Search

Long-term stability of TlBr detectors with Tl electrodes was evaluated in this study. Tl/TlBr/Tl detectors were fabricated and tested with a charge sensitive preamplifier and a spectroscopy amplifier and a gated integrator. Significant improvement of energy resolutions was realized by applying the gated integrator to the TlBr detectors. The full-energy peak positions, counts and resolutions for 662 keV gamma-rays from 137 Cs were monitored during the course of measurements (10 days). By changing the polarity of the applied bias voltages every 24 hours, the Tl/TlBr/Tl detectors exhibited stable spectroscopic performance for totally over 240 hours.

Multi-Pixel Photon Counter (MPPC) is a Hamamatsu's new Si avalanche photodiode. The main features of the MPPC are very high gain (105 to106) and very fast operation. The MPPC offers considerable advantages over widely used photomultiplier tubes (PMT) for scintillation detection, especially due to their high quantum efficiency, low cost, compactness, low operating voltage, mechanical robustness and insensitivity to magnetic fields. One of the most attractive applications of MPPCs is positron emission tomography (PET). In the present study, the MPPCs were coupled to inorganic scintillators including LSO, BGO and YAP in order to evaluate the scintillator/MPPC devices in terms of energy and timing resolutions for PET applications. The scintillation detector consisted of an LSO scintillator (2 mm × 2 mm × 4 mm) coupled to an MPPC exhibited energy resolutions of 196 keV FWHM and 189 keV FWHM for 511 keV and 662 keV gamma-rays, respectively. The MPPC was operated at a bias of 71.4 V and at room temperature. In order to investigate timing properties of the LSO/MPPC device, coincidence timing spectra between a reference scintillation detector which consisted of a BaF2 crystal coupled to a PMT and the LSO/MPPC device were measured with 511 keV positron annihilation gamma-rays from a 22Na source. A coincidence timing resolution of 3.2 ns FWHM was obtained with the LSO/MPPC system.

A simple x-ray computed tomography (CT) system utilizing a cadmium telluride detector and its application to enhanced iodine K-edge angiography are described. The CT system is of the first generation type and consists of an x-ray generator, a turn table, a translation unit, a motor drive unit, a cadmium telluride detector, an interface unit for the detector, and a personal computer (PC). Tomography was performed by the repetition of the translation and rotation. Narrow-photon-energy bremsstrahlung x-rays with a peak photon energy of approximately 35 keV is very useful for performing enhanced K-edge angiography because these rays are absorbed effectively by iodine-based contrast media with a K-edge of 33.2 keV. The tube voltage, the tube current, and the aluminum filter thickness were 60 kV, 1.5 mA, and 3.0 mm, respectively. Holes filled with iodine media in phantoms are visible with high contrasts, and the CT system can be applied to photon-counting and fluorescent x-ray CT systems.

Detection of high-dose-rate pulse x-rays from a samarium plasma flash x-ray generator utilizing a multipixel photon counter is described. Monochromatic K-series characteristic x-rays are detected by a plastic scintillator, and fluorescent lights are lead to the photon counter through a 10-m-length plastic fiber. The reverse bias was 70.0 V, and x-ray outputs were recorded by a digital storage scope. The samarium plasma flash x-ray generator is useful for performing high-speed enhanced K-edge angiography using cone beams because K-series characteristic x-rays from the samarium target are absorbed effectively by iodine-based contrast media. In the flash x-ray generator, a 150 nF condenser is charged up to 80 kV by a power supply, and flash x-rays are produced by the discharging. Since the electric circuit of the high-voltage pulse generator employs a cable transmission line, the high-voltage pulse generator produces twice the potential of the condenser charging voltage. At a charging voltage of 80 kV, the estimated maximum tube voltage and current are approximately 160 kV and 40 kA, respectively. When the charging voltage was increased, the K-series characteristic x-ray intensities of samarium increased. Bremsstrahlung x-ray intensity rate decreased with increasing the charging voltage, and K lines were produced with a charging voltage of 80 kV. The x-ray pulse widths were approximately 100 ns, and the time-integrated x-ray intensity had a value of approximately 500 μGy at 1.0 m from the x-ray source with a charging voltage of 80 kV. Angiography was performed using a filmless computed radiography (CR) system and iodine-based contrast media. In the angiography of nonliving animals, we observed fine blood vessels of approximately 100 μm with high contrasts.

