S.V. Guru

An imaging nuclear survey system

A combined video and gamma ray imaging system has been developed to rapidly determine the location, distribution, and intensity of gamma ray sources. This instrument includes both a conventional video camera and a gamma ray imaging system, which is based upon a position sensitive photomultiplier tube, a scintillator, and a pinhole collimator. The gamma camera records the position and energy of each interaction, determining the energy spectrum and count rate from each direction. The design of the instrument and results of preliminary field tests will be presented. We have used a prototype of such an instrument in preliminary field tests to image radioactive sources with gamma ray energies between 120 keV and 2.4 MeV. This new system achieves an angular resolution for the nuclear image of 60 with an efficiency of 3/spl times/10/sup -6/ at 1 meter, a performance suitable for many nuclear applications. The sensitivity of the system is sufficiently high that, in a low background environment, a 1 mCi /sup 137/Cs source at 5 meters can be located in <30 seconds. Alternatively, higher spatial resolution can be attained at lower efficiency and longer imaging times.

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. >

PORTABLE HIGH ENERGY GAMMA RAY IMAGERS

Abstract To satisfy the needs of high energy gamma ray imagers for industrial nuclear imaging applications, three high energy gamma cameras are presented. The RMD-Pinhole camera uses a lead pinhole collimator and a segmented BGO detector viewed by a 3 in. square position sensitive photomultiplier tube (PSPMT). This pinhole gamma camera displayed an energy resolution of 25.0% FWHM at the center of the camera at 662 keV and an angular resolution of 6.2° FWHM at 412 keV. The fixed multiple hole collimated camera (FMCC), used a multiple hole collimator and a continuous slab of NaI(Tl) detector viewed by the same PSPMT. The FMCC displayed an energy resolution of 12.4% FWHM at 662 keV at the center of the camera and an angular resolution of 6.0° FWHM at 412 keV. The rotating multiple hole collimated camera (RMCC) used a 180° antisymmetric rotation modulation collimator and CsI(Tl) detectors coupled to PIN silicon photodiodes. The RMCC displayed an energy resolution of 7.1% FWHM at 662 keV and an angular resolution of 4.0° FWHM at 810 keV. The performance of these imagers is discussed in this paper.

A portable gamma camera for radiation monitoring

A compact gamma camera capable of imaging high energy gamma rays (up to 2 MeV) is currently being developed for nuclear industrial applications. This camera uses a three inch square position sensitive photomultiplier tube (PSPMT) [Hamamatsu, R-2487-05] coupled to a NaI(Tl) scintillator. The incoming gamma ray direction is determined by a segmented collimator. The camera exhibits energy resolutions of 17% FWHM at 122 keV and 12% FWHM at 662 keV. With the collimator design used, an angular resolution at the source plane of 6/spl deg/ FWHM was measured for 412 keV. Images have been obtained using gamma ray sources up to 1 MeV using this system. The image reconstruction technique used is presented in detail. Corrections for the non-uniformities across the face of the PSPMT are implemented to improve the spatial and spectral information content in the final image. >

A high energy gamma camera using a multiple hole collimator and PSPMT

Abstract A high energy gamma camera using a parallel hole tungsten collimator and a 7.62 cm square Position Sensitive Photomultiplier Tube (PSPMT) has been assembled and tested. The measured energy resolution is 12–20% FWHM for 137 Cs (662 keV) gamma rays, and 17–23% FWHM for 57 Co (122 keV). The measured spatial resolution measured is 2.8–3.2 mm for 122 keV and 3.4–5.3 mm for 662 keV. The variation in the energy and spatial resolution is due to the continuously varying gain across the PSPMT. Measured energy spectra and images obtained are presented.

Monte Carlo modelling of a multiple-hole collimator for high energy gamma-ray imaging

To develop a second generation gamma-ray camera for industrial applications, various multiple-hole collimator designs that meet high-energy, high-intensity and far-field imaging requirements have been studied. The collection of parallel holes each parallel to one another (parallel-hole, parallel collimator) is ineffective for imaging high energy far-field sources, whereas a collimator with parallel holes arranged in a diverging manner (parallel-hole, diverging collimator) is a good candidate for our imaging system. The response of a proposed collimator to different source conditions is modelled and presented in this work. >

A position sensitive β-γ coincidence technique for multiplexed gamma spectrometry of many small samples

Abstract A technique based on β-γ coincidence has been developed to perform multiplexed gamma-ray spectrometry of small samples using a single gamma-ray detector and a position sensitive beta detector. A system is described that uses a position sensitive photomultiplier tube coupled to a thin plastic scintillator as the beta imaging detector. Multiplexed gamma-ray spectrometry is demonstrated by results obtained with this system for a 4 × 4 array of Au, Co and Ag samples. The advantages of this technique over gamma spectrometry on individual samples are the substantial reduction in total counting time and the reduction in background, which are especially significant in neutron activation analysis of particles.

Instantaneous flux measurements using the background signal of the rhodium self-powered neutron detector

Abstract The background current produced by a rhodium self-powered neutron detector (RSPND) with an integral background lead has been investigated to determine its relationship to the neutron flux seen by the detector. The background current and steady state flux were measured at the core midplane for all fuel assemblies, and good linearity was seen. Under transient conditions, the background current, when delayed fission product decay contributions are included, follows the flux reasonably well although with a slight lag. This opens the possibility of using the RSPND during transient conditions.

Portable wide-angle γ-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 70 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/, 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 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.<<ETX>>