocessor unit. The HV unit is a down-regulator and requires an external HV supply. The preamplifier also has a sum output, which can be used for timing pick-off and energy discrimination. With LYSOs, the timing performance was evaluated using flexible printed cables of two different lengths. Set in a copper shield box, energy spectra and flood images were evaluated with a 3T-MRI Very little interference was observed during simultaneous MRI measurements.

Current status and optimization of handy compton camera using 3D position-sensitive scintillators

After the Japanese nuclear disaster in 2011, a large amount of radioactive isotopes was released and still remains a serious problem in Japan. To help identify radiation hotspots and ensure effective decontamination operation, we are developing a novel Compton camera weighing only 1.9 kg and measuring just 14×14×15 cm3 in size. Despite its compactness, the camera realizes a wide 180° field of vision, Δθ ~ 10°(FWHM) angular resolution, and offers excellent sensitivity that can image a hotspot producing a 5 μSv/h dose at a distance of three meters, every 10 sec. Our key technology using 3D position-sensitive scintillators coupled with thin monolithic MPPC arrays has made this innovation possible for the first time. In this paper, we present the detailed optimization and simulation of the Compton camera currently under production with Hamamatsu Photonics.