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A large-area hybrid photodiode interfaced to scintillating crystals is being evaluated to determine its application as part of a positron camera dedicated to imaging the breast, positron emission mammography (PEM). The sensitive photocathode area of the HPD is 72 mm in diameter and the diode is divided into 61 separate pixels. The principal of the detector for PEM will be the use of analogue information from the individual diodes to provide 2D positional information. Simulations of the device attached to a low noise amplifier system show that a spatial resolution of a few mm should be possible. Estimates of the spatial resolution, sensitivity and energy resolution from different scintillating crystals detecting annihilation photons will be presented.

High-resolution scintillation spectrometers based on the use of large-volume spherical CsI(Tl) spectrometers viewed by conventional 1 cm/sup 2/ PIN diode have been shown to have excellent performance characteristics. In particular, their unique geometry, the small area of contact between the diode and the crystal's surface, and the high reflectivity of the packing material combine to provide a detector in which there is a very small variance in the signal generated by energy deposits in different regions of the crystal. In view of the unique spectral-resolution achieved using the first prototype Scintispheres (S100-PIN and S300-PIN) to be manufactured, it was natural to explore the boundaries within which this technology might be useful. This paper presents the results of a series of modeling exercises and experimental investigations aimed at finding the range of detector volumes that can usefully be constructed based on the use of PIN photodiodes. For a given amplifier performance, the predicted spectra-resolution of a range of Scintisphere volumes has been estimated as a function of energy. This study was made both for the CsI(Tl) material used in the original devices and for other materials which may be important when the speed of operation and stopping power are important design parameters. These studies also considered the performance limits that might be achievable in small volume Scintispheres. This paper also presents the results obtained when our deconvolution algorithm is applied to a wide range of standard scintillation spectrometers. These demonstrate the high spectra-resolution and sensitivity that can be achieved using this technique.

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 5.7 cm was polished and packed in dry MgO powder. The scintillation light from the crystal was viewed using a single 1 � 1c m 2 silicon PIN diode. A low-noise preamplifier was also integrated within the detector housing. The measured noise level was equivalent to B800 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 3 in. NaI scintillation counter, this feature leads to a much-improved energy resolution, particularly for photon energies above 1 MeV. The results presented in this paper clearly demonstrate that a spherical CsI-photodiode detector could be used as an ideal replacement for the standard 3 in. NaI detectors in many applications, but especially when the incident gamma-ray spectra extend up to B10 MeV as in neutron-activated gamma-ray analysis. r 2001 Elsevier Science B.V. All rights reserved.

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 (DOI) information whilst also preserving good energy and coincidence-time resolution. A prototype detector was built using a 4/spl times/4 crystal array coupled to 61-pixel hybrid photodiodes. The measurements demonstrate a DOI resolution of 5/spl sim/7 mm FWHM in a 30 mm deep 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 FWHM was estimated based on indirect measurements. 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 impact of the photons scattered in the object have been studied using a Monte Carlo simulation of the proposed small animal PET imager.

A design for a large area, position-sensitive gamma-ray spectrometer for use in imaging explosive materials is described. The design has been optimised for use in the energy range from 2 to 12 MeV. At 5 MeV, the spectral resolution of each CsI(Tl)-photodiode pixel is better than 3% FWHM. The multi-element detector system, when used in conjunction with a coded-aperture mask, is able to provide a “multi-colour” image of the scene when illuminated by a neutron source. The feasibility of using such a system to identify the unique elemental composition and location of the explosive materials is discussed.

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 0.5 mm 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.

This paper describes the results obtained from various scintillators when viewed by a Hybrid PhotoDiode (HPD) with several different scintillation crystals. The energy loss spectra produced by gamma-ray photons having energies in the range of 20 to 662 keV were measured using a commercially available HPD. An excellent single photo-electron response was measured using a low-noise preamplifier. Measurements of the number of photo-electrons produced and the energy resolution of the full-energy peak as a function of the gamma-ray energy and the acceleration voltage were made. Finally, optical Monte Carlo simulations were used to estimate the light loss in the entrance window and to optimise its geometry. The potential advantage of incorporating a BGO entrance window for some applications was also explored.

