Neutron-induced235U fission spectrum measurements using liquid organic scintillation detectors

Abstract

Measurements have been performed at Los Alamos Neutron Science Center to acquire neutron fission spectra of 235 U in the energy range between 0.5 and 10 MeV. These new data complement the currently available experimental data, which are not well known in the energy range below 1 and above 5 MeV. Organic liquid scintillation detectors (EJ-309s) were used together with a digital data-acquisition system. The EJ-309 detectors show excellent pulse shape discrimination capabilities and this is vital for identifying only neutron pulses and rejecting γ -ray pulses. The measurement data show the dependence of average fission neutron energy as a function of the inducing neutron energy. The data agree well with previously published measurements and Watt spectra fits for energies up to tens of MeV. At high fission-inducing neutron energies the spectrum deviates from Watt-spectra fits. The increased energy deposited in higher energy neutron-induced fissions does not significantly increase emitted fission neutron energies for up to 10 MeV. A significant decrease in neutron energy around second-chance fission cannot be observed beyond the current measurement uncertainties.

A new method to reduce the statistical and systematic uncertainty of chance coincidence backgrounds measured with waveform digitizers

“…In this case, the two most forward angle neutron detectors (not included in fig. 7) are very sensitive to direct scattering of neutrons from the beam and would show up in the two-dimensional analysis as a strong enhancement of the horizontal band near t n − t 0 ≈ 0µs, and the 1D peak arises from chance coincidences between the scattered beam neutrons and fissions [10]. Similarly, the step in the background in the lower right panel of fig.…”

Prompt Fission Neutron Spectra for Neutron-Induced Fission of 239Pu and 235U

“…Early attempts at such measurements were necessarily primitive, but interest in these spectra was such that the first numbered report from the then secret program at Los Alamos was titled A Discussion of the Fission Neutron Spectrum [2]; this document is no longer classified. More recently, there has been a renewed interest in improved PFNS data, with numerous experimental efforts (see, for example, [3][4][5][6][7][8][9][10][11]) and related evaluations [12][13][14] for neutron-induced fission of the major actinides. Improved data on PFNS are needed, particularly for fast neutrons, to improve nuclear data libraries [15,16] used in neutronics calculations for fast systems.…”

Review on the progress in nuclear fission

“…In this domain, the coupled-channels model, the dispersive optical potential, and an heuristic model for resonant transmission are of eminent importance. On the experimental side there have been several publications on refinements of existing [109,110,111,112,113,114] or the development of novel techniques [115,116] as well as on new experimental findings [117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,…”