Demonstration of a collimated in situ method for determining depth distributions using γ-ray spectrometry

In situ γ-ray spectrometry uses a portable detector to quantify radionuclides in materials. The main shortcoming of in situ γ-ray spectrometry has been its inability to determine radionuclide depth distributions. Novel collimator designs were paired with a commercial in situ γ-ray spectrometry system to overcome this limitation for large area sources. Positioned with their axes normal to the material surface, the cylindrically symmetric collimators limited the detection of unattenuated γ-rays from a selected range of polar angles (measured off the detector axis). Although this approach does not alleviate the need for some knowledge of the γ-ray attenuation characteristics of the materials being measured, the collimation method presented in this paper represents an absolute method that determines the depth distribution as a histogram, while other in situ methods require a priori knowledge of the depth distribution shape. Other advantages over previous in situ methods are that this method does not require multiple γ-ray emission energies, provides the capability for characterizing complex depth distributions. The disadvantages are that collimation adds weight to the in situ system and increases the counting time by reducing the detectorʹs field of view. Using a high-purity germanium spectrometry system, collimated in situ measurements of large area sources buried in attenuating materials were performed in laboratory settings and demonstrated the methodʹs capability to yield accurate depth information.