A pinhole camera based on the use of a 3 in. square position-sensitive photomultiplier (PSPMT) to view a two-dimensional segmented CsI(Tl) scintillation crystal has been designed to operate in the range 2 to 200 keV. It has also been used to generate extended test images by rotating two radioactive sources of different energies in the the field of view. The optical characteristics of the system have been simulated using a Monte-Carlo package in order to optimise the detector crystal geometry. The detector performance has also been investigated experimentally as a function of crystal dimensions, and measurements made using different readout techniques are presented. Also outlined is a design for an auroral imager based on the use of an array detector modules which will form part of the Auroral Imaging Observatory (AURIO), selected for inclusion within the payload complement of ENVISAT II. This will be located on a polar orbiting platform at an altitude of 800 km.

Use of 1 cm 2 CsI(T1) scintillators with photodiode readout in the hard X-ray band is currently limited to above ∼ 40 keV by the lack of intrinsic gain in the photodiode and the electronic noise in the readout system. However, below this energy the photodiode itself becomes a practical X-ray detector. We report on the use of a pulse-shape analysis technique in an attempt to separate the signals from direct X-ray interactions in the photodiode and from scintillation events in the CsI(T1). We have demonstrated that separation of the two types of event is certainly possible, and the use of an appropriate spectral reconstruction algorithm enables us to normalise the signals from the two detectors. In this way we have constructed a hybrid detector capable of operating in the energy range 10 keV to 1 MeV. We discuss, using both simulations and laboratory measurements, the optimisation which can be carried out by changing the thickness of the silicon photodiode. We present details of the pulse-shape analysis system, demonstrate the identification and separation of the two types of signal, and present energy spectra for a range of energies.

A detailed study of small, discrete, CsI(Tl)—PIN photodiode detectors has been carried out for the INTEGRAL project. Optical Monte Carlo simulations and noise calculations show that the maximum signal-to-noise ratio may be obtained by using photodiodes whose sensitive area covers approximately 50–60% of the scintillator output surface area. The measured performance for a range of photodiode sizes coupled to a 1 cm 2 scintillator has been compared with the results of the simulations.

A novel multichannel readout technique has been developed for a position-sensitive photomultiplier with a crossed-wire anode. A new charge-sensitive shift register has been used to record the charge collected by each of the anode wires. The performance of a position-sensitive scintillation counter as a function of photon energy and position has been studied using this new technique. A variety of algorithms for reconstructing the energy and position of each event using the individual anode signals are discussed and the potential benefits of this more complex readout system will be illustrated. Compared with the traditional centroiding technique, the individual anode readout method promises better spatial linearity over the entire photomultiplier sensitive area as well as a significant reduction in the low energy threshold.

A position-sensitive phoswich detector designed to cover a wide energy band from 15 keV to 1 MeV has been investigated. The detector concept is based on the use of an array of discrete 1 × 1 × 5 cm phoswich scintillation bars, the top 3 cm of which is CsI(Na) whilst the rear 2 cm is a GSO (Cerium-doped Gadolinium Orthosilicate Gd 2 SiO 5 (Ce), [1,2]) veto crystal. The array is viewed by a single position-sensitive photomultiplier tube (PSPMT). The test results show that energy resolution ranges from 31% FWHM at 60 keV to 13% FWHM at 511 keV. The FWHM of the point-spread function for the location of each event is smaller than the cross section of a single bar over the entire energy range. This guarantees good separation between energy deposits in adjacent bars. The outputs from the CsI(Na) main detectors and GSO veto crystals can be distinguished by a simple pulse-shape discriminator (PSD). Those multi-bar events which deposit energy only in the CsI(Na) array can be recognised using the position sensing capability of the tube. When used in conjunction with a coded-mask imaging system, an array of such detector modules could provide a competitive telescope for a future gamma-ray astronomy mission.

