skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Effect of pre-equilibrium spin distribution on neutron induced 150Sm cross sections

Abstract

Prompt {gamma}-ray production cross section measurements were made as a function of incident neutron energy (En = 1 to 35 MeV) on an enriched (95.6%) {sup 150}Sm sample. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the Los Alamos Neutron Science Center (LANSCE) facility. The prompt-reaction {gamma} rays were detected with the large-scale Compton-suppressed Germanium Array for Neutron Induced Excitations (GEANIE). Above E{sub n} {approx} 8 MeV the pre-equilibrium reaction process dominates the inelastic reaction. The spin distribution transferred in pre-equilibrium neutron-induced reactions was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). These preequilibrium spin distributions were incorporated into the Hauser-Feshbach statistical reaction code GNASH and the {gamma}-ray production cross sections were calculated and compared with experimental data. Neutron inelastic scattering populates 150Sm excited states either by (1) forming the compound nucleus {sup 151}Sm* and decaying by neutron emission, or (2) by the incoming neutron transferring energy to create a particle-hole pair, and thus initiating the pre-equilibrium process. These two processes produce rather different spin distributions: the momentum transfer via the pre-equilibrium process tends to be smaller than in the compound reaction. This difference in the spin population hasmore » a significant impact on the {gamma}-ray de-excitation cascade and therefore in the partial {gamma}-ray cross sections. The difference in the partial {gamma}-ray cross sections using spin distributions with and without preequilibrium effects was significant, e.g., for the 558-keV transition between 8{sup +} and 6{sup +} states the calculated partial {gamma}-ray production cross sections changed by 70% at E{sub n} = 20 MeV with inclusion of the spin distribution of pre-equilibrium process.« less

Authors:
; ; ; ; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
908099
Report Number(s):
UCRL-PROC-230015
TRN: US0703616
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Conference
Resource Relation:
Conference: Presented at: International Conference on Nuclear Data for Science and Technology 2007, Nice, France, Apr 22 - Apr 27, 2007
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; CROSS SECTIONS; DE-EXCITATION; DISTRIBUTION; EXCITED STATES; GERMANIUM; INELASTIC SCATTERING; LANL; MOMENTUM TRANSFER; NEUTRON EMISSION; NEUTRON SOURCES; NEUTRONS; PRODUCTION; SPALLATION; SPIN

Citation Formats

Dashdorj, D, Kawano, T, Mitchell, G E, Becker, J A, Agvaanluvsan, U, Chadwick, M, Cooper, J, Devlin, M, Fotiades, N, Garrett, P E, Kunieda, S, Nelson, R O, Wu, C Y, and Younes, W. Effect of pre-equilibrium spin distribution on neutron induced 150Sm cross sections. United States: N. p., 2007. Web.
Dashdorj, D, Kawano, T, Mitchell, G E, Becker, J A, Agvaanluvsan, U, Chadwick, M, Cooper, J, Devlin, M, Fotiades, N, Garrett, P E, Kunieda, S, Nelson, R O, Wu, C Y, & Younes, W. Effect of pre-equilibrium spin distribution on neutron induced 150Sm cross sections. United States.
Dashdorj, D, Kawano, T, Mitchell, G E, Becker, J A, Agvaanluvsan, U, Chadwick, M, Cooper, J, Devlin, M, Fotiades, N, Garrett, P E, Kunieda, S, Nelson, R O, Wu, C Y, and Younes, W. Mon . "Effect of pre-equilibrium spin distribution on neutron induced 150Sm cross sections". United States. doi:. https://www.osti.gov/servlets/purl/908099.
@article{osti_908099,
title = {Effect of pre-equilibrium spin distribution on neutron induced 150Sm cross sections},
author = {Dashdorj, D and Kawano, T and Mitchell, G E and Becker, J A and Agvaanluvsan, U and Chadwick, M and Cooper, J and Devlin, M and Fotiades, N and Garrett, P E and Kunieda, S and Nelson, R O and Wu, C Y and Younes, W},
abstractNote = {Prompt {gamma}-ray production cross section measurements were made as a function of incident neutron energy (En = 1 to 35 MeV) on an enriched (95.6%) {sup 150}Sm sample. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the Los Alamos Neutron Science Center (LANSCE) facility. The prompt-reaction {gamma} rays were detected with the large-scale Compton-suppressed Germanium Array for Neutron Induced Excitations (GEANIE). Above E{sub n} {approx} 8 MeV the pre-equilibrium reaction process dominates the inelastic reaction. The spin distribution transferred in pre-equilibrium neutron-induced reactions was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). These preequilibrium spin distributions were incorporated into the Hauser-Feshbach statistical reaction code GNASH and the {gamma}-ray production cross sections were calculated and compared with experimental data. Neutron inelastic scattering populates 150Sm excited states either by (1) forming the compound nucleus {sup 151}Sm* and decaying by neutron emission, or (2) by the incoming neutron transferring energy to create a particle-hole pair, and thus initiating the pre-equilibrium process. These two processes produce rather different spin distributions: the momentum transfer via the pre-equilibrium process tends to be smaller than in the compound reaction. This difference in the spin population has a significant impact on the {gamma}-ray de-excitation cascade and therefore in the partial {gamma}-ray cross sections. The difference in the partial {gamma}-ray cross sections using spin distributions with and without preequilibrium effects was significant, e.g., for the 558-keV transition between 8{sup +} and 6{sup +} states the calculated partial {gamma}-ray production cross sections changed by 70% at E{sub n} = 20 MeV with inclusion of the spin distribution of pre-equilibrium process.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Apr 16 00:00:00 EDT 2007},
month = {Mon Apr 16 00:00:00 EDT 2007}
}

