Influence Of Hpge Detector Energy Resolution On The Results Of Isotopic Analysis Of Plutonium By Mga And Fram Software Codes

For the uranium enrichment verification and plutonium isotopic composition determination, portable HPGe spectrometers accomplished by the isotopic analysis software codes MGA, MGAU and FRAM are commonly used by nuclear safeguards. The codes use absolute branching ratios of radionuclides and intrinsic relative detection efficiency curves for the deconvolution of spectral lines in the X-ray region of the uranium or plutonium spectrum. Therefore, the most important characteristic of HPGe spectrometers in the context of U/Pu measurements is the energy resolution of the detector, which basically defines the operability of the codes. Deterioration of energy resolution caused for example by the degradation of vacuum in the cryostat with a time, or because of the vibrations produced by the cryocooler, may significantly influence the quality of spectrometric data. The purpose of this work was to establish practical limits on the energy resolution of HPGe detectors that are still suitable for plutonium isotopic composition verification by nuclear inspectors. To evaluate the ability of MGA and FRAM software codes to operate outside the normal window of energy resolution and to define measurement uncertainties the following was done: a) Spectra of 4 plutonium reference standards were measured using planar HPGe detector having optimal energy resolution of 550-eV at 122-keV (680‑eV at 185.7-keV); b) Four spectra of Pu standards were measured with a planar HPGe detector at different amplifier shaping time constant to get the range of energy resolution from 550 to 850-eV at 122-keV; c) Equivalent spectra were simulated using Monte Carlo radiation transport code MCNP6 and were validated by the comparison with the real ones;d) Adjusting the energy resolution parameter in MCNP (GEB function of F8 tally) with an increment of 100-eV a set of spectra of Pu84 standard have been simulated covering the energy resolution range from 450-eV to 1750-eV at 122‑keV; e) Simulated spectra had been processed using isotopic analysis codes MGA 9.65 and FRAM 5.2, and measurement uncertainties have been determined. In the result, practical limitations on the HPGe detector energy resolution which is still suitable for the verification activities of the nuclear safeguards for the analysis of Pu have been described.