James J. Egan

Prompt fission neutron energy spectra induced by fast neutrons

Abstract Prompt fission neutron energy spectra for 235 U and 239 Pu have been measured for fission neutron energies greater than the energy of the incident neutrons inducing fission. The measurements were undertaken to investigate the shape dependence of the fission neutron spectra upon both the incident neutron energy and the mass of the nucleus undergoing fission. Measurements were made for both nuclides at incident neutron energies of 0.50, 1.50, 2.50 and 3.50 MeV. The data are presented either as relative yields or as ratios of measured spectra to that of 235 U at 0.50 MeV. Incident neutrons were produced by the 7 Li(p,n) 7 Be reaction using a pulsed, bunched proton beam from the 5.5 MV Vand de Graaff accelerator at the University of Massachusetts Lowell Radiation Laboratory. Fission neutrons were detected by a thin liquid scintillator with good time resolution capabilities. Neutron energies were determined by time-of-flight techniques with pulse-shape discrimination to reduce gamma-ray background levels. The measurements are compared to calculations using the Los Alamos model of Madland and Nix to test the predictive capabilities of the model. The calculations were performed prior to the measurements. The data are fit by the Watt equation to determine the mean energy of the spectra and facilitate comparison of the results to previous measurements.

Spin and parity assignments for 35Cl levels from the 31P(α, p)34S reaction

Abstract 35 Cl states at excitation energies between 9.9 and 11.8 MeV have been identified through sharp resonances in the 31 P(α, p o ) 34 S excitation functions at 25°, 105° and 155° for E α = 3.25–5.50 MeV. Forty-eight on-resonance angular distributions, normalized to an absolute cross section scale, have been subjected to single-level and two-level analyses resulting in spin and parity assignments for each resonance. Approximately half the resonances were of the pure single-state type, having unique angular distribution shapes. Data from 12 resonances of an earlier experiment 1 ) were analyzed with the same theory, extending the diagnostics down to a 35 Cl excitation energy of 9.1 MeV. A set of optical potentials consistent in all four reactions that this experimental program encompasses has been incorporated in the present analysis. Validity of the optical potential is demonstrated for α-particles elastically scattered by 31 P.

Fast neutron ineleastic scattering cross sections of /sup 238/U for states between 680 and 1530 keV

Neutron inelastic scattering cross sections for /sup 238/U levels between 680- and 1530-keV excitation energy have been measured in the incident neutron energy range from 0.9 to 2.2 MeV. The (n,n) time-of-flight (TOF) technique was used to obtain direct differential inelastic cross sections. Neutrons were generated using the /sup 7/Li(p,n)/sup 7/Be reaction. Experimental parameters were optimized to achieve an energy resolution of <15 keV. Level cross sections were deduced from the measured 125-deg differential scattering cross sections. The validity of this procedure was confirmed by measuring the angular distributions for nine levels at En=1.5 and 2.0 MeV. Background due to fission induced by fast neutrons was subtracted. The TOF spectra were unfolded using the method of the response function. The data were corrected for multiple scattering and neutron attenuation in disk scattering geometry using an analytic method. Theoretical calculations of the cross sections were carried out using reaction models appropriate to the description of compound nucleus and direct interaction processes. The data are compared to (n,n..gamma..) results and the ENDF/B-V evaluation.

Spin and parity assignments for 23Na levels from the 19F(α, p)22Ne reaction

Abstract Resonances were found in the 120° and 160° excitation functions for the 19 F(α, p o ) 22 Ne reaction between E = 2.5 and 5.0 MeV corresponding to 23 Na levels at excitation energies between 12.56 and 14.51 MeV. Twenty-one on-resonance angular distributions were analysed with single-level and two-level theory to extract 23 Na spin and parity information. The results of an earlier experiment were analysed by the same procedure, extending the diagnostics down to a 23 Na excitation energy of 11.55 MeV. The analysis incorporated an optical potential for the α-particle consistent with previous channel-spin- 1 2 reaction studies and α-induced reaction data.

