D. Ramsden

An inter-comparison of three spectral-deconvolution algorithms for gamma-ray spectroscopy

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 optimisation of small CsI(Tl) gamma-ray detectors

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. >

The non-invasive inspection of baggage using coherent X-ray scattering

An inspection technique based on angular dispersive X-ray diffraction (ADXRD) has been developed to chemically specific detection of explosives for screening applications. The technique identifies explosives from benign materials using the characteristic Bragg features seen in coherently scattered X-rays. To implement this technique in a prototype screening system, a detector has been designed which, when coupled with the appropriate analysis algorithm, provides chemically specific material identification. Profile analysis has been performed using the Singular Value Decomposition (SVD) maximum likelihood technique. The detector system combines a custom designed collimator with an X-ray image intensifier tube and is capable of acquiring diffraction profiles from several independent volume elements (voxels) within the region of interest. The acquisition of spatially and energy resolved diffraction profiles for both benign and explosive materials using a separate cooled germanium detector, has allowed the data analysis algorithm to be optimised. Results from the prototype detector system show that explosives such as Semtex, RDX, and PETN have sufficient order to produce unique diffraction profiles, which may then be differentiated from benign materials.

Multi-channel readout of crossed-wire anode photomultipliers

Abstract 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.

Pixellated CsI(T1) arrays with position-sensitive PMT readout

Abstract The position and energy resolution characteristics of three scintillation detectors viewed by a 3 in. square position-sensitive photomultiplier tube have been measured as a function of photon energy. Pixellated detectors having a pitch that ranges between 1.5 mm and 3.5 mm have been studied. The FWHM of the distribution in measured positions was as little as 0.9 mm at 122 keV. In this case, the tube was read out using individual amplifiers to record the charge detected on each individual anode wire and the location found using a peak-fitting algorithm. Comparative measurements were also made using the conventional hardware-centroiding technique.

The application of pinhole and coded aperture imaging in the nuclear environment

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]3.5]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 12.8i. The pinhole diameter of 6 mm provides an angular resolution of &2i 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 &1i 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. ( 1999 Elsevier Science B.V. All rights reserved. PACS: 07.85.!m; 29.40.Me; 07.88.#y

A 5 inch diameter position-sensitive scintillation counter

Recent developments in photomultiplier technology have enabled the construction of 5-in-diameter position-sensitive scintillation counters. A 5-in-diameter hard X-ray imager has been built and a position resolution of 4.0 mm full width half maximum (FWHM) has been obtained at 122 keV using a standard 3-mm-thick, 125-mm-diameter NaI(Tl) crystal with a 100-mm-diameter exit window. The spatial resolution can be improved by making some simple changes in the crystal design. The results show that the light output and spatial resolution vary considerably over the surface of such a detector. In order to make the best use of the imaging capability of these tubes, individual photon events must be corrected both in energy and position. The results of a 300-point detailed survey of the characteristics of the position-sensitive detector demonstrate its overall performance. A thyroid phantom image was obtained using this imager. >

Portable wide-angle /spl gamma/-ray vision systems

The characteristics of two portable /spl gamma/-ray vision systems, which could be transported by a robot, have been explored and compared. The detector of the first system (CSPMT) consists of an array of 37 CsI(Na) scintillation crystals viewed by a single 5 inch diameter position-sensitive photomultiplier tube (PSPMT), while the second system (CSPD) employs an array of 40 CsI(Tl) scintillation detectors coupled to PIN silicon photodiodes. These devices are designed to operate in the energy range from 50 keV to 1.5 MeV, which encompasses most energies of /spl gamma/-ray radiation from the radioactive nuclides of interest to the nuclear industry. These systems have good angular resolutions of about 3/spl deg/ FWHM at the central field of view of 10/spl deg//spl times/10/spl deg/ or better when image reconstruction is employed, and coarser angular resolutions of about 10/spl deg/ FWHM elsewhere within a wide field of view of 50/spl deg//spl times/50/spl deg/. The energy resolution of both systems have been tested using individual detector elements, and the imaging performance of proposed full systems have been simulated using a prototype. Our results show that these devices should be good candidates for the next generation portable /spl gamma/-ray imaging systems. >

Images obtained with a compact gamma camera

Abstract A design for a compact gamma camera based on the use of a position-sensitive photomultiplier is presented. Tests have been carried out on a prototype detector system, having a sensitive area of 25 cm 2 , using both a simple pinhole aperture and a parallel collimator. Images of a thyroid phantom are presented, and after processing to reduce the artefacts introduced by the use of a pinhole aperture, the quality is compared with that obtained using a standard Anger camera.

Hybrid photodiodes in scintillation counter applications

Abstract 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.