Developed Imaging Detector Research Articles

Development of an ultrahigh resolution 1 mm Si-PM array based GGAG alpha particle imaging detector with gamma or X-ray separation capability

For precise distribution measurements of alpha particles, as required in alpha radionuclide therapy research, a high-resolution alpha particle imaging detector is desirable. While combining a thin scintillator with a photodetector array, such as a position-sensitive photomultiplier (PSPMT), is a potential method to develop an event-by-event-based alpha particle imaging detector, the spatial resolution is currently limited to around 0.8 mm FWHM. To overcome this limitation, we utilized a small-sized Si-PM array combined with a thin GGAG plate to create an alpha particle imaging detector and evaluated its performance. In the developed alpha particle imaging detector, a Si-PM array with a channel size of 1 mm × 1 mm arranged in an 8 × 8 configuration was optically coupled to a 1 mm thick GGAG plate, with a 1-mm-thick light guide between them. Since the decay times of GGAG differ between alpha particles and gamma photons or X-rays, we were able to separately measure the distributions of alpha particles and gamma photons using pulse shape discrimination. The spatial resolution of the developed alpha particle imaging detector was 0.19 mm FWHM, and the energy resolution was 11% FWHM for 5.5 MeV alpha particles. Additionally, we were able to separately measure the distributions of alpha particles and gamma photons for simultaneously irradiated phantoms using pulse shape discrimination. The developed imaging detector shows promise for distribution measurements of alpha particles, as well as gamma photons or X-rays, where high spatial resolution and separation of gamma photons or X-rays are required.

Development of a phoswich detector composed of ZnS(Ag) and YAP(Ce) for astatine-211 imaging

For the research on alpha emitters used for targeted alpha particle therapy such as astatine-211 (211At), alpha particle as well as X-ray imaging are required for autoradiography and phantom imaging or in vivo imaging of small animals. Consequently, two imaging detectors for different types of radiations are required in such research. However, preparing two types of imaging detectors needs cost and space. If a single imaging detector could image alpha particles as well as X-rays for 211At, it would be an advance in the imaging research for targeted alpha particle therapy. Consequently, we developed a detector that can be used for both alpha particle and X-ray imaging. The developed imaging detector is a phoswich type made of a ZnS(Ag) sheet and a cerium doped YAlO3 (YAP(Ce)) plate optically coupled to each other with a 1-mm-thick light guide between scintillators. These scintillators were optically coupled to a 1-inch-square position-sensitive photomultiplier tube (PSPMT) to form an imaging detector. The ZnS(Ag) sheet detects alpha particles while the YAP(Ce) plate detects X-rays from 211At. Due to the pulse height difference between alpha particles and X-rays, a different high voltage was supplied to the PSPMT for alpha particle and low-energy X-ray detection. Discrimination between alpha particles and X-rays were conducted by pulse height as well as pulse shapes of ZnS(Ag) and YAP(Ce). The intrinsic spatial resolution for 5.5-MeV alpha particles was better than 0.6 mm the full width at half maximum (FWHM), while that for 60-keV gamma photons was ∼1.2 mm FWHM. Energy resolution for 5.5-MeV alpha particles was 60% FWHM, while that for 60-keV gamma photons was 25% FWHM. With a 4-mm-thick, 0.6-mm-diameter parallel hole collimator mounted on the detector, the system spatial resolution for 60-keV gamma photons was ∼2 mm FWHM within 5 mm from the collimator surface, and its sensitivity was 0.5%. An image of the 211At alpha particle distribution of a sliced kidney as well as an X-ray image of a paper soaked with 211At solution could be obtained. We conclude that the developed detector is promising for single-detector imaging of alpha particles as well as X-rays from 211At.

Development of a Gd₂Si₂O₇ (GPS) Scintillator-Based Alpha Imaging Detector for Rapid Plutonium Detection in High-Radon Environments

We developed a Gd2Si2O7 (GPS) scintillator-based alpha imaging detector and demonstrated its effectiveness by evaluating the actual Pu particle and 222Rn progeny. The GPS scintillator plate was prepared by a sintering method. The outer dimensions of the GPS scintillator plate were $5\times 5$ cm, and the scintillator layer was approximately $50~\mu \text{m}$ on a 3-mm-thick high-transparency glass. The plate was optically coupled to a position-sensitive photomultiplier tube (Hamamatsu H8500, Hamamatsu, Japan) with silicone grease. The developed imaging detector exhibited good uniformity; Pu particle activities were accurately evaluated at 14 different positions and the difference in activity was within ±6%. The radon-222 progeny counts were reduced by 65.3% by applying an energy window. Although the Pu/222Rn progeny activity ratio was 1/51, the Pu particle was successfully identified among the 222Rn progeny within the 5-min measurement time. The imaging detector has an excellent ability for detecting Pu among the 222Rn progeny. Thus, this detector is useful for alpha contamination monitoring in high-radon-background environments.