A fluorescent x-ray tomography system is useful in performing fluorescent x-ray analysis for target atoms in biomedical objects utilizing a drug deliverly system. This tomography system is employed in order to measure iodine distribution in objects, and the system consists of a cerium x-ray generator, a 58-μm-thick stannum filter, a tungsten collimator, and a computed radiography system. Because K-series characteristic x-rays from the cerium target are absorbed effectively by iodine-based contrast media, iodine fluorescent x-rays from iodine atoms in the objects are produced. In the tomography system, when the objects are exposed by fan beams, the stannum filter easily transmits iodine Kα rays from a slice plane, and tomograms are obtained using the CR system and the collimator.

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.

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.

Pixellated mercuric iodide (HgI2) detectors with thickness of 5 and 10 mm have been investigated using depth sensing single polarity charge sensing technique at room temperatures. By acquiring energy spectra of 137Cs as a function of the interaction depth, an insensitive region exhibiting poor spectroscopic performance was observed in the vicinity of the cathode surface of a 10 mm thick HgI2 detector operated at 2500 V, while the other regions of the detector present clear photo peaks with good energy resolutions. The thickness of the insensitive region is estimated to be around 2 mm. The electron mobility-lifetime (mutau) product was estimated based on the change of photopeak amplitude at different bias voltages. The value of electron mutau product was estimated to be around 1.9times10-3 cm2/V which is smaller than the value of 7.4times10-3 cm2/V reported earlier

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.

We have been studying a /spl gamma/-ray detector which uses a lead iodide PbI/sub 2/ crystal. The detector fabricated from a PbI/sub 2/ crystal grown by 10-pass zone travelling molten zone (TMZ) method. The /spl mu//spl tau/product of this crystal are estimated to be about 1/spl times/10/sup -6/ cm/sup 2//V for electrons and about 6/spl times/10/sup -7/ cm/sup 2//V for holes. The resistivity of the PbI/sub 2/ crystal is estimated to be about 10/sup 12/ /spl Omega/cm. FWHM energy resolution of the detector for a photoelectric peak for the 59.5 keV /spl gamma/-rays is found to be about 5 keV.

Cadmium telluride (CdTe) radiation detectors applied three-electrode and pixel structure have been fabricated from commercially available CdTe crystals (2 mm thick). In case of a three-electrode detector, the cathode was 6/spl times/6 mm/sup 2/, the anode was 0.5 mm in diameter and the third electrode surrounding the anode was I mm width. The gap between the third electrode and the anode was 0.25 mm. In case of pixel detectors, the anodes were 9 pixels and the guardring surrounded the pixels. One pixel detector has rectangular 1.35/spl times/1.35 mm/sup 2/ pixels with 1.45 mm pitch and the other pixel detector has rectangular 0.7/spl times/0.7 mm/sup 2/ pixels with 0.8 mm pitch. All electrodes were formed on the crystals by vacuum evaporation of Au. Pulse height spectra of gamma-rays emitted from a /sup 137/Cs source were acquired with the fabricated detectors. The three-electrode detectors were operated with the anode voltage of +10 V, the cathode voltage of -540 V and the third electrode grounded. An energy resolution of 6.0 keV FWHM was recorded for 662 keV gamma-rays at room temperature. The pixel detectors with 1.35/spl times/1.35 mm/sup 2/ pixels and with 0.7/spl times/0.7 mm/sup 2/ pixels have exhibited energy resolutions of 16.7 keV and 12.3 keV FWHM at 662 keV, respectively. The pixel detectors were operated at 600 V and at room temperature. Detailed discussions about the device fabrication, operation and characterization are given in this paper.

