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.

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.

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.

A gamma-ray imaging system optimised for use in the nuclear environment has been developed. The system allows the simple selection of either pinhole or coded aperture image formation techniques depending on the nature of the scene. The detector portion of the instrument is the same for both options and comprises a 3 " diameter PSPMT coupled to an array of CsI(Tl) crystals each with dimensions 3.5 x 3.5 x 25 mm on a 3.8 mm pitch. The detector is mounted within a tungsten shell, which provides at least 35 mm of shielding to sources outside of the FOV. The field of view for both imaging techniques is 12.8 degrees. The pinhole diameter of 6 mm provides an angular resolution of similar to 2 degrees at 662 keV. The coded aperture comprises a 127 hexagonal-URA produced by machining 3 mm holes on a 3.5 mm pitch in a 3 mm deep tungsten sheet and provides an angular resolution of similar to 1 degrees at 662 keV. Both imaging techniques have been used to view a range of radiation scenes to determine their relative performance. As expected the coded aperture showed better sensitivity in high background environments however for scenes with low and moderate background rates with several sources in the FOV the pinhole system performed better. The results suggest that a lightweight system, which allows both coded aperture and pinhole imaging, can provide excellent sensitivity and dynamic range coverage for a wide range of radiation scenes. (C) 1999 Elsevier Science B.V. All rights reserved.

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.

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.<>