The new scintillation detectors, CsI(Tl) + Si PIN photodiode, were developed in the Kurchatov Institute. The current state of the developed spectrometric probes are large volume (10-300cm3) scintillation detectors intended for gamma-ray spectroscopy. These detectors consist of a spherical CsI(Tl) crystal packed in MgO powder within a cylindrical aluminum housing and a Hamamatsu photodiode. Environmental devices based on the above probes uses methods of field spectrometry. They include spectrometric electronics and algorithms for spectra processing to obtain particular characteristics of environmental radioactivity or radioactive contamination. The description of measurement techniques used with devices, their performance parameters and results of their practical applications are presented.

This paper presents details of the design and performance of three prototype Scintisphere detectors for use in gamma-ray spectroscopy. In these detectors, spherical CsI(Tl) scintillation crystals, having diameters ranging from 3.4 cm (20 cm/sup 3/) to 8.3 cm (300 cm/sup 3'/), were polished and packed in dry MgO powder. The scintillation light from the crystal was viewed using a single 1 /spl times/ 1 cm/sup 2/ silicon PIN diode. A low-noise preamplifier was also integrated within the detector housing. The measured noise level was equivalent to /spl sim/800 electrons (FWHM). Such a configuration results in a relatively high light-collection efficiency. For example, in the 300 cm/sup 3/ (8.3 cm diameter) detector, an average of /spl sim/10 electron/hole pairs were generated for each keV deposited in the crystal. One of the key features of this detector design is that it minimizes the spatial variations in the light-collection efficiency throughout the detector. Compared with a standard 3-in NaI(Tl) scintillation counter, this leads to a much-improved energy resolution, particularly for photon energies above 1 MeV The results presented in this paper clearly demonstrate that these spherical CsI-photodiode detectors could be used as an ideal replacement for the standard 3-in NaI(Tl) detectors in many applications.

This paper presents details of the design and performance of a prototype large-volume scintillation detector used for gamma-ray spectroscopy. In this detector, a spherical CsI(Tl) scintillation crystal having a diameter of 5.7cm was polished and packed in dry MgO powder. The scintillation light from the crystal was viewed using a single 1×1cm2 silicon PIN diode. A low-noise preamplifier was also integrated within the detector housing. The measured noise level was equivalent to ∼800 electrons (FWHM). Such a configuration provided a very good light collection efficiency, which resulted in an average of 20 electrons being generated per keV of energy deposited in the crystal. One of the key features of the detector design is that it minimises spatial variations in the light collection efficiency throughout the detector. Compared with a standard 3in. NaI scintillation counter, this feature leads to a much-improved energy resolution, particularly for photon energies above 1MeV. The results presented in this paper clearly demonstrate that a spherical CsI-photodiode detector could be used as an ideal replacement for the standard 3in. NaI detectors in many applications, but especially when the incident gamma-ray spectra extend up to ∼10MeV as in neutron-activated gamma-ray analysis.

In this paper, we present the design and the results of a feasibility study of a novel, ultra-fast and high-resolution PET detector. It is based on the use of an LSO crystal array read-out by a Multi-pixel Hybrid PhotoDiode (M-HPD) using an encoded fiber light-guide. The fiber encoding method enables one to readout more than 400 discrete crystal elements using a single M-HPD tube having 61 pixels. One of the key features of the detector is that the encoded fiber light-guide, used in conjunction with a digital readout system, eliminates the need for the use of ADCs in finding the address of the crystal in which interaction occurred. The readout time of a detector comprising around four hundred 2x2x10mm discrete crystals, defined as the time between identifying a coincidence and providing the address of the crystal hit, can be reduced to less than 0.1mus. As a consequence, one could significantly improve the noise-effective-count-rate (NECR) performance of a PET system based on this detector design. Another advantage of this detector design is that the parallel readout scheme used, greatly simplifies the readout electronics by eliminating the use of ADC in the readout system. The feasibility of this detector design has been confirmed by measurements using a prototype detector module based on a 5x5 array of 2x2x10mm LSO crystals. A signal level of around 200 photoelectrons has been measured for 511keV energy deposited in the detector. An energy-resolution of similar to30% and a timing resolution of less than 4 ns were achieved in this study.

Recent developments in the application of maximum likelihood expectation maximisation (MLEM) techniques to the deconvolution of low-resolution gamma-ray spectra have demonstrated a substantial improvement in both the energy-resolution and sensitivity achievable with the industry-standard 3" NaI scintillation counter. This technique can be used to determine the specific activities of the radioisotopes present in low-activity materials. In this paper, a practical application of this method for measuring the cement content of concrete in real-time is presented. However, the technique can be applied in many other fields

In this paper, the performance characteristics of a small prototype gamma-camera system based on the use of the recently developed Multi-pixel Hybrid Photodiode (M-HPD) are described. A compact read-out system has been developed to record the signals from all 61 pixels in the device. Single photo-electrons can be detected in each pixel. This provides an excellent means for calibrating the signal detected in each pixel in terms of the number of photo-electrons. The position-resolution of the detector has been measured as a function of the number of photo-electrons by using a finely collimated beam of light. The spatial resolution (FWHM) was measured to be 0.5mm for a signal level of 100 photo-electrons. The M-HPD has also been used in a miniature Anger-camera system to locate the position of interaction of gamma-ray photons in a scintillator. The light-pool generated by each event in a continuous scintillation crystal is spread in the 2mm thick entrance window of the M-HPD so that the photo-electron cloud is detected by a small cluster of anode pixels. The light-spread information has been measured and is compared with optical Monte Carlo simulations.

The results of a feasibility study for a high-resolution PET detector system are presented. The detector is based on the use of etched LSO scintillation crystals viewed from just one end. The authors have demonstrated that this configuration yields Depth-of-Interaction information whilst also preserving good energy and coincidence-time resolution. Prototype detectors have been built using 6/spl times/6 crystal arrays coupled to 61-pixel hybrid photodiodes. The measurements demonstrate a DOI resolution of 5/spl sim/7 mm in a 30 mm long detector. The signal level was measured to be in the range 250 to 600 photoelectrons depending on the location of the 511 keV energy deposit in the crystal. A resolving-time of less than 5 ns was also achieved. The technique depends on the assumption that all of the events detected above a selected threshold value are, in fact, full-energy deposits. The validity of this assumption and the contribution from photons scattered in the object, have been carefully studied using a well-calibrated Monte Carlo simulation of the proposed small animal PET imager.

Recent measurements of the performance of the newly available multi-pixel hybrid photo-detector (M-HPD) have demonstrated their particular value in the detection of very low light-level signals. The single and multiple photo-electron response characteristics of these devices is unmatched by any other room-temperature device. This characteristic, coupled with their speed of response and the availability of an internally-generated trigger signal when any one of the pixels detects an event, makes them well suited for the detection of minimum ionizing particles (MIPs). These detectors may therefore be expected to find application in particle-tracking applications both in High-Energy Physics and Gamma-ray Astronomy. The result of tests made using single and double-clad plastic fibres have confirmed the usefulness of these devices. The technique used to read-out 61 channels of data is described along with a way of viewing as many as 2000 fibres with just two 61-pixel M-HPDs.