Development of an event-by-event based radiation imaging detector using GGAG: A ceramic scintillator for X-ray CT

For a radiation imaging detector, low cost, high light output and short decay time scintillator is required. Although ceramic scintillators developed for X-ray computed tomography (CT) are possible materials for this purpose, the performance of the scintillation imaging detector using ceramic scintillators was not reported. For this purpose, we tested a ceramic scintillator originally developed for X-ray CT, Gd3(GaAl)5O12:Ce (GGAG), combined with a position sensitive photomultiplier (PSPMT) to test whether an event-by-event based radiation imaging detector is possible to develop. The radiation imaging detector consists of a 0.1 mm thick GGAG plate, light guide and a 1-inch square PSPMT. The GGAG plate was optically coupled to the PSPMT with a 1 mm thick light guide between them. Anger principle was used for the position calculation of the radiations. We also conducted the comparison of the imaging detectors using 0.5 mm thick GGAG and single crystal Ce doped Gd3(Ga, Al)5O12 (GAGG) plates. The spatial resolution and energy resolution of the developed imaging detector were 0.53mm FWHM and 11.5 % FWHM for 5.5 MeV alpha particles, respectively. The uniformity of the imaging detector at the central part of the field of view (FOV) was ±11%. We could obtain variable phantom images with the developed imaging detector for alpha particles. Reasonable performance was also obtained for beta particles, low energy gamma photons and X-ray. The spatial resolution of the imaging detector used GGAG plate was twice better and distortion was smaller than that of GAGG. We conclude that GGAG ceramic scintillator is promising for the development of radiation imaging detectors.

Development of ultrahigh resolution alpha particle imaging detector using 1 mm channel size Si-PM array

For precise distribution measurements of alpha particles, a high-resolution alpha particle imaging detector is required. Although combining a thin scintillator with a silicon photomultiplier (Si-PM) array is a promising method for achieving high resolution, the spatial resolution is limited. Reducing the size of the Si-PM array is a possible approach to improving the spatial resolution of the alpha particle imaging detector. Consequently, we employed a 1 mm channel size Si-PM array combined with a thin ZnS(Ag) sheet to form an alpha particle imaging detector and evaluated the performance. For the developed alpha particle imaging detector, an Si-PM array with 1 mm x 1 mm channel size arranged 8 x 8 was optically coupled to a ZnS(Ag) sheet with a 1-mm-thick light guide between them. The size of the alpha particle imaging detector was 9.5 mm x 9.5 mm. The spatial resolution of the developed alpha particle imaging detector was 0.14 mm FWHM, and the energy resolution was 74% FWHM for 5.5 MeV alpha particles. The uniformity of the imaging detector at the central part of the field of view (FOV) was ±4.7%. The background count rate was 0.06 counts/min. We obtained various high-resolution phantom images for alpha particles with the developed system. We conclude that the developed imaging detector is promising for high-resolution distribution measurements of alpha particles.

Development of a GSO positron/single-photon imaging detector

We have developed and tested a GSO (gadolinium oxyorthosilicate) position-sensitive gamma detector which can be used with positron and single-photon radionuclides for imaging breast cancer or sentinel lymph node detection. Because GSO has a relatively good energy resolution for annihilation gammas as well as low energy gamma photons, and does not contain any natural radioisotopes, it can be used for positron imaging and lower energy single-photon imaging. The imaging detector consists of a GSO block, 2 inch square multi-channel position-sensitive photo-multiplier tube (PSPMT), and associated electronics. The size of a single GSO element was 2.9 mm × 2.9 mm × 20 mm and these elements were arranged into 15 × 15 matrixes to form a block that was optically coupled to the PSPMT. It was possible to separate all GSO crystals into a two-dimensional position histogram for annihilation gammas (511 keV) and low energy gamma photons (122 keV). The typical energy resolution was 24% FWHM and 37% FWHM for 511 keV and 122 keV gamma photons, respectively. For the positron imaging, coincidence between the imaging detector and a single gamma probe is measured. For the single-photon imaging, a tungsten collimator is mounted in front of the imaging detector. With this configuration, it was possible to image both positron radionuclides and low energy single-photon radionuclides. We measured spatial resolution and sensitivity as well as image quality of the developed imaging detector. Results indicated that the developed imaging detector has the potential to be a new and useful instrument for nuclear medicine.