As part of the INTEGRAL (International Gamma-Ray Astrophysical Laboratory) project, CsI(Tl) scintillation elements with a cross-sectional area of 1 cm/sup 2/ and between 1 and 6 cm in length have been developed to form the basis of the imager detector plane. The crystal preparation and wrapping, crystal geometry, optical coupling, and matching to the photodiode have all been optimized in order to maximize the light output from the crystal, and hence produce the lowest energy threshold and best spectral resolution for any given readout electronics. Energy resolutions of 22% of 122 keV and 7% at 662 keV have been obtained using a 1 cm/sup 3/ crystal on a 10*10 mm crystal. With standard laboratory electronics, a low energy threshold of approximately 40 keV has been obtained. >

Imaging telescopes for hard X-ray astronomy require either the use of a spinning telescope to facilitate the use of a rotating modulation collimator or a good 2D position-sensitive scintillation counter. The background in such a space-borne detector can be significantly reduced if an anti-Compton veto crystal can be combined with the primary detector using a phoswich technique. The design and performance of a position-sensitive phoswich detector based on the use of a position-sensitive photomultiplier will be described.

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 2/spl times/2/spl times/10 mm discrete crystals, defined as the time between identifying a coincidence and providing the address of the crystal hit, can be reduced to less than 0.1 /spl mu/s. 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 5/spl times/5 array of 2/spl times/2/spl times/10 mm LSO crystals. A signal level of around 200 photoelectrons has been measured for 511 keV energy deposited in the detector. An energy-resolution of /spl sim/30% and a timing resolution of less than 4 ns were achieved in this study.

Arrays of small scintillation crystals are being used increasingly for high-resolution imaging applications in nuclear medicine. Although the degree of pixellation now available is high for some scintillation materials, this spatial resolution is often achieved at the expense of degraded energy resolution due to the lower, and more variable, light-collection efficiency. The energy resolution of a detector is, however, especially important in nuclear medicine, where events that have been scattered in the body need to be rejected efficiently. The light output from a range of CsI(Tl) arrays was measured in terms of the number of photoelectrons; detected, using a hybrid photodiode (HPD). This data, used in conjunction with the measured energy resolution and an estimate of the intrinsic energy resolution of CsI(Tl), were used to assess the magnitudes of the various contributions to the overall energy resolution of these detectors. This information suggested that there is an opportunity to improve their energy resolution by carefully choosing the geometry and reflector material to minimize the variance in the light collection. If the nature of this variance is understood sufficiently well, there may be an opportunity to apply a postprocessing technique similar to that that has dramatically improved the performance of other standard scintillation detectors. This possibility depends on the use of either the uniform quantum efficiency of the multipixel HPD photocathodes or a monolithic array of PIN diodes. This would ensure that no additional or indeterminate variance in the light collection is introduced. Measurements made using fine BGO arrays for higher energy applications and columnar grown CsI(Tl) for X-ray imaging will also be presented.

High-resolution scintillation spectrometers based on the use of large-volume spherical CsI(Tl) spectrometers viewed by conventional 1 cm/sup 2/ PIN diodes, have been shown to have excellent performance characteristics. In particular, their unique geometry, the small area of contact between the diode and the crystal's surface and the high reflectivity of the packing material, combine to provide a detector in which there is a very small variance in the signal generated by energy deposits in different regions of the crystal. In view of the unique spectral-resolution achieved using the first prototype Scintispheres (S100-PIN and S300-PIN) to be manufactured, it was natural to explore the boundaries within which this technology might be useful. This paper presents the results of a series of modelling exercises and experimental investigations aimed at finding the range of detector volumes that can usefully be constructed based on the use of PIN photodiodes. For a given amplifier performance, the predicted spectral-resolution of a range of Scintisphere/sup /spl reg// volumes have been estimated as a function of energy. This study was made both for the CsI(Tl) material used in the original devices and for other materials which may be important when the speed of operation and stopping power are important design parameters. These studies also considered the performance limits that might be achievable in small volume Scintispheres. This paper also presents the results obtained when our deconvolution algorithm is applied to a wide range of standard scintillation spectrometers. These demonstrate the high spectral-resolution and sensitivity that can be achieved using this technique.

