Measurement of various U and Pu samples by gamma-ray spectrometry were performed at the Institute of Physics and Power Engineering to support physical-inventory-taking activities under the Joint US-Russian MPC and A Program. The resulting data was analyzed by several different methods which included Canberra's MGA9.63 (Pu and MOX analysis) and MGAU (U analysis), EG and G Ortec's MGA++ (Pu and MOX analysis) and U235 (U analysis), and FRAM v2.2 (U and Pu analysis) provided by Los Alamos. The results indicate that all of these codes are capable of performing the isotopic analysis adequately. However, some additional modifications may be required to permit better measurement of some of the more unusual components in the Institute of Physics and Power Engineering (IPPE) inventory to meet the demands of inventory-taking activities.

Isotopic measurements of items stored in shielded shipping containers presents a challenge to standard, nondestructive high-resolution gamma spectroscopy analysis. For example, some plutonium oxide material that will be shipped from Rocky Flats will be packaged in a combination of containers that places more than 12 mm of lead and 25 mm of steel between the material and the detector. This shielding effectively eliminates gamma rays below approximately 300 keV. Spectra were taken through simulated containers and analyzed using FRAM version 4.0 and a parameter set developed for use with highly attenuated items. The results indicate that 10% precision in measured 24°Pu content should be achievable with 2-hour measurements.

The gamma ray spectrum of plutonium contains measurable gamma rays ranging in energy from 60 keV to above 1 MeV. The FRAM gamma ray isotopic analysis code can analyze data from all types of HPGe detectors in this energy range typically using planar detectors in the energy range 60-210 keV or 120-451 keV and using coaxial detectors in the energy ranges 120-451 keV or 200-1001 keV. The statistical measurement precision depends upon the detector/energy range combination as well as the characteristics of the sample and any addition filters. In this paper we carry out the optimization of measurement precision for the important case of a multi-kg sample of low bumup PuO2 contained in a DOE 3013 Standard-compatible long-teml storage container.

We describe the new capability of and.present measurement results from the PC/FRAM plutonium isotopic analysis code. This new code allows data acquisition from a single coaxial germanium detector and analysis over an energy range from 120 keV to above I MeV. For the first time we demonstrate a complete isotopic analysis using only gamma rays greater than 200 keV in energy. This new capability allows the measurement of the plutonium isotopic composition of items inside shielded or heavy-walled containers without having to remove the items from the container. This greatly enhances worker safety by reducing handling and the resultant radiation exposure. Another application allows international inspectors to verify the contents of items inside sealed, long-term storage containers that may not be opened for national security or treaty compliance reasons. We present measurement results for traditional planar germanium detectors as well as coaxial detectors measuring shielded and unshielded samples.

The Gamma-Ray multi-group analysis code MGA developed at Lawrence Livermore National Laboratory has been widely used in the area of gamma-ray non-destructive plutonium assay. This plutonium isotopic analysis code de-convolutes the complicated, 100-keV x-ray and gamma-ray region to obtain the ratio of Pu isotopes. Calibration of the detector efficiency is not required, but is determined intrinsically from the measured spectra. The code can either analyze low-energy gamma-ray spectrum taken using a high-resolution HPGe detector for energies below 300 keV, or analyze the low-energy spectrum combined with a high-energy spectrum (up to 1 MeV) in the two-detector analysis mode. In the latter case, the use of two detectors has been mandated by the conflicting requirements: excellent resolution at low energies (characteristic of small planar detectors) with good high-energy efficiency (characteristic of coaxial detectors). Usually, a high-energy spectrum taken using a coaxial Ge detector will not provide sufficient energy resolution for 100-keV plutonium isotopic analysis, while the small planar used at low energies has inadequate high-energy efficiency. An optimized-geometry ORTEC HPGe detector has been developed which combines good energy resolution at 100 keV combined with acceptable high-energy ((almost equal to) 1 MeV) efficiency in a single detector. It has been used to gather spectra of both low- and high-energy regions of plutonium spectra simultaneously, for analysis by MGA in the two-detector mode. Five Pu gamma-ray calibration standard sources were used in this study of this special detector.