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Neutron Activation of Molybdenum Isotopes


The effective cross sections for neutron activation of diverse molybdenum (Mo) isotopes were measured in the neutron energy range between 7 MeV and 15 MeV. Due to its excellent material properties at high temperatures, it is envisaged to use molybdenum as a component of the alloy of materials used in modern nuclear plants. Compared to current nuclear reactors, the temperatures, mechanic strains, neutron energies and neutron fluxes that will occur in future, 4th generation fission reactors as well as in fusion reactors and in accelerator-driven systems, will be considerably higher. By improving the experimental data of Mo isotopes, the parameterization of nuclear modelling parameters can be developed systematically and theoretical predictions can be checked experimentally. What is particularly interesting about the integrity of structure materials is the production of hydrogen and helium by (n,p) and (n,α) processes.

These reaction channels were investigated along with the (n,2n) channels in an experiment performed at PTB’s cyclotron. The neutrons were generated in a gas target with the reaction D(d,n)3He and the neutron energy was determined by means of the time-of-flight method. The neutrons were detected by means of NE213 detectors at a distance of 12 m from the gas target. The disc-shaped samples of 1 mm thickness and 10 mm diameter were made of natural molybdenum with 99.9 % purity. The samples were mounted in front of a fission chamber at an angle of 0° to the deuteron beam. The distance to the gas target was 6 cm. The neutron fluence was monitored by means of a 238U fission chamber (mass of the 238U: 101.6 ± 0.6 µg, degree of enrichment: 99.98 %) and also by means of an aluminium foil positioned in front of the molybdenum sample.

For neutron energies above 8 MeV, it is particularly difficult to measure activation cross sections with the D(d,n) neutron source, since the energy distribution contains, besides monoenergetic neutrons from the D(d,n)3He reaction, also low-energy break-up neutrons which contribute to the measured reaction rates. At each energy, one measurement was performed with a filled gas target and another with an evacuated gas target (empty target), and the standardized empty-target measurement was then subtracted. The experimentally determined spectral distributions were compared with spectra calculated by means of the Monte Carlo code SINENA. This code contains a database for the double differential cross sections for the D(d,np) break-up reactions which have been determined at PTB in the course of a detailed investigation in the neutron energy range from 5.3 MeV to 13.3 MeV and for emission angles of up to 15°. The calculated spectral distributions were used to determine corrections for the activation caused by break-up neutrons. The effective cross sections required for this purpose for the 238U(n,f) and the 27Al(n,α)24Na reactions as well as for the investigated activation reactions on Mo were taken from existing evaluations.

The activity of the irradiated samples was measured by means of an HPGe detector with a volume of 300 cm3 and a detection probability of 70 %. To this end, the samples were positioned 15 mm from the entry window of the detector. By performing additional measurements at a distance of 135 mm, the summation effect could be determined. The comparison of the measured summation corrections with the calculated ones yielded an agreement in the range of 1 %. In general, the most intensive gamma lines were used for the evaluation. Only for the 96Mo(n,p)96Nb reaction measured with samples of natural isotopic composition, did an interference of the 778.224 keV gamma line from the decay of 96Nb with the 777.921 keV gamma line from the decay of 99Mo occur, so that another - less intensive - line had to be used.

All effective cross sections were determined as a function of the 238U(n,f) and 27Al(n,α)24Na standard effective cross sections from the ENDF/B-VII data library. Corrections for the time-dependent variation of the neutron flux, the geometry of the experiment, dead-time effects and summation effects were taken into account. As an example, the figure shows a comparison of the effective cross sections determined in this way with previous experimental data and current, evaluated data.

Figure : Preliminary results of the measured effective cross sections of the reaction 92Mo(n,α)89Zr.