Search

A 0.3-mm thin YA1[O.sub.3] (Ce) scintillating crystal plate of 22 mm diameter was mounted inside an electrostatic-focussed hybrid photomultiplier tube (HPMT). The photocathode was evaporated directly on this scintillating plate opposite to the HPMT-anode. The HPMT was vacuum-sealed with a 1-mm thick Be radiation entrance window. Photoelectron numbers at their peak energies of photoabsorption and energy resolutions were measured between 3.69 and 59.6 keV. The average number of photoelectrons produced per keV radiation energy was determined to be 4.457 phel/keV and the energy resolution at 59.6 keV was 17%. Introducing in the same way such a thin YA1[O.sub.3](Ce) plate inside an imaging silicon pixel array (ISPA) tube would improve its spatial resolution, since it depends on the thickness of the X-ray detector. Index Terms--29.30 spectroscopic techniques, 29.40 radiation detectors, 85.60.H photodetectors, hybrid photon detectors, YAP(Ce).

The imaging silicon pixel array (ISPA) tube is an opto-electronic camera with single photon detection capability at room temperature and ~ 1 Mhz detection rates. To further improve the ISPA spatial resolution, its radiation entrance window is made of scintillating YAl[O.sub.3](Ce), which serves at the same time as its gamma detector. The photocathode is coated at its vacuum surface and the escaping photoelectrons are guided by a parallel electric field (proximity focusing) toward a pixelized silicon anode. With this new window configuration, we achieved 0.25 mm ISPA spatial resolution with 0.35 mm collimator holes. This agrees with 0.22 mm expected spatial resolution, deduced from the evaluated hit-number of photoelectrons on the anode pixels and the photoelectron cluster residuals. Index Terms--29.30 spectroscopic techniques, 29.40 radiation detectors, 85.60.H photodetectors, 87.59 medical imaging, hybrid photon detectors, ISPA tube.

An Imaging Silicon Pixel Array (ISPA)-tube with nunfiber optic quartz window has been tested with respect to spatial and energy resolutions. The 122-keV [Gamma]-quanta of a [Co.sup.57] source was converted in a planar YAP-disc or in optically separated YAP-pillars of 0.6 x 0.6-[mm.sup.2] or 0.3 x 0.3-[mm.sup.2] cross sections. The achieved intrinsic spatial resolution is worsened when compared to the ISPA-version with fiber optic glass window but is still in the submillimeter region. As the YAP emission spectrum is fully covered by the spectral transmission of the quartz window and well adapted to the photocathode quantum efficiency spectrum, the resulting improved energy resolution allows for triggering of ISPA-events within a selected energy window at the 122-keV total absorption peak. This mode of operation, since it strongly reduces the number of unwanted events, is essential for the application of a - camera in environmental conditions where a wider energy range from other background sources may be present.

Several articles have been reviewed on the scintillation response of NaI(Tl). Those publications which report results obtained with radioactive sources either dispersed throughout the NaI(Tl)-lattice or located inside the wells of NaI(Tl)-crystals match within a [+ or -]0.9% standard deviation of a linear scintillation response between 3 MeV and 3 keV. Several authors who placed their sources outside the crystals measured nonlinear responses of NaI(Tl) with up to [+ or -]7% of standard deviations between 700 and 4.5 keV. The presented analysis suggests that the observed nonlinearity is not an intrinsic effect of NaI(Tl) but could be caused, at least partly, by effects occurring at the radiation entrance surfaces of the crystals. Index Terms - Gamma spectroscopy, inorganic scintillators.

We have developed a new type of beta camera based on an imaging silicon pixel array (ISPA) tube combined with planar plastic scintillators or with Si[Y.sub.2][O.sub.5](Ce) scintillating powder. The ISPA tube consists of a photocathode viewed at 3 cm distance by a silicon anode divided into 1024 rectangular (75 [[micro]meter] x 500 [[micro]meter]) detector pixels, each bump-bonded to its equally sized electronic pixel. Depending on the beta detector thickness we achieved spatial resolutions (FWHM) between 105-[[micro]meter] (63Ni source and 30-[[micro]meter]-thick plastic scintillator) and 240-[[micro]meter] (90Sr-90Y source and 120-[[micro]meter]-thick plastic scintillator) by covering the detectors with brass templates. With their four 60-[[micro]meter]-wide silts oriented parallel to the long pixel edges, we simulated small-sized beta strips. The impact of detector thickness is explained by multiple scattering, angular aperture of the template slits, and scintillating light distribution at the ISPA photocathode. Beta detection sensitivities were measured with calibrated 3H (tritium)- and 14C-Amersham microscale sources. They amount to 0.1 Bq (3H) with 150 min counting time and to 0.025 Bq (14C) with 180 min counting time.

This paper presents our research and development work on particle tracking with scintillating fibers. We have developed new fiber dyes, more efficient fiber cladding, coherent fiber bundles with improved packing fraction, and a new fiber readout technique (ISPA-tube). Altogether, these new developments increased the hit density of fine grain (60 [[micro]meter]) fibers by about seven times. This results in mini-tracks per 2.5-mm fiber layer rather than in single hits only and enhances the track reconstruction efficiency to nearly 100%. Compared with competing tracking methods (silicon strips, micro strip gas chambers), our scintillating fibers are superior in hit numbers per radiation length and in the two-track resolution. They require much less readout channels and consequently no cooling provisions to remove their electronic heat.

The impact of photoelectron backscattering on spectral distributions measured with Hybrid Photodetector Tubes has been calculated. The calculations are based on the backscattering coefficient [micro], the average number of photoelectrons [[bar]N.sub.phel] emitted from the photocathode, and on the distribution of the fractional photoelectron energy q absorbed in silicon during the backscattering process. We obtained the following results: the average number of absorbed (measured) photoelectrons [[bar]N.sub.meas] in the silicon anode amounts to ~88% of the incident [[bar]N.sub.phel]. Photoelectron- and gamma-absorption peaks are broadened by a factor 1.043 due to backscattering. As an example, for Photo Multiplier Tubes, this broadening can amount to an average factor of 1.18 due to statistic and gain fluctuations on the dynode chain. Index Terms--Backscattering, hybrid photodetector tubes.

A YAP(YAl[O.sub.3]: Ce)-scintillating window, coated on its inner surface with an S20-photocathode, seals a cross-focusing hybrid photomultiplier tube (HPMT) equipped with a small p-i-n anode of 2-mm diameter. This new radiation detector separates X-ray lines down to about 2-kev peak energy from the HPMT noise. Its detection efficiency for high gamma energies depends on the YAP-window thickness and amounts to about 18% attenuation at 400-kev energy in the present version. Competitive radiation detectors like silicon photodiodes and silicon drift chambers are discussed and compared to our prototype, with particular attention given to their energy resolution and noise performance, which limits their active area considerably. Index Terms--Hybrid photodetector tubes, scintillating crystals, X- and [Gamma]-ray detection.

Presents a study that reported an imaging silicon pixel array (ISPA) tube equipped with a scintillating YAP(Ce)-radiation entrance window. Description of an ISPA; Classifications of hybrid photon detectors; Discussion on where the ISPA contribution comes from.

Presents a study which calculated the impact of photoelectron backscattering on spectral distributions measured with Hybrid Photodetector Tubes. Overview of the backscattering process; How backscattering affects the energy resolution; Conclusion.