The non-destructive assay of materials through neutron-activation analysis requires the use of gamma-ray detectors which are both sensitive up to /spl sim/11MeV and able to provide an adequate spectral-resolution to identify the range of materials of interest to the user. However, in many such applications, it is not practical to use a cooled HPGe spectrometer and in a number of currently available systems, the manufacturers offer a variety of detector types appropriate to their particular type of neutron source and the range of elements that they wish to identify in the material to be analysed. In this paper, we examine the impact that our recently-developed spectrum-processing technology can have when applied to a range of NaI(Tl) and BGO spectrometer sizes. This information is useful in assessing which detector option is the most appropriate for a particular NAA application. The improved energy-resolution and lower cost of a NaI detector must be traded-off against the better stopping power of BGO and its poorer light-yield. In this paper we present results from NaI(TI) and BGO detectors with sensitivities at high energies up to 5 times that of a standard 5/spl times/6" NaI detector.

A technique for searching for contraband materials that may be hidden behind a barrier is described. This imaging technique is based on the detection of one of the two annihilation photons produced by a positron-emitter that has scattered in the target, in coincidence with the detection of the second photon in a position-sensitive scintillation counter. The results of a feasibility study will be presented in which a 1mCi source was used to illuminate a 1times1 m 2 area to provide a two dimensional image of the line-of-sight density within this object. The spatial-resolution achievable is -1times1 cm. Details of the detector design envisaged for this application will be described and the images generated through Monte Carlo simulations presented.

Symetrica's spectrum-processing software improves both the spectral-resolution and sensitivity of scintillation spectrometers. This paper demonstrates the robustness of this algorithm when applied to spectra having poor statistical quality. The error in the location of the deconvolved 609keV full-energy peak, within a /sup 226/Ra spectrum; was less than 2keV for a peak area containing fewer than 150 counts. For the raw data, this accuracy was only achieved for peak areas greater than 1300 counts. Processing the spectra in this way also improves the accuracy, by factor of between 2 and 2.6, with which the full-energy peak areas could be measured. This paper has also shown that, by using the processed data, a hidden /sup 137/Cs source having a count rate at the level of just 2% that of a /sup 60/Co masking source, was always detectable in the limited number of spectrum recorded.

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.

EPSILON is a position-sensitive detector system for gamma ray imaging applications in the nuclear industry. This system is based on the use of a 3 degrees circular position-sensitive photomultiplier tube (Hamamatsu R2486) coupled to a segmented CsI(TI) array and is optimised for imaging photons in the energy range 60 keV to 662 keV. The detector when mounted on a pan and tilt head combines the flexibility of a scanning system with the sensitivity and high angular resolution of an imager.This paper describes the design and optimisation of the collimator, detection system and readout for imaging the radiation environment. The performance of the imager has been evaluated using Cs137 point sources at distances of 2 to 5 metres. The radiation scene may be mapped with a 0.8 precision across an 11 degrees diameter field of view. The sensitivity of the system and the improvements in image quality achievable by image processing are discussed.

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.

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. This data, used in conjunction with the measured energy-resolution and an estimate of the intrinsic energy-resolution of CsI(Tl), was 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 minimise the variance in the light collection. If the nature of this variance is understood sufficiently well, there maybe an opportunity to apply a similar post-processing technique to that which has dramatically improved the performance of other standard scintillation detectors. This possibility depends on the use of either the uniform quantum efficiency of the M-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.

Many different gamma-ray imaging systems have been proposed for, or used in the nuclear radiation environment for the localization of radioisotopes. Examples include the simple scanning collimator, pinhole cameras, radon transform imagers and coded aperture imagers. In this paper we present the results of a series of Monte Carlo simulations aimed at quantifying the relative merits of these quite different imaging techniques. In particular the paper compares image quality with respect to angular resolution and sensitivity for a range of test scenes which includes both point-like and extended/structured sources. Whilst one technique may be preferred for technical reasons such as size or mass constraints, the comparison presented here provides a valuable insight into which imaging system is optimum for certain observation scenarios.

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.

