Neutron Cross Section Measurements

Abstract

The main emphasis during the past year has been on analyzing and publishing results and the preparation for the impending start-up of the newly modified Nevis cyclotron. Papers on the separated isotopes of Yb and W have been submitted to the Physical Review. The published results on Er confirm Dyson's theory of level spacing systematics. The total cross section, resonance parameters and spin-parity values of Na below 300 keV have been submitted for publication. A report on the evaluation of the T's of U 238 has been distributed. We now have extensive results on the Er isotopes, 232 U, 152 > 15lt Sm, 151 , 153 Eu, Fe, La, In, Ta, F, Mg, Al, S, CI, K and Ca. A paper on the precision measurement of the 2200 m/sec neutron-proton capture cross section has been submitted for publication. The background for fission measurements has been extensively analyzed and modifications to the neutron source of the Nevis cyclotron will be made for the next run. New methods of analyzing low count rate data have been developed to eliminate observation of spurious levels. The captive cross sections of thulium and rhodium have been analyzed and the results reported. During the report period, our main emphasis has been on analyzing and publishing data, and the preparation for the impending startup of the newly modified Nevis cyclotron. A number of major papers and reports on our results have been published or submitted for publication. Papers on the neutron resonance spectroscopy and systematics of the separated Yb isotopes and the separated W isotopes have been submitted to the Physical Review. These papers round out our recent work on many of the rare earth isotopes, 150<_ A< 190, with a paper on the separated even Gd isotopes in preparation and tne separated Dy isotopes expected to be ready soon. The published paper on the Er isotopes, confirming Dyson's theories concerning level spacings systematics, has led to extensive Monte-Carlo theoretical studies by our group, partly on the basis of interactions with Professor Freeman Dyson. These studies have extended the understanding of the implications of the statistical orthogonal ensemble theory and have permitted a detailed comparison with our results for many even-even nuclei, 150 < A< 190, which supports this theory well for the first time. "The total cross section, resonance parameters and spin-parity values for Na below 300 keV has been submitted to Nuclear Science and Engineering for publication. An evaluation report on the total radiation widths of the important reactor material 238 U has concluded that there is no convincing evidence for quasi-periodic structure in r versus energy (apart from a statistical level to level fluctuation of a high chi-squared number of degrees of freedom). The fluctuations in r present in many data sets are most likely due to experimental uncertainties in the r values themselves. Our best evaluated average radiation width was taken to be <r > = 23.55 + 0.16 meV. T We now have.extensive results for the resonance parameters and systematics for a large ...

Neutron reactions in astrophysics

“…When ORELA was closed in 2012, GELINA at Geel/Belgium [239,240] remained the only electron-driven, moderated facility for neutron studies in the energy regime of the s process. The accelerator delivers a 140 MeV electron beam with a pulse width of 1 ns onto a rotating U target, and the water-moderated neutron spectra from 25 up to 20 MeV can be accessed at 10 flight paths from 10 to 400 m. The astrophysics activities are essentially carried by visiting groups and are devoted to (n, γ ) cross sections of isotopes on the s process path, e.g., Si [241], Kr [242], Nd [243] and Ba/Pb [244].…”

Windowed multipole for cross section Doppler broadening

“…A reaction channel is defined as a pair of interacting particles (α) and their quantum state, respectively the orbital and intrinsic angular momentum of the two particles, the total angular momentum and its projection on the z axis: c ≡ {α, , j, J, m J }. A given reaction can thus be written as: α → A * → α [21]. For each channel, space is divided in two regions: an internal region of radius a c , corresponding to the inner nucleus potential (considered as a black box), and an external region, where the nuclear potential is null (if relevant, the Coulomb potential exists).…”

Homogenization of the linear Boltzmann equation with a highly oscillating scattering term in extended phase space

“…One of the main difficulties in the mathematical study of models describing composite materials is the strongly oscillating character of some physical quantities, such as the neutron capture cross-section of Uranium-235 as a function of the energy of the incoming neutron in the range between 10 2 and 10 3 eV [5].…”