Title: Gamma-rays from a 241AmO2 Source in an Al2O3 Matrix
Publisher: Publications Office of the European Union
Publication Year: 2011
JRC N°: JRC64615
ISBN: 978-92-79-20244-5 (print), 978-92-79-20245-2 (pdf)
ISSN: 1018-5593 (print), 1831-9424 (pdf)
Other Identifiers: EUR 24818 EN
OPOCE LA-NA-24818-EN-C (print), LA-NA-24818-EN-N (pdf)
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC64615
DOI: 10.2787/43520
Type: EUR - Scientific and Technical Research Reports
Abstract: Americium is a minor actinide making an important component of high level nuclear waste. A considerable number of studies have been performed or are ongoing to determine cross sections for neutron-induced reactions on 241Am. Recently, two measurements of the neutron-induced capture reaction on 241Am were performed at the n_TOF facility of CERN. One of these measurements used the C6D6 detectors, the other used the BaF2 calorimeter. In both cases, a sample from IRMM was used that had been prepared at ITU [1]. This sample consisted of 241AmO2 which was dispersed in a matrix of Al2O3. The material was pressed into a disk, calcined and enclosed in an aluminium container. It contained about 40 mg of 241Am. The samples had been prepared for measurements of the 241Am(n,2n)240Am reaction cross section [2]. Further details about the sample and these measurements may be found in [1,2]. During the measurements at CERN it was noted that several high energy gamma-rays were emitted by the sample. This presented the question as to the exact energies and origin of these gamma-rays. For this purpose the sample was returned to IRMM and gamma-ray spectroscopy with a high purity germanium (HPGe) detector was performed. The energy and origin of most gamma-rays was determined in this way. Here we report about these measurements paying attention only to gamma-rays that are not known from the decay of 241Am [3] and to the gamma-ray energy range from 844 keV to 13 MeV. There are two mechanisms leading to gamma-ray emission. First there is the natural activity of 241Am and the three known actinide impurities: 237Np (0.021), 233-236,238U (0.000094) and 239,240Pu (0.0017; fractions by weight). Of these 241Am dominates the spectrum, even after applying absorbers to completely stop the 59 keV transition. From the main impurity, 237Np, no gammas are found but there are those of its daughter, 233Pa. For the other actinide impurities and their descendants no gamma-rays were found in the measurement. The second source of gamma-rays are alpha-induced reactions. For energies below the maximum alpha energy of 5.485 MeV, Q-values, thresholds and main characteristic gamma-rays are given in table 1 for the likely candidate reactions. Reactions conclusively identified are 27Al(alpha,alpha’gamma)27Al, and 27Al(alpha,p)30Si and these explain nearly everything besides the 241Am and 233Pa gammas already discussed. There is a clear indication for the 27Al(alpha,n)30P reaction, but for the 27Al(alpha,gamma)31P reaction the evidence is not conclusive due to an overlap with gammas from 30Si. No evidence was found for alpha-induced reactions on the isotopes of oxygen. The measurements are described in the section Experiment. A table with gamma-ray energies and figures with the gamma-ray spectra are given in the section Results. The origin of these gammas is indicated there as well. Only three gamma-rays remain unattributed. A spectrum taken at CERN with a germanium detector showing many additional lines cannot be confirmed. Most likely this was taken under very poor background conditions.
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