Neutron-Induced Partial Gamma-Ray Cross-Section Measurements on Uranium

Precision measurements of 238U(n,n'g) and 235,238U(n,2ng) partial cross sections have been performed at Triangle Universities Nuclear Laboratory (TUNL) to improve crucial data for the National Nuclear Security Administration's (NNSA) Stockpile Stewardship Program. Accurate neutron-induced reaction cross-section data are required for many practical applications, including nuclear energy and reactor technology, nuclear transmutation, and explosive nuclear devices. Due to the cessation of underground nuclear testing in the early 1990s, understanding of the performance of nuclear devices is increasingly dependent on precise model calculations which are, in turn, themselves reliant on accurate reaction data to serve as benchmarks for model codes. Direct measurement of (n,n') and (n,2n) reaction cross sections for uranium is extremely difficult due to large neutron background from fission and very close nuclear level spacing. Previous direct measurements of the cross sections are incomplete and/or discrepant over the energy range of interest. However, the (n,n'g) and (n,2ng) partial gamma-ray cross-section data obtained in the present work can be combined with model calculations to infer total (n,n') and (n,2n) reaction-channel cross sections.

A pulsed and monoenergetic neutron beam was used in combination with high-resolution gamma-ray spectroscopy to measure these partial cross sections for incident neutron energies between 5 and 14 MeV. Gamma-ray yields were measured with high-purity germanium (HPGe) clover and planar detectors. Neutron fluxes were determined from the well-measured 2+ -> 0+ transition in 56Fe to be on the order of 10^4 n/cm^2/s. Detector efficiency and attenuation of gamma rays in the target were simulated using the MCNPX Monte-Carlo radiation transport code.

Measured partial cross sections were compared with previous measurements and calculations from GNASH and TALYS Hauser-Feshbach statistical-model codes. Results are generally in good agreement with existing data and provide cross-section data for transitions in energy regions where none previously existed. Total reaction-channel cross sections are inferred from statistical-model calculations and compared with existing direct measurement data.