Determination of 242Pu abundance in plutonium samples using self-fluorescent X-ray emission

Non-destructive gamma spectrometric measurements of the isotopic composition of plutonium are routinely carried out in the framework of nuclear safeguards inspections. High-resolution spectra of Pu obtained from planar Ge detectors are de-convoluted by means of special codes such as MGA or FRAM and these results are then applied when determining the total plutonium mass using neutron coincidence measurement techniques. The radiation emission of Pu-242 is so weak that its abundance cannot be measured directly; hence isotopic correlations are utilized to estimate it in low and medium burn-up Pu material. The present work investigates the feasibility of determining the total Pu/Am ratio, exploiting X-ray fluorescence lines present in the plutonium spectrum. Combining the total Pu/Am ratio with individual Pu isotopic ratios to Am-241 then allows an estimate of the Pu-242 to be made. To this end, reference samples of plutonium, with an Am-241 content comprised between 5 and 7 %, were measured using a planar germanium detector. The spectra were processed using MGA followed by quantification of the X-ray fluorescence peak of Americium, and then further calculations, including MCNP simulations. The conclusion of the present work is that, when the americium content of the sample is high enough to produce an Am K-alpha 1 line which can be exploited, it is possible to indirectly determine the Pu-242 abundance by analysing the X-ray fluorescent lines present in the plutonium gamma spectrum, thus raising the prospect to apply this method to cases for which the algorithms based on empirical correlations cannot be applied.