An energy-discriminating K-edge X-ray computed tomography (CT) system is useful for increasing the contrast resolution of a target region utilizing contrast media and for reducing the absorbed dose for patients. The CT system is of the first-generation type of detector using cadmium telluride (CdTe). CT is performed by repeated translations and rotations of an object. Penetrating X-ray photons from the object are detected by a CdTe detector, and event signals of X-ray photons are produced using charge-sensitive and shaping amplifiers. Both photon energy and energy width are selected out using a multichannel analyzer, and the number of photons is counted by a countercard. To perform energy discrimination, a low-dose-rate X-ray generator for photon counting was developed. Its maximum tube voltage and minimum tube current were 110 kV and 1 µA, respectively. In energy-discriminating CT, the tube voltage and tube current were 100 kV and 20 µA, respectively, and the X-ray intensity was 2.98 µGy/s at a distance of 1.0 m from the source and a tube voltage of 100 kV. The demonstration of enhanced gadolinium K-edge X-ray CT was carried out by selecting photons with energies just beyond the gadolinium K-edge energy of 50.3 keV.

Blue sensitive photodetectors have been fabricated from thallium bromide (TlBr) crystals for scintillation detection. Lu2SiO5 (LSO)/TlBr detectors consisting of 3 mm×4 mm TlBr photodetectors coupled to 2 mm×2 mm×4 mm long LSO scintillators have demonstrated energy resolutions of 34.5% and 25.5% full width at half maximum for 511 keV and 662 keV γ rays, respectively.

An energy-discriminating x-ray camera is useful for performing monochromatic radiography using polychromatic x rays. This x-ray camera was developed to carry out K-edge radiography using iodine-based contrast media. In this camera, objects are exposed by a cone beam from a cerium x-ray generator, and penetrating x-ray photons are detected by a cadmium telluride detector with an amplifier unit. The optimal x-ray photon energy and the energy width are selected out using a multichannel analyzer, and the photon number is counted by a counter card. Radiography was performed by the detector scanning using an x-y stage driven by a two-stage controller, and radiograms obtained by energy discriminating are shown on a personal computer monitor. In radiography, the tube voltage and current were 60 kV and 36 µA, respectively, and the x-ray intensity was 4.7 µGy/s. Cerium K-series characteristic x rays are absorbed effectively by iodine-based contrast media, and iodine K-edge radiography was performed using x rays with energies just beyond iodine K-edge energy 33.2 keV.

In this study, TlBr detectors were irradiated with 25 MeV protons accelerated by an AVF cyclotron. Isothermal annealing was performed to restore the performance of the detectors. In order to characterize the radiation damage and thermal annealing effects on the TlBr detectors, we measured current-voltage (I-V) characteristics, mobility-lifetime (/spl mu//spl tau/) products and spectrometric responses. The I-V and /spl mu//spl tau/ measurements suggest that electron traps have been induced by 25 MeV protons in the TlBr crystals. X- and /spl gamma/-ray energy spectra were measured for two different electronic conditions: the electric signals induced mainly by electron carriers traversing the crystal were used for one case and the signal induced by hole carriers were used in the other case. After irradiation of 25 MeV protons, the /sup 241/Am X- and /spl gamma/-ray spectra obtained in the former showed significantly degraded energy resolution. No degradation of energy resolution, however, was observed in the latter case. Noticeable improvements of the degraded detector performance have been observed after the thermal annealing.

The radiation detectors were fabricated from the TlBr crystals grown by TMZ method and the FWHM and transit time of electrons and holes were measured as a function of the detector temperature. The TlBr radiation detector shows the best response characteristics at about 320 K for holes. At the temperature of 330 K or above, the transit time and FWHM for the /sup 241/Am /spl alpha/-particle peak become worse. On the other hand, in the case of electron, the spectrum becomes worse with increasing the temperature form 150 K to 380 K. In the case, we can deduce an activation energy of about 0.20 eV for holes and about 0.10 for electron from the temperature dependence of the carrier transit time.

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.

Bismuth tri-iodide (BiI/sub 3/) is an attractive material for room temperature radiation detectors because of its wide bandgap energy and high photon stopping power. In this study, BiI/sub 3/ crystals have been grown by the vertical Bridgman technique using commercially available powder. The grown crystals have been characterized in terms of their structural properties and stoichiometry. Room temperature radiation detectors have been fabricated from the crystals and tested by measuring their leakage currents and spectroscopic responses. A clear peak corresponding to 5.48 MeV /spl alpha/-particles (/sup 241/Am) was recorded with a BiI/sub 3/ detector 82 /spl mu/m thick with 0.8 mm/sup 2/ palladium electrodes. The energy resolution of the 5.48 MeV peak was measured to be 2.2 MeV FWHM.