Assessment of M X-ray line energies of 237Np emitted by 241Am

Complete text of publication follows. In the course of a systematic investigation to accurately characterize the photon spectrum of 241Am in the 2-130 keV energy range in order to use it for simultaneous energy, resolution and efficiency calibration of X-ray detectors, we have observed significant deviation of the energy values of certain Np L and practically all M X-ray lines relative to literature data on elemental 237Np. Our experimental data was taken with two liquid nitrogen cooled Si(Li) detectors and one Peltier cooled Si SDD detector having energy resolutions between 145-160 eV, energy linearity within ±2 eV and each having been carefully characterized for their response in the energy range of interest beforehand. The 241Am source used was a commercial thin α-type source, supposedly in the form of 241AmO2, deposited on a stainless steel backing. This had the advantage of simultaneously acquiring accompanying fluorescent X-ray lines from Cr, Fe and Ni which could serve as built in energy references, but unfortunately Cr K lines masked the highest energy Np M-O transitions. While Np L X-ray lines from 241Am are somewhat better studied, data on M X-rays are rather scarce. Therefore spectra were first fitted using transitions identified during a high resolution crystal spectrometer measurement of EPMA excited M-lines from 237NpO2. Such fits using Lorentzian widths from resulted in energies and Gaussian widths well in excess to, and our prior calibration values, respectively. In the next approach, therefore, experimental Gaussian width values were used, and for the three most intense transitions allowance was made for the presence of high energy satellite lines with parameters - for the sake of simplicity - identical to those of diagram lines. Results similar to the one in the figure, taken with a Si(Li) detector, were consistently obtained with other detectors too. Satellites (distinguished by shading in the figure) have intensities ∼1/3 of corresponding diagram lines and are shifted ∼70 eV upwards, whereas diagram line energies are still ∼9 eV up relative to, compared with ∼24 eV before, without satellites. Note that contrary to intershell transitions, the L1L3 intrashell transition of Np showed neither energy shift, nor extra Gaussian widening larger then ∼ ±2 eV each, therefore no need was for the inclusion of satellites, similar to neighbour Cr, Fe, and Ni K lines mentioned above. Although the resolving power of the detectors used, combined with the statistics acquired has made it possible to unambiguously identify the presence of high energy satellites of the main Np M lines during the α-decay of 241Am, more favourable experimental condition and theoretical explanation are necessary to get details of the physics behind. (author)