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T.Kawano, P.Talou, and S.Hilaire

We solve the Schrödinger equation for an arbitrary one-dimensional  potential energy to calculate the transmission coefficient in the  fission channel of compound nucleus reactions. We incorporate the  calculated transmission coefficients into the statistical  Hauser-Feshbach model calculation for neutron-induced reactions on 235,238U and 239Pu.  The one-dimensional model reproduces the evaluated fission cross  section data reasonably well considering the limited number of model  parameters involved. A resonance-like structure appears in the  transmission coefficient for a double-humped fission barrier shape that  includes an intermediate well, which is understood to be a quantum  mechanical effect in the fission channel. The calculated fission cross  sections for the neutron-induced reactions on 235,238U and 239Pu all exhibit a similar structure.

Physical Review. C 109, 044610 (2024)

K.J.Kelly, M. Devlin, J. M. O'Donnell, D. Neudecker, A. E. Lovell, R. C. Haight, C.  Y. Wu, R. Henderson, E. A. Bennett, T. Kawano, J. L. Ullmann, N.  Fotiades, J. Henderson, S. M. Mosby, T. N. Taddeucci, P. Talou, M. C.  White, J. A. Gomez, and H. Y. Lee

With the recent emergence of fast nuclear reactors, there has been a corresponding increasing interest in U238-related  nuclear data. However, while existing literature data span much of the  energy ranges of interest for the prompt fission neutron spectrum (PFNS)  for neutron-induced fission of U238,  most literature data sets are highly correlated, and thus new,  independent measurements of this quantity are needed. In this work, we  report the results of a new measurement of the U238 PFNS at the Los Alamos Neutron Science Center for incident neutron  energies from 1.5–20.0 MeV, and outgoing neutron energies of 0.01–10.0  MeV. With some notable exceptions, the present results generally agree  with existing literature data, especially with regard to features  relating to multichance fission and pre-equilibrium features in the  PFNS, thus adding confidence to existing nuclear data evaluations and  filling in gaps of knowledge at previously unmeasured incident neutron  energies. This result is the third in a series of PFNS measurements by  the Chi-Nu collaboration now spanning all three major actinides, Pu239, U235, and U238.  Thus, for the first time, we report reliable experimental PFNS ratios  and average PFNS energy comparisons for measurements of all three of  these isotopes including accurate correlations between the different,  but correlated experiments.

Physical Review C 108, 024603 (2023)

T. Kawano, A. E. Lovell, S. Okumura, H. Sasaki, I. Stetcu, and P. Talou

Prompt and β-delayed  fission observables, such as the average number of prompt and delayed  neutrons, the independent and cumulative fission product yields, and the  prompt γ-ray energy spectra for the photonuclear reactions on U235,238 and Pu239, are calculated with the Hauser-Feshbach fission fragment decay (HF3D)  model and compared with available experimental data. In the analysis of  neutron-induced fission reactions to the case of photo-induced fission,  an excellent reproduction of the delayed neutron yields supports a  traditional assumption that the photo fission might be similar to the  neutron-induced fission at the same excitation energies regardless of  the spin and parity of the fissioning systems.

Physical Review C 107, 044608 (2023)

Nathan P. Giha, Stefano Marin, James A. Baker, Isabel E. Hernandez,  Keegan J. Kelly, Matthew Devlin, John M. O'Donnell, Ramona Vogt, Jørgen  Randrup, Patrick Talou, Ionel Stetcu, Amy E. Lovell, Olivier Litaize,  Olivier Serot, Abdelaziz Chebboubi, Ching-Yen Wu, Shaun D. Clarke, and  Sara A. Pozzi

We study γ-ray emission following Pu239(n,f) over an incident neutron energy range of 2<Ei<40 MeV. We present the first experimental evidence for positive  correlations between the total angular momentum generated in fission and  the excitation energy of the compound nucleus prior to fission. The γ-ray multiplicity increases linearly with incident energy below the second-chance fission threshold with a slope of 0.085±0.010MeV−1. This linear trend appears to hold for the average excitation energy of the compound nucleus between 9<〈Ex〉<19 MeV. Most of the multiplicity increase comes from an enhancement around a γ-ray energy of 0.7 MeV, which we interpret as stretched quadrupole γ rays that indicate an increase in total fission-fragment angular momentum with excitation energy.

Physical Review C 107, 014612 (2023)

K. J. Kelly, J. A. Gomez, M. Devlin, J. M. O'Donnell, D. Neudecker, A.  E. Lovell, R. C. Haight, C. Y. Wu, R. Henderson, T. Kawano, E. A.  Bennett, S. M. Mosby, J. L. Ullmann, N. Fotiades, J. Henderson, T. N.  Taddeucci, H. Y. Lee, P. Talou, and M. C. White

The characterization of fission-driven nuclear systems primarily relies  on calculations of neutron-induced chain reactions, and these  calculations require evaluated nuclear data as input. Calculation  accuracy heavily depends on input nuclear data evaluation accuracy, and  thus high precision on the experimental input to the nuclear data  evaluation is essential for fundamental quantities like the energy  spectrum of neutrons emitted from neutron-induced fission (i.e., the  prompt fission neutron spectrum, PFNS). Despite decades of measurement  efforts, prior to the measurements described in this work there were  only three literature data sets for the U235(n,f) PFNS at incident neutron energies above 1.0 MeV considered reliable for  inclusion in nuclear data evaluations and no reliable data sets above  3.0 MeV incident neutron energy. In this work we report on new  measurements of the U235(n,f) PFNS spanning a grid of 1.0–20.0 MeV in incident neutron energy and  0.01–10.0 MeV in outgoing (PFNS) neutron energy. These measurements were  carried out at the Weapons Neutron Research facility at the Los Alamos  Neutron Science Center and used a multifoil parallel-plate avalanche  counter target with both a Li-glass and a liquid scintillator detector  array in separate experiments to span the quoted outgoing neutron energy  ranges. The PFNS results are shown in terms of the energy spectra  themselves as well as the average PFNS energy (〈E〉) and ratios of 〈E〉 at forward and backward angles. The results are compared with  literature data and selected nuclear data evaluations. Generally, the  data agree with the ENDF/B-VIII.0 evaluation below 5.0-MeV incident  neutron energy and more closely with the JEFF-3.3 evaluation above 5.0  MeV, though no evaluations considered for comparison in this work agree  with the data across all of the incident and outgoing neutron energies  shown, especially in regions where the third-chance fission process  becomes available. Additionally, we show a ratio of the present PFNS  results for U235(n,f) with a recent and highly correlated experiment to measure the Pu239(n,f) PFNS at the same experimental facility and with nearly identical  equipment and analysis procedures. Many observations reported in this  work are the first of their kind and represent significant advancements  for knowledge of the U235(n,f) PFNS.

Physical Review C 105, 044615 (2022)

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