Toshiyuki Onodera

TlBr Gamma-Ray Spectrometers Using the Depth Sensitive Single Polarity Charge Sensing Technique

Pixellated thallium bromide (TlBr) gamma-ray spectrometers were fabricated and evaluated with the depth sensitive single polarity charge sensing technique. The detectors exhibited energy resolutions of 2.22%, 1.22% and 1.11% for 356 keV, 662 keV and 1.33 MeV, respectively, at room temperature. Direct measurements of electron mobility-lifetime products in TlBr detectors were performed. An electron mobility-lifetime product of 4.1times10-3 cm2/V was obtained from a TlBr detector.

TlBr gamma-ray spectrometers using the depth sensitive single polarity charge sensing technique

Pixellated thallium bromide (TIBr) gamma-ray spectrometers were fabricated and evaluated with the depth sensitive single polarity charge sensing technique. The detectors exhibited energy resolutions of 2.22%, 1.22% and 1.11% for 356 keV, 662 keV and 1.33 MeV, respectively, at room temperature. Direct measurements of electron mobility-lifetime products in TIBr detectors were performed. An electron mobility-lifetime product of 4.1 times 10-3 cm2/V was obtained from a TIBr detector.

Timing Performance of TlBr Detectors

The timing performance of TlBr detectors was evaluated at room temperature (22 °C). 0.5-mm-thick planar TlBr detectors with Tl circular electrodes with a diameter of 3 mm were fabricated from TlBr crystals grown by the traveling molten zone method using a zone-purified material. The pulse rise time of the TlBr detector was measured using a digital oscilloscope as the cathode surface of the device was irradiated with a 22Na gamma-ray source. Coincidence timing spectra were obtained between the TlBr detector and a BaF2 scintillation detector when both detectors were irradiated with 511 keV positron annihilation gamma-rays. The timing resolution of the TlBr detector was found to be inversely proportional to the applied bias voltage. The TlBr detector, in coincidence with the BaF2 detector, exhibited timing resolutions characterized by a 6.5 ns full width at half maximum (FWHM) and an 8.5 ns FWHM with and without an energy window of 350 keV-560 keV, respectively.

TlBr Capacitive Frisch Grid Detectors

Capacitive Frisch grid detectors with the dimensions of 2 mm × 2 mm × 4.4 mm were fabricated from TlBr crystals grown by the travelling molten zone method using zone-purified materials. Spectroscopic performance of the detectors was evaluated at room temperature (24°C), - 5°C and - 20°C. The TlBr detector exhibited an energy resolution of 1.6% FWHM at 662 keV with a peak-to-Compton ratio of 8.7 with the depth correction at room temperature. Improvement of the detector performance was achieved by cooling the detector-preamplifier system. The energy resolutions of 3.4%, 1.5%, 1.0% and 0.9% FWHM at 122 keV, 356 keV, 662 keV and 1332 keV, respectively, were recorded with the TlBr detector without the depth correction at - 5°C. An energy resolution of 0.9% FWHM at 662 keV with a peak-to-Compton ratio of 18.2 was obtained from the TlBr detector with the depth correction at -20°C. An upper limit for the Fano factor of TlBr was estimated to be 0.43 at -20°C.

Characterization of Thallium Bromide Detectors Made From Material Purified by the Filter Method

Thallium bromide (TlBr) has been regarded as candidate detector materials for the gamma-ray spectrometers operating at room temperature. In this study, a simple and rapid method, the filter method, was performed to purify a raw TlBr material used for fabrication of TlBr detectors. The material was loaded on shards of crashed quartz and installed in a Pyrex tube, and was melted using a furnace. A purified material passing through interspaces of the shards of quartz was collected in a quartz ampoule located at the outlet of the Pyrex tube. After the purification, impurities colored black extracted from the raw material remained. TlBr crystals were then grown by the travelling molten zone method both from the raw material and the purified material. TlBr detectors were fabricated from the grown crystals, and were characterized by measuring mobility-lifetime products (μτ) for carriers and gamma-ray spectra ( 137Cs) at room temperature. μτ for electrons of a TlBr detector fabricated from the purified material was around 5 times higher than that of a detector fabricated from the raw material.