A pinhole camera based on the use of a 3 inch square position-sensitive photomultiplier (PSPMT) to view a two-dimensional pixellated CsI(Tl) scintillation crystal has been designed to operate in the range 2 keV to 200 keV. It has also been used to generate extended test images by rotating two radioactive sources of different energies in the the field of view. The optical characteristics of the system have been simulated using a Monte-Carlo package in order to optimise the detector crystal geometry. The detector performance has also between investigated experimentally as a function of crystal dimensions, and measurements made using different readout techniques are presented. Also outlined is a design for an auroral imager based on the use of an array of such detector modules, for inclusion as part of the Auroral Imaging Observatory (AURIO), selected for inclusion within the payload complement for ENVISAT II. This will be located on a polar orbiting platform at an altitude of 800 km. >

A description of a prototype X-ray imaging system based upon a rotating one dimensional silicon microstrip detector is provided along with simulations of the detector's performance. Two-dimensional images have been reconstructed from a series of one-dimensional projections using a fast version of the Maximum Entropy algorithm. A one-dimensional silicon detector, having 256 strips with a pitch of 0.3 mm, is rotated in steps through 180 degrees in the image plane. The charge deposited in each strip for each angular position provides the series of projections from which the image may be reconstructed. Simulated images of a range of test scenes have been obtained at 256 by 256 resolution. The prototype detector described has an intended operating energy range of 5-30 keV.

A new position-sensitive gamma-ray detector to cover the energy band from 15 keY to 1 MeV has been investigated. The detector concept is based on the use of an array of discrete 1 x 1 x 5 cm phoswich scintillation bars viewed by position-sensitive photomultiplier tube(PSPMT). The test results show that the energy resolution, about 40%FWHM at 60 keY and 12% FWHM at 511 keY, can be easily obtained when using CsI(Na) crystals. The point-spread function for the location of each event is smaller than the cross section of a single bar over the entire energy range. This means that there is virtually no ambiguity in locating signals from different bars. The outputs from the CsI(Na) and GSO veto crystals can be distinguished by a pulse-shape discriminator designed especially for thissystem. Those multi-site events which deposit energy only in the CsI(Na) array can be recognised using the positionsensing capability of the tube. 1. INTRODUCTION In order to image hard X-rays, some telescopes have incorporated multi-wire proportional counters as the position-sensitive detector1 . Whilst their energy and position-resolutions are superior to that of scintillation detectors, their

Normal phoswich detector systems use a combination of NaI(Tl) and CsI(Na) scintillators and require the application of careful pulse-shape discriminator techniques to resolve the two components in the scintillation light output which have decay constants of 250 and 630 ns respectively. These techniques provide a good anticoincidence veto efficiency for a relatively narrow range in the ratio of energy deposits in the two crytals and for a detector system whose temperature is carefully controlled. This paper describes the performance of a simple phoswich which makes use of the fast UV signal from a BaF2 crystal to provide a prompt veto signal. The performance to be expected from various combinations of a BaF2 anticoincidence crystal with other primary detectors is presented. These simulations have been verified by simple experimental tests.

A balloon-borne hard X-ray telescope is described that has been designed to make highly sensitive observations of cosmic sources in the energy range 15 to 300 keV. The payload is characterized by a combination of NaI(Tl) based detector of a novel design and spectroscopic proportional counters, providing a total sensitive area of 5000 cm2 and a 1 standard deviation sensitivity at the level of about 4 × 10−6 photons cm−2 s−1 keV−1. All of the telescope sub-systems are described including the micro-processor based orientation platform that provides a pointing stability of better than 10 arc min. The physical characteristics of the detectors are included along with a summary of the telescope performance during a balloon flight in 1982.

A phoswich detector incorporating interleaved strips of NaI and CsI scintillator crystals has been used as the detection plane of a multipitch rotating modulation collimator (RMC) which has produced high-quality images of both multiple point sources and extended objects. The particular characteristics of the point spread function of such a telescope have been examined in some detail in order to optimise the design of a telescope for a possible future space mission.

A compact CsI (T1)/photodiode gamma-ray detector is described which is capable of locating the point of interaction of incident gamma-ray photons in the spectral region around 1 MeV. Laboratory tests are used to quantify both the spectral and positional resolutions of the detectors. Their likely application in space gamma-ray astronomy is also discussed.

A γ-ray telescope incorporating an acoustic spark chamber is included in the payload of the OGO-5 spacecraft. The performance of the instrument, which is sensitive to photons of energy 25 to 100 MeV, is discussed.