Feeding of the 11/2{sup -} isomers in stable Ir and Au isotopes

Journal Article · Thu Oct 15 00:00:00 EDT 2009 · Physical Review. C, Nuclear Physics

DOI:https://doi.org/10.1103/PHYSREVC.80.044612· OSTI ID:21293960

Fotiades, N; Nelson, R O; Devlin, M; Holloway, S; Kawano, T; Talou, P; Chadwick, M B; Becker, J A; Garrett, P E ^[1]

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1. Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)

Excited states in {sup 191}Ir, {sup 193}Ir, and {sup 197}Au were studied and absolute partial {gamma}-ray cross sections were measured using the (n,n{sup '}{gamma}) reaction. A Compton-suppressed germanium-detector array (GEANIE) for {gamma}-ray spectroscopy was used for the measurement and the broad-spectrum pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility provided energetic neutrons. The energy of the incident neutrons was determined using the time-of-flight technique. Absolute partial {gamma}-ray cross sections were measured up to incident neutron energy of 20 MeV for several transitions feeding directly the 11/2{sup -} isomers and ground states in {sup 191}Ir, {sup 193}Ir, and {sup 197}Au. The feeding of the 11/2{sup -} isomers, which originate from the odd proton occupying the h{sub 11/2} orbital, was found for the three targets to be very similar and increasing relative to the feeding of the corresponding ground state with increasing neutron energy up to E{sub n}{approx}10 MeV. Above this neutron energy the opening of the (n,2n) reaction channel strongly affects the population of the isomers and leads to a decrease of their relative population compared to the population of the ground states. The experimental results are compared with theoretical predictions from the GNASH reaction model calculation implementing a version of the spin distribution for the pre-equilibrium reaction piece with either a compound nucleus spin distribution (CN-GNASH) or a Feshbach-Kerman-Koonin (FKK-GNASH) quantum mechanical spin distribution. The effects of the spin cutoff parameter values on the population of states are examined. Evidence is presented that FKK-GNASH provides a description of the experimental data that mitigates the need for adjustment of the level density parameter to fit the data.

• OSTI ID: 21293960
• Journal Information: Physical Review. C, Nuclear Physics, Vol. 80, Issue 4; Other Information: DOI: 10.1103/PhysRevC.80.044612; (c) 2009 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 0556-2813
• Country of Publication: United States
• Language: English

Related Subjects

73 NUCLEAR PHYSICS AND RADIATION PHYSICS
COMPOUND NUCLEI
CROSS SECTIONS
DISTRIBUTION
ENERGY-LEVEL DENSITY
EQUILIBRIUM
EXCITED STATES
GAMMA DECAY
GAMMA RADIATION
GAMMA SPECTROSCOPY
GE SEMICONDUCTOR DETECTORS
GOLD 197
GROUND STATES
IRIDIUM 191
IRIDIUM 193
ISOMERS
MEV RANGE 10-100
NEUTRON REACTIONS
NEUTRON SOURCES
NEUTRONS
PROTONS
QUANTUM MECHANICS
SPECTRA
SPIN
TIME-OF-FLIGHT METHOD