The Next Generation Tomographic Gamma Scanner

ABSTRACT

The goal of Non-Destructive Assay (NDA) of radioactive waste is to accurately identify and quantify the radionuclides present in the waste stream. Gamma ray scanning instruments such as the Segmented Gamma Scanner (SGS) have found widespread use in the NDA community. The Tomographic Gamma Scanner (TGS) aims to improve the accuracy of assay results for difficult cases such as a non-uniform distribution of radioactivity in a heterogeneous matrix. The TGS combines high resolution gamma ray spectrometry with 3-dimensional single photon attenuation coefficient images (Transmission image) and single photon Emission images.

CANBERRA developed its prototype TGS in collaboration with Los Alamos National Laboratory (LANL). The prototype CANBERRA TGS was used as a Research and Development tool to conduct an in-depth study of the TGS technique. The results of these studies have been reported elsewhere.

Recently, CANBERRA has built its next generation of TGS systems implementing many advanced features. The new features include an automated variable collimator aperture, an automated detector slide mechanism for adjusting the sample to detector distance, automated attenuator mechanism, and an automatic selection of assay geometry based on the dose rate and density of the assayed sample. The collimator aperture is formed by interleaved layers of tungsten that can be opened and closed to change either the spatial resolution of the TGS measurement or to switch over to an SGS measurement mode. The automated slide mechanism allows the detector to move further back from the high activity drums extending the dynamic range of the system. Alternately, for use with small samples such as cans, the can can be placed on sample pedestal to allow the detector to move closer to the container for improved spatial resolution in either the SGS or TGS modes. The automated attenuator assembly is intended to allow system operation for drums with high surface exposure rates. The assembly is mounted to the front face of the detector shield and collimating aperture.

The paper discusses the features and performance of the next generation TGS.

INTRODUCTION

The Non Destructive Assay (NDA) of special nuclear materials in waste containers for inventory and safeguards purposes is challenging because of the potentially high degree of variability from item to item. This variability may allow the deliberate addition or removal of special nuclear materials to be made to waste containers to go undetected. These materials could be used in a clandestine nuclear weapons program. The Tomographic Gamma Scanning (TGS) technique combines transmission and emission reconstructive tomography techniques in an effort to improve the accuracy of high resolution gamma-ray spectroscopic measurements. In particular the goal is to reduce the uncertainties associated with the unknown material and density distribution of the waste matrix and also with the unknown distribution of SNM and other radioactive materials present. HRGS is retained in order to unravel the complex spectrum and in order to provide 'good geometry' results - that is peak areas which are relatively insensitive to coherent and small angle scattering

In addition to improving the quality of the assay results the intermediate transmission (linear attenuation map as a function of energy) and emission (by nuclide or line) images, although only of low spatial resolution, can provide powerful additional visual verification and confirmation information about the items.

Reconstructive tomography involves scanning the item in a series of layers or slices from many different orientations or views and using the information to reconstruct a picture of the interior of the object free from the interference effects from underlying and overlying planes. The idea is that if we know the attenuating properties of the interior along with the distribution of activity that a better matrix attenuation correction factor can be derived. The objective is to improve the accuracy of the assay while still retaining a useful detection limit and throughput at reasonable cost. Thus for a 200 liter drum, say, a representation of the item comprising 16 layers each of 88 volume elements (or voxels) is used. This contrasts with the familiar segmented gamma scanner approach which may use 8 to 16 segments, say, with each individual segment being treated as distinct but as homogeneous in matrix and uniform in activity (on a per unit mass of matrix basis). The TGS scan sequence generates a series of data grabs or views which over determine the contents of the voxel grids. Algebraic reconstruction in real space is used to extract a best fit solution consistent with the data and as free from spurious features as possible. The quality of the reconstructed images may be judged by the correspondence between the measured and actual distributions of test cases. The reconstruction model takes into account the changing collimation, attenuation and inverse square law affects at each view. These physical processes acting together provide the modulation and contrast in pattern of view data that allow the images, free from cross talk, to be formed.

The experimental realization of the TGS discussed here has been outlined elsewhere [^1-3]. Here we wish to concentrate on the next generation of TGS that has incorporated in it, many new features. These improvements make the instrument suitable for assaying waste streams with a wider range of matrices and activities. We include some preliminary results generated from the new system.