Thallium lead iodide radiation detectors

Thallium lead iodide (TlPbI/sub 3/) is a compound semiconductor characterized with wide band gap (2.3 eV) and high photon stopping power. Thus, TlPbI/sub 3/ is an attractive material for fabrication of room temperature radiation detectors. In this study, TlPbI/sub 3/ crystals were grown by the vertical Bridgman technique using zone-purified materials. The starting materials for the crystal growth were synthesized from commercially available TlI and PbI/sub 2/ powders with nominal purity of 99.99%. Powder X-ray diffraction analysis was performed to evaluate chemical composition of the synthesized TlPbI/sub 3/. In order to fabricate radiation detectors, the grown crystals were cut into several wafers using a wire saw. The wafers were then polished using Al/sub 2/O/sub 3/ abrasives. Electrodes were formed on the wafers by vacuum evaporation of gold. The resultant TlPbI/sub 3/ radiation detectors were evaluated by measuring their current-voltage characteristics and spectral responses. Most of TlPbI/sub 3/ detectors exhibited resistivities more than 10/sup 11/ /spl Omega/cm. The TlPbI/sub 3/ detectors were irradiated with /spl alpha/-particles (5.48 MeV) from a /sup 241/Am source or /spl gamma/-rays (662 keV) from a /sup 137/Cs source. The TlPbI/sub 3/ detectors exhibited a clear peak of 5.48 MeV /spl alpha/-particles. Although the 662 keV peak was not resolved in the energy spectra, increased counts above the noise spectrum were observed for the detectors. To our knowledge, it is the first time that TlPbI/sub 3/ detectors exhibit /spl alpha/-particle spectra with a clear peak and /spl gamma/-ray spectra.

Characterization of thallium bromide chloride crystals for radiation detectors

Thallium bromide chloride (TlBrxCl1−x) crystals have been evaluated as a material used for fabrication of room temperature radiation detectors. In this study, TlBrxCl1−x crystals with various chlorine (Cl) concentrations were grown by the travelling molten zone method and the detectors were fabricated from the crystals. The optical properties of the crystals were evaluated by measuring the transmittances. The charge transport properties were characterized by the Hecht analysis. The band gap energy of the crystals proportionally increased with Cl concentration. Mobility-lifetime products (μ(τ) of the crystals decreased with increasing Cl concentration.

Evaluation of LGSO scintillators with Si PIN photodiodes

Low-Lu LGSO scintillators (Lu0.4Gd1.6SiO5: 2.0 mol% Ce) were evaluated with Si PIN photodiode. An LGSO scintillator (2 mm times 2 mm times 4 mm) was coupled to a Si PIN photodiode (HAMAMATSU S1337-33BR). Spectroscopic and timing performance of the LGSO/Si PIN detector was investigated at room temperature. The LGSO/Si PIN detector was irradiated with a 22Na source. The energy resolutions of LGSO/Si PIN detector for the 511 keV full-energy peak was measured to be 17.7% (90.7 keV FWHM). The 511 keV peak position was found to correspond to 3793 e- (conversion factor of 7.4 e-/keV). A coincidence timing spectrum was acquired with the LGSO/Si PIN detector and the reference BaF2 detector. The coincidence timing resolution was measured to be 57 ns FWHM.

Cerium bromide scintillation detectors

In this study, CeBr3 crystals were grown by the vertical Bridgman method from the starting material with nominal purity of 99.999%. The size of the single crystal was approximately 60 mm in length and 8 mm in diameter. The grown crystals were cut into small pieces. The crystal was connected to a photodetector (Si APD or Si photodiode) and energy spectra were measured by irradiating them with 662 keV gamma-rays at room temperature. An FWHM energy resolution of 662 keV gamma-rays obtained with the CeBr3 coupled to the APD was measured to be 40 keV (6.1%), which is 2 times better than that obtained with a GSO scintillator at the same condition.

Pixellated thallium bromide X- and gamma-ray detectors

Thallium bromide is a very promising compound semiconductor for fabrication of X- and gamma-ray detectors due to its wide bandgap (2.68 eV) and higher photon stopping power than other semiconductor materials used for radiation detector fabrication such as CdTe and HgI/sub 2/. Our studies made an effort to fabricate pixellated TlBr detectors with sufficient detection efficiency and good charge collection efficiency. In this study, pixellated X- and gamma-ray detectors were fabricated from TlBr crystals. The TlBr crystal was purified by the multipass zone-refining method and grown by the traveling molten zone method. The pixellated TlBr detector has a continuous cathode over one crystal surface and 3 /spl times/ 3 pixellated anodes (0.57 /spl times/ 0.57 mm/sup 2/ each) surrounded by a guard ring on the opposite side. The electrodes were prepared by vacuum evaporation of palladium through a shadow mask. Typical thickness of the detectors was around 2 mm. Spectrometric performance of the pixellated TlBr detectors was tested by irradiating them with /sup 241/Am (59.5 keV), /sup 57/Co (122 keV) and /sup 137/Cs (662 keV) gamma-ray sources. Each of the anode pixels of the detector was connected to each charge sensitive preamplifier and the guard ring was grounded. Typical energy resolutions (FWHM) obtained for a pixel of the TlBr detector at -20/spl deg/C were 4 keV, 7 keV and 10 keV for 59.5 keV, 122 keV and 662 keV gamma-rays, respectively.