Conference:
Other availability
Please see Document Availability for additional information on obtaining the full-text document. Library patrons may search WorldCat to identify libraries that hold this conference proceeding.

Save / Share:
  • Cross section measurements were made of prompt gamma-ray production as a function of neutron energy using the germanium array for neutron induced excitations (GEANIE) at LANSCE. Measuring the prompt reaction gamma rays as a function of incident neutron energy provides more precise understanding of the spins populated by the pre-equilibrium reaction. The effect of the spin distribution in pre-equilibrium reactions has been investigated using the GNASH reaction code. Widely used classical theories such as the exciton model usually assume that the spin distribution of the pre-equilibrium reaction is the same as the spin distribution of the compound nucleus reaction mechanism.more » In the present approach, the pre-equilibrium reaction spin distribution was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). This pre-equilibrium spin distribution was incorporated into the GNASH code and the gamma-ray production cross sections were calculated and compared with experimental data. Spin distributions peak at lower spin when calculated with the FKK formulation than with the Compound Nuclear theory. The measured partial gamma-ray cross sections reflect this spin difference. Realistic treatment of the spin distribution improves the accuracy of calculations of gamma-ray production cross sections.« less
  • No abstract prepared.
  • Cross sections were measured for the /sup 235/U(n,3n) reaction from 14.8 to 21 MeV, for /sup 238/U(n,2n) from 14.7 to 19 MeV, and for /sup 238/U(n,3n) from 14.7 to 22 MeV with /sup 3/H(d,n) neutrons and a large liquid scintillator tank, and (n,xn) and fission cross sections from 4 to 22 MeV were calculated for both nuclei using a statistical, preequilibrium model. Also calculated were the individual spectra of the two (n,2n) and three (n,3n) neutrons. 15 references.
  • Neutron-induced cross sections on tungsten isotopes were calculated in the energy range between 1 and 20 MeV using preequilibrium-statistical model techniques. The success of these calculations, which form part of an effort to improve the evaluated neutron and gamma-ray production cross sections for tungsten appearing in ENDF/B, depends strongly on the determination of consistent input parameter sets applicable over the entire range of interest. For example, neutron optical model parameters were derived through a simultaneous analysis of total cross sections, resonance data, and angular distributions. These parameters, when used in multistep Hauser-Feshbach calculations, produce good agreement with varied experimental datamore » such as neutron inelastic scattering excitation functions and (n, 2n) cross sections. Likewise, gamma-ray strength functions were determined through fits to neutron capture data that produce calculated results that compare well to measured gamma-ray production cross sections. A description of the techniques used in such parameter determinations as well as comparison of calculated results to experimental data will be presented.« less