Neutron inelastic scattering cross sections for states above 700 keV in /sup 232/Th

Fast neutron inelastic scattering cross sections for levels between 700- and 1400-keV excitation energy in /sup 232/Th have been measured using the (n,n) time-of-flight (TOF) technique. Measurements of 125-deg differential cross sections were made using neutrons with a typical energy spread of 8 to 10 keV, generated by the /sup 7/Li(p,n)/sup 7/ Be reaction. The incident neutron energies covere three regions: (a) 950 to 1550 keV in 50-keV intervals with the TOF spectrometer optimized to detect 200 to 600-keV scattered neutrons, (b) 1200 to 2000 keV in 100-keV intervals with the spectrometer optimized to detect 400- to 800-keV scattered neutrons, and (c) 1700 to 2100 keV in 100-keV steps with the spectrometer optimized for 800- to 1300-keV scattered neutrons. Throughout the experiment, an overall energy resolution of <15 keV was maintained. Level cross sections were deduced from the 125-deg differential scattering cross sections and are compared with (n,n..gamma..) measurements and the ENDF/B-V evaluation. Angular distributions for states in the 700- to 900-keV region have been measured at 1.2, 1.5, and 2.0 MeV.

Studies of a Poenitz-type black neutron detector as a neutron flux monitor

Abstract A black neutron detector of novel design has been constructed and tested as a neutron flux monitor. The neutron time-of-flight (TOF) technique was used to measure the zero degree neutron spectrum from a subnanosecond pulsed proton beam on a thick metallic lithium target. We describe the detector, discuss the shielding arrangement and electronics, determine the low-energy cut-off. We calculate the efficiency, show spectra and compare measured zero degree differential cross-section of the 7 Li(p, n 0 ) 7 Be reaction with recommended published cross-sections.

Neutron scattering angular distributions in 239Pu at 570 and 700 keV

Neutron elastic and inelastic scattering in 239 Pu have been studied via the time-of-flight technique. Neutrons were generated by the 7 Li(p, n) 7 Be reaction at the University of Massachusetts Lowell 5.5-MV Van de Graaff Accelerator Laboratory. Angular distributions were measured at 570 and 700 keV for two level groups, ground state plus first excited state, and second plus third excited state. The plutonium sample was disk-shaped with a mass of 28. 7 g. The angle-integrated cross sections obtained for the two scattered neutron groups, one corresponding to the elastic plus 7.9 keV level and the other corresponding to the inelastic 57 keV plus 76 keV levels, were 5864 ± 264 mb and 570 ± 42 mb, respectively, for 570-keV incident neutrons and 5060 ± 308 mb and 518 ± 62 mb, respectively, for 700-ke V incident neutrons. The results are compared with ENDF/B-VI and with the measurement of Haouat et al. at 700 keV.

Neutron scattering cross sections up to 2. 4 MeV for the ground and first two excited states of /sup 232/Th

Differential cross sections have been measured at the University of Lowell for the ground and first two excited states of /sup 232/Th via the neutron time-of-flight technique. The following results are presented for neutron scattering cross sections: (a) excitation functions of /sup 232/Th in the incident energy range from 185 to 2400 keV for the 0/sup +/ ground state and the 2/sup +/, 49-keV state, and in the 480- to 2400-keV range for the 4/sup +/, 162-keV state; (b) angular distributions for the 0/sup +/ and 2/sup +/ states at 185 kev and for the 0/sup +/, 2/sup +/, and 4/sup +/ states at 550 keV. Level cross sections were obtained using these and previously measured angular distributions.

55Mn(n, n'γ) cross-section studies for En = 1.0–3.6 MeV

Abstract Absolute 55 Mn(n, n′γ) γ-ray production cross sections have been measured for 19 transitions from levels up to and including the 2429 keV state in 55 Mn over the energy range E n = 1.0–3.6 MeV. Angular distributions were also measured for 6 of the transitions. Branching ratios were extracted and total inelastic neutron cross sections were inferred for these 55 Mn excited states. The measured and inferred cross sections are compared with calculated cross sections using the statistical compound nucleus theory.

Thulium-169 neutron inelastic scattering cross section measurements via the 169Tm(n,n′γ) reaction

Abstract Neutron inelastic scattering from thulium-169 has been studied for states above 100 keV via the (n,n′γ) reaction at incident energies in the 0.2- to 1.0-MeV range. A high-resolution Ge spectrometer in conjunction with the time-of-flight technique was utilized. Thirty-six gamma-ray transitions from 16 levels were observed. Gamma-ray angular distributions were measured at E n =750 keV and excitation functions at 125 degrees were measured in 50 keV steps over the range of incident energies. Differential gamma-ray production cross sections and gamma-ray branching ratios were obtained. Inferred neutron inelastic level cross sections of the four lowest ground-state rotational band (K π =1/2 + ) members are compared to the sum of calculated compound nucleus and direct interaction cross sections. For the remaining levels, measurements are compared to compound nucleus calculations only. The comparison shows generally good agreement particularly near threshold.