This work presents preliminary measurements taken with the first prototype Hamamatsu (R6970) 8-inch Position Sensitive Photomultiplier Tube (PSPMT). These measurements are compared with similar ones obtained using a 5 inch PSPMT (Hamamatsu R3292). The new 8 inch PSPMT has 12 dynode stages, 11 with a proximity mesh structure whilst the last one is a back reflector. The entrance window is 7.5mm thick and has an active area of 180mm diameter. The crossed-wire anode of the 8 inch tube consists of 36x36 wires on a 4 mm pitch. These are paired together to give an 18x18 wire outputs. Both PSPMTs were coupled to a 110mm diameter, 3mm thick CsI(TI) scintillating array in which each pixel has dimensions of 2 x 2 mm(2).Two read-out methods are compared in this paper. The first being the conventional resistive-divider technique. The second method uses a new multi-wire readout technique in which, the charge on each anode wire is individually read out and digitized. Measurements of the spatial resolution, position linearity, energy resolution and intrinsic charge distribution were carried out for both tubes using both read-out systems.Spatial resolution values of approximately 2 mm FWHM were obtained using the g-inch PSPMT and the multiwire read-out technique. The other measured characteristics were similar to those obtained using the 5-inch PSPMT. These results obtained using the prototype 8-inch PSPMT underline the potential of this detector in the field of imaging in Nuclear Medicine.

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.

This paper presents a comparison of three deconvolution techniques, Maximum Likelihood, Maximum Entropy and Linear Regularisation for the unconstrained deconvolution of gamma-ray spectra. These convert the raw energy-loss spectra obtained using a standard scintillation counter, into a good representation of the incident gamma-ray spectrum. This work is based on the use of an industry-standard 3/spl times/3 inch NaI detector. Both simulated and measured data have been deconvolved using the three algorithms to provide a direct comparison between the qualities of the deconvolved spectra. For applications in which it is important to derive an accurate estimate of the number of counts in a particular full-energy peak, the Maximum Likelihood Method has been shown to be superior.

The basis for a design of a high-resolution PET imaging system, utilising a wavelength-shifting (WLS) fibre readout technique is presented here. Scintillation light is transported from the individual LSO pixels in the detector array through the WLS fibres to a multi-pixel hybrid photodiode (M-HPD). Each detector array consists of a number of 2/spl times/2/spl times/10 mm LSO crystals viewed by two orthogonal ribbons of WLS fibres. An array of as many as 30/spl times/31 crystals can thus be read-out by a single, small 61-pixel M-HPD tube. The design is both compact, inexpensive and suitable for application not only in the field of small-animal PET or in a dedicated PEM system, but could also operate in a combined PET/MRI system. Initial tests have shown that the signal level generated at the detector by a single 511 keV energy-deposit, is 20-25 photoelectrons. Separate tests have indicated that, at this signal-level, a resolving time of /spl sim/25 ns could be achievable.

The excellent properties of the recently developed, multi-pixel hybrid photodiodes, will be easier to exploit following the development of the new hybrid-circuits described in this paper. This system will enable all of the required read-out functions to be accommodated on a single board into which the M-HPD is plugged. The design and performance of a versatile system is described in which a trigger-signal, derived from the common-side of the silicon anode in the M-HPD, is used to trigger the readout of the 61-anode pixels in the M-HPD. The multi-channel amplifier section is based on the use of a new, commercial VLSI chip, whilst the read-out sequencer uses a chip of our own design. The common anode signal is processed by a fast amplifier and discriminator to provide a trigger signal when a single event is detected. In the prototype version, the serial analogue output data-stream is processed using a PC-mounted, high speed ADC. Results obtained using the new read-out system in a compact gamma-camera and with a small muon tracking-chamber demonstrate the low-noise performance of the system. The application of this read-out system in other position-sensitive or multi-anode photomultiplier tube applications are also described.

This paper describes the design of a fast imaging system aimed at providing the accurate location of the centroid of a sentinel lymph node. Through the proposed use of a new 80mm diameter multi-pixel hybrid photo-diode, equipped with a fibre-optic window, a CsI(T1) scintillation crystal and a specially optimised collimator, we expect to achieve a precision of +/- 3mm within 20 s for the location of a lymph node. It has been assumed that the affected gland has a 10mm diameter and had accumulated 3 mu Ci of Tc-99 through drainage of the region of the breast. The paper discusses the likely imaging quality of the larger tube based on measurements made with a smaller prototype detector. The design of two collimators is presented; one to provide a rapid location of the centroid of the SLN and the other to provide the surgeon with it measure of the depth of the potentially infected organ. A method for relating the image information to the patient's anatomy to guide the surgeon is also presented.