Hidehito Nakamura

A prototype Si/CdTe Compton camera and the polarization measurement

A Compton camera is the most promising approach for gamma-ray detection in the energy region from several hundred kiloelectronvolts to megaelectronvolts, especially for application in high energy astrophysics. In order to obtain good angular resolution, semiconductor detectors such as silicon, germanium and cadmium telluride(CdTe) have several advantages over scintillation detectors, which have been used so far. Based on the recent advances of high resolution CdTe and silicon imaging detectors, we are working on a Si/CdTe Compton camera. We have developed 64-pixel CdTe detectors with a pixel size of 2 mm /spl times/ 2 mm and double- sided Si strip detectors(DSSDs) with a position resolution of 800 /spl mu/m. As a prototype Si/CdTe Compton camera, we use a DSSD as a scatterer and two CdTe pixel detectors as an absorber. In order to verify its performance, we irradiate the camera with 100% linearly polarized 170 keV /spl gamma/-rays and demonstrate the system works properly as a Compton camera. The resolution of the reconstructed scattering angle is 22/spl deg/(full-width at half-maximum). Measurement of polarization is also reported. The polarimetric modulation factor is obtained to be 43%, which is consistent with the prediction of Monte Carlo simulations.

Radiation measurements with heat-proof polyethylene terephthalate bottles

This study demonstrates that the energy resolution of a newly developed 100 per cent pure polyvinyltoluene (PVT) plate allows its use as a base material for a plastic scintillator. The energy resolution, which is a key element for high-performance radiation detectors, was ΔE/E=8.41±0.07% (full width at half maximum (FWHM)) for 976u2009keV K-line conversion electrons from a 207Bi source. On the basis of results from 207Bi and 137Cs sources, the observed energy resolution of the PVT plate, ΔE/E=8.2/E1/2% (FWHM), was slightly better than that of a typical plastic scintillator (BC-408), ΔE/E=8.7/E1/2% (FWHM), with E in units of MeV. These results prompted us to search for other new base materials for plastic scintillators. In this study, we examined polyethylene terephthalate (PET) bottles, a common source of domestic plastic waste. We demonstrated that a lump of heat-proof PET bottles is fluorescent; moreover, there is excellent compatibility of the fluorescence with the quantum efficiency of typical photomultiplier tubes. This inexpensive source of plastic appears suitable for radiation measurements and as a base material for plastic scintillators. Future studies on the radiation response of plastics should lead to the development of higher performance and more eco-friendly radiation detectors.

Gamma-ray polarimetry with Compton telescope

Compton telescope is a promising technology to achieve very high sensitivity in the soft gamma-ray band (0.1-10 MeV) by utilizing Compton kinematics. Compton kinematics also enables polarization measurement which will open new windows to study gamma-ray production mechanism in the universe. CdTe and Si semiconductor technologies are key technologies to realize the Compton telescope in which their high energy resolution is crucial for high angular resolution and background rejection capability. We have assembled a prototype module using a double-sided silicon strip detector and CdTe pixel detectors. In this paper, we present expected polarization performance of a proposed mission (NeXT/SGD). We also report results from polarization measurements using polarized synchrotron light and validation of EGS4 MC simulation.

Large area gamma-ray imaging detector based on high-resolution CdTe diode

We are developing a large array detector composed of 1024 individual CdTe diodes. Each detector has the dimensions of 1.2 mm /spl times/ 5.0 mm and a thickness of 1.2 mm. An edge-on geometry is used for the injection of /spl gamma/-rays, to obtain a cross-section thickness of 5 mm. With this geometry, the distance between the two electrodes can be kept small, and we can therefore apply the high electric field which is necessary to achieve a high energy resolution (by reducing the low energy tail) and also to sustain the long-term stability of the CdTe diode. Signals from each detector element are fed into newly developed low noise ASICs. We use 32 chips for the readout of 1024 elements. In this paper, we report the basic characteristics of the individual detectors and the overall performance of the gamma-camera. Design of the readout electronics system is also described.

A data readout system with high-speed serial data link for balloonborne X-ray detectors

Data size of recent balloonborne detector is growing. High speed data readout is one of the key technologies for those detectors. A parallel data bus has been used for sending data with high speed. However, it causes problems in noise, mechanical size and power consumption. We apply the serial data link for our apparatus as a replacement of the parallel data link to solve those problems. We designed the hardware and firmware based on the SpaceWire protocol. Because of simple and flexible routing protocol of SpaceWire, the system was compact and has very high modularity. The protocol is implemented on Alteras FPGAs, APEX20KE and Cyclone. The speed of the serial data link was 48-53 Mbps. The maximum data rate to the computer was 4.5 MB/s.

