Jamie C. Adams

Determination of the Depth of Localized Radioactive Contamination by 137Cs and 60Co in Sand with Principal Component Analysis

A method to determine the depth of buried localized radioactive contamination nonintrusively and nondestructively using principal component analysis is described. The γ-ray spectra from two radionuclides, cesium-137 and cobalt-60, have been analyzed to derive the two principal components that change most significantly as a result of varying the depth of the sources in a bespoke sand-filled phantom. The relationship between depth (d) and the angle (θ) between the first two principal component coefficients has been derived for both cases, viz. d(Φ) = x + y log(e) Φ where x and y are constants dependent on the shielding material and the γ-ray energy spectrum of the radioactivity in question, and φ is a function of θ. The technique enables the depth of a localized radioactive source to be determined nonintrusively in the range 5 to 50 mm with an accuracy of ±1 mm.

Depth profiling 137Cs and 60Co non-intrusively for a suite of industrial shielding materials and at depths beyond 50 mm.

A phantom has been used to position two radiation sources, separately, when buried under dry-silica sand at depths between 5 and 50 mm. A γ-ray energy spectrum was then measured at every 1 mm depth. Principal component analysis has been conducted, which has led to a non-linear fit being established, allowing the depth of entrainment to be accurately inferred. The technique has been expanded for additional shielding media: water, aggregate and both wet and dry soil. The technique has also been expanded beyond the previous depth constraint of 50 mm.

The advancement of a technique using principal component analysis for the non-intrusive depth profiling of radioactive contamination

A non-intrusive technique using principal component analysis, to infer the depth of the fission fragment caesium-137, when it is buried under silica sand has been described. Using energy variances within different γ-ray spectra, a complete depth model was produced for a single caesium-137 source buried under 1mm depths ranging between 5–50 mm. This was achieved using a cadmium telluride detector and a bespoke phantom. In this paper we describe the advancement of the technique by further validating it using blind tests for applications outside of the laboratory, where not only the depth (z) but also the surface (x, y) location of γ-ray emitting contamination is often poorly characterised. At present the technique has been tested at the point of maximum activity above the entrained γ-ray emitting source (where the optimal x, y location is known). This is not usually practical in poorly characterized environments where the detector cannot be conveniently placed at such an optimal location to begin with and scanning at multiple points around the region of interest is often required. Using a uniform scanning time, the point of maximum intensity can be located by sampling in terms of total count rate, and converging on this optimal point of maximum intensity.

A phantom for research studies of radiologically-contaminated land

A phantom designed for the study of radiologically-contaminated land is described. The phantom comprises a bespoke, 1 m3 outer tank and an inner tube matrix for the deployment of γ-ray emitting radioactive sources. The phantom was filled to a depth of 900mm with dry sandy loam to mimic soils found in areas surrounding nuclear legacy facilities in the U.K. A series of non-intrusive spectral measurements were taken using an automated scanning rig and a cadmium telluride γ-ray detector. The detector was clamped at a constant height above the phantom, face-down and sequenced over a pre-defined grid above the phantom, using a scanning rig. A small caesium-137 source was positioned at a depth of 150mm in one of the tubes at random. Scans were then conducted at 30 minute intervals at twelve separate measurement points, to allow sufficient counting time to isolate the γ-ray caesium-137 662 keV photopeak from the background. A CAD model was then built of a simplified albeit geometrically correct representation of the phantom and the reading points. The combination of this model and the imported γ-ray spectra has enabled the inverse modelling technique N-Visage™ to be validated against the accurate surface location for the source without prior knowledge.

The effect of high-scatter shielding geometries in validating the dose inferred by N-Visage.

The aim of this paper is to further validate the physical capability of N-VisageTM under more challenging shielding geometries, when the number of mean free paths is greater than one. N-VisageTM is a recently established technique developed at REACT Engineering Ltd. The software locates radionuclide sources and contours radiation magnitude. The N-VisageTM software uses a geometric computer model combined with measured spectra. The software is able to estimate source locations through shielding materials by using mass attenuation coefficients to calculate the number of unscattered gamma photons arriving at the detector, and build-up factors to estimate scatter contribution to dose rate. The experiments described in this paper were carried out in a high-scatter environment using cobalt-60 and caesium-137 sources, these two sources are the primary sources of radiological contamination found in the nuclear industry. It is hoped that this will further assist in the identification, characterisation and removal of buried radiologically contaminated waste.

Inverse radiation modelling for plant characterisation

A requirement to actively manage radiation dose is often the biggest single constraint in activities associated with the operation and decommissioning of nuclear facilities. Often this management involves the minimisation of accrued dose for employees and contractors who work in the vicinity of the managed area. An important tool for evaluating the effectiveness of remediation of this dose is environmental modelling; however results are often inaccurate or misleading due to poor characterisation of the underlying source activity. In this paper we present a method for semi-automatically calculating source terms using an inverse modelling approach. This approach has been used as a basis for a new software package called N-Visage™ which has been used to demonstrate the performance of the method in both controlled experiments and real-world applications.