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Determination of the alpha emission probability of plutonium-240

04.01.2010

The radionuclide plutonium-240 (240Pu) is produced in nuclear reactors by means of multiple neutron capture of uranium-238 (238U) over plutonium-239 (239Pu). Less than 6 ppm of the 240Pu atoms are subject to spontaneous fission, so that the emission of alpha particles with a simultaneous formation of uran-236 (236U) represents the main decay process. The alpha particles occurring thereby display eleven energy levels between 4.2 MeV and 5.2 MeV having different emission probabilities.

These nuclear data are required to be increasingly precise, as, ever since nuclear energy has been in use, slight traces of 240Pu have been found in the environment. The data are also used in nuclear technology for the purpose of quality measurements.

In 2009, PTB’s Working Group "Unit of Activity", together with four partners of the European Association of National Metrology Institutes (EURAMET), completed Project 749. This project was launched in order to determine as many alpha emission probabilities as possible directly from the measurement of the alpha particles and to, thus, clarify discrepancies which have occurred in the past.

A 240Pu source made available by the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium) - the initiator of the project - was measured at PTB by means of an alpha-sensitive silicon detector in a vacuum chamber at one millionth atmospheric pressure (0.1 Pa). In this way, we obtained information on approx. 360 million decays of 240Pu atoms over a measuring period of 100 days.

The information which was available in the form of spectra now had to be analysed. For this purpose, it was necessary to extend existing PTB software for spectral deconvolution for this particular case in which over 20 alpha peaks of low quantities of alpha-emitting impurities present in 240Pu had to be taken into account. Furthermore, the influence of conversion electrons emitted coincidentally with the alpha decay on the measurements had to be investigated and quantitatively described.

Our work led to the determination of four alpha emission probabilities with clearly lower uncertainties than those indicated by the Decay Data Evaluation Project (DDEP) and is about to be published. The final result of the EURAMET project has already been presented at the International Conference on Radionuclide Metrology and its Applications (ICRM 2009) and will also be published.