PC/FRAM: a code for the nondestructive measurement of the isotopic composition of actinides for safeguards applications

The Nuclear Safeguards Program at the Los Alamos National Laboratory has developed and fielded techniques for the gamma-ray spectrometry measurement of the isotopic composition of plutonium and other actinides for over 20 years, ever since Parker and Reilly first proposed a practical method for the measurement of the arbitrary sample. Their procedures, incorporating internal (to the measured gamma-ray spectrum) or ‘intrinsic’ self-determination of the relative efficiency function of the sample-detector measurement system, are widely applied today. The PC/FRAM code is the most recent and most highly developed Los Alamos code developed specifically for the nuclear safeguards applications of this technique. We will describe the measurement principles that allow accurate measurements to be taken on samples of arbitrary size, shape, and measurement geometry—and of arbitrary physical and chemical composition—through the use of known nuclear decay data (half-lives and branching intensities). Subsequently, we will describe the analysis methodology, which is driven by an easily edited parameter file that frees the user from dependence on a dedicated programmer for analyses of special cases. This methodology relies on internal gamma-ray peaks from the spectrum under analysis to self-calibrate the unknown spectrum for energy and peak shape (energy dependence of full width at half maximum (FWHM) and tailing parameters). The program uses these parameters to calculate response functions that are fit to the analysis peaks requested in the parameter file. The structure of the code and its Windows 3.1 user interface allows use with equal ease by the experienced spectroscopist or operator-level personnel in a working facility. The code has been applied primarily to the analysis of gamma-ray spectra from plutonium with the isotopic results used to calculate the parameters 240Pueff for interpretation of neutron coincidence counting assays and Peff (W/g Pu) for the interpretation of calorimetry measurements. These measurement combinations form the backbone of nondestructive assay measurements for materials control and accountability both domestically and internationally. We will present examples of the results of measurements of these parameters with PC/FRAM and discuss the wide range of materials measured, including uranium samples and samples with lead shielding.