Measurements of Neutron Photoproduction Cross Sections for ²³²Th and ²³⁸U Using Capture Gamma Rays

Neutron photoproduction studies for ²³²Th and ²³⁸U were carried out from 5.61 to 10.83 MeV, by using up to 30 neutron capture gamma rays with high resolution in energy (4 to 20 eV), produced in an experimental arrangement at the IPEN-IEA-R1 2-MW research reactor. Samples of U₃O₈ depleted to 0.34% in ²³⁵U and natural ThO₂ were irradiated inside a 4 sr long-counter neutron detector system, 520.5 cm away from the capture target. The gamma-ray flux was determined by means of a coaxial solid state Ge(Li) detector (EG&G ORTEC, 25 cm³, 5%) previously calibrated with capture gamma rays from a standard target of nitrogen (melamine). The compound neutron photoproduction cross section was measured for the gamma-ray spectrum produced by each capture target. Two methods to unfold the set of experimental data were proposed in order to obtain the differential cross sections at the main gamma line energies: the iterative and the least-squares methods. The calculated neutron photoproduction cross sections for ²³²Th and ²³⁸U were compared with experimental data reported by other authors who have employed different gamma-ray sources. A good overall agreement was observed among the experimental data, however, marked discrepancies were identified for some data points, indicating the possibility of narrow structures showing up at these excitation energies.