Pu Response Functions for Hand-Held NaI Radionuclide Identifiers

Hand-held gamma-ray spectrometers employing right cylindrical crystals of NaI(Tl) scintillator are used in search and identification applications. In this paper we consider the deployment of such a portable, medium resolution spectrometer for the detection and identification of plutonium. This is a challenge for spectral analysis because of the complex spectrum from the mixture of plutonium isotopes and americium present in these special nuclear materials (SNM). The prominent peaks cannot all be resolved. Self absorption in the material and varying shielding attenuation can severely distort the spectrum due to preferential absorption of the low energy lines and the build-up of scattered radiation. We attempt to address the problem experimentally by gathering spectra using a set of Plutonium Isotopic Determination Intercomparison Exercise (PIDIE) samples and a Canberra Inspector 1000 (IN1K) hand-held spectrometer. The PIDIE specimens comprise compressed pellets of PuO2 contained in stainless steel jackets. Seven items with varying isotopic mix cover a range of burn-up (BU) from low BU (characteristic of weapons grade material) with a 239Pu/Pu weight ratio of 94% to high BU (characteristic of reactor grade material) with a 239Pu/Pu weight ratio of 64%. The high BU material also has (by virtue of the age of the items) a high Am-241/Pu ratio (approximately 7%) and the associated 60 keV emission can dominate the detector response for lightly attenuated measurement configurations. The measurements were performed in a controlled area at Canberra’s Harwell facility and involved measuring each PIDIE sample with a variety of attenuators between the source and the detector. In this way the spectral distortion that might be anticipated by SNM hidden in a larger item or deliberate shielding could be simulated. A description of the experimental work is presented. The salient results are discussed from the perspective of applying the information in the database of measured spectra to the problem of reliable automated radionuclide identification by non expert users under field conditions. An effective spectral analysis is rooted in the development of carefully chosen library entries.