A new method for calibration and response measurement of a scintillation detector using radioisotope sources.

Abstract Nakamura, H., Ejiri, H. and Kitamura, H. A New Method for Calibration and Response Measurement of a Scintillation Detector Using Radioisotope Source. Radiat. Res. 170, 811–814 (2008). A new method for calibration was developed that takes the energy deposited by radioisotope sources into account; it was validated with a plastic scintillation plate using a 207Bi source. The energy deposits of the conversion electrons in the 207Bi source led to an energy difference of a few keV for the 976 keV monoenergetic peak. Calibration with high accuracy is essential to estimate radiation doses as well as to evaluate detector performance. The method will be available not only for use in scintillation detectors but also for other detectors.

Study of Statistical and Non-statistical Components of Energy Resolution for High-sensitive Beta Camera

Energy resolution of a scintillation detector was studied from the viewpoints of statistical and non-statistical components. The statistical component is caused mainly by fluctuation of the number of photoelectrons, N, collected at the anode of a photodetector and the non-statistical component comes from intrinsic property of the scintillator. A new method identifying these two components was developed to study the resolution of detector as beta camera. The beta camera consists of a 6 cm times 6 cm times 1 cm plastic-scintillator plate (BC-408 provided by SAINT GOBAIN) and 4 photomultiplier tubes covered each side face of the plastic-scintillator. The statistical component is found to be well reproduced by 1/radicN. The non-statistical component of the plastic-scintillator is sigmastat = 4.0 plusmn 0.3 % (DeltaE/E = 9.3 plusmn 0.8 % in FWHM) at the 976 keV region. The present method is useful for investigating statistical and non-statistical components of the resolution, and thus to improve and monitor them.

MOON for spectroscopic studies of double beta decays and the present status of the MOON-1 prototype detector

The MOON (Molybdenum Observatory Of Neutrinos) project, as an extension of ELEGANT V, aims at spectroscopic studies of double beta decays from 100Mo with a sensitivity of the Majorana neutrino mass around 30 meV. Measurements with good energy and position resolutions enable one to select true signals and to reject background ones. A prototype MOON detector (MOON Phase-1A) with 142 g 100Mo was built and is running at the Oto underground laboratory. The present report describes briefly the outline of the MOON project and the present status of MOON-1.

Energy Resolution of Plastic Scintillation Detector for Beta Rays

Many experiments on neutrinoless double beta decays have been proposed recently for determining the effective mass of neutrinos. Some of them use a plastic scintillator for measuring the beta-ray energy. Achieving a high energy resolution is critical for neutrinoless double beta decay experiments (e.g., MOON and superNEMO). The motivation of this study is to investigate the energy resolution of a plastic scintillation detector in terms of ldquocomponentsrdquo in the MeV-electron region. It is known that the total energy resolution of a plastic scintillation detector for a monoenergetic spectrum consists of two components: statistical and intrinsic energy resolution. In this paper, these two components were investigated for electrons. Experiments on protons were also performed for the purpose of comparison. Three additional experiments were performed to determine (1) the uniformity of the plastic scintillator materials by scanning a 2.8-MeV proton microbeam, (2) the energy spread of electrons and protons, and (3) the effect of the electron-beam size on the set-up. These additional experiments were performed to ensure that these factors did not affect measurements of the two above-mentioned components.

Prototype of Compton camera using high resolution Si/CdTe detectors Si/CdTe Compton camera as a polarimeter

To bring a breakthrough in the observation of the gamma-ray universe, we are working on the development of Semiconductor Multi-Compton Telescope(SMCT). In the SMCT, all the energy, positional and timing resolution must be high to ensure high sensitivity. The imaging device based on the high resolution CdTe diode and Si, such as CdTe pixel detectors and Double-sided Si Strip Detectors (DSSDs), are promising candidates for the components of the SMCT. Here we report the results of a CdTe pixel detector connected with a low noise analog ASIC. We obtained an energy resolution of 2.5 keV(FWHM) at 122 keV, with a positional resolution of 2 mm. Performance of the first prototype Compton Camera using a DSSD and two CdTe pixel detectors is also described. We irradiated 100% linearly polarised 170 keV /spl gamma/-ray line to the system and obtained the polarimetric modulation factor of 43%.