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

Title: High-Resolution Neutron Capture and Total Cross-Section Measurements, and the Astrophysical 95Mo(n,gamma) Reaction Rate at s-process Temperatures

Abstract

Abundances of Mo isotopes predicted by stellar models of the s process are, except for {sup 95}Mo, in good agreement with data from single grains of mainstream presolar SiC. Because the meteorite data seemed sound and no reasonable modification to stellar theory resulted in good agreement for {sup 95}Mo, it has been suggested that the recommended neutron capture reaction rate for this nuclide is 30% too low. Therefore, we have made a new determination of the {sup 95}Mo(n,{gamma}) reaction rate via high-resolution measurements of the neutron-capture and total cross sections of {sup 95}Mo at the Oak Ridge Electron Linear Accelerator. These data were analyzed with the R-matrix code SAMMY to obtain parameters for resonances up to E{sub n} = 10 keV. Also, a small change to our capture apparatus allowed us to employ a new technique to vastly improve resonance spin and parity assignments. These new resonance parameters, together with our data in the unresolved range, were used to calculate the {sup 95}Mo(n,{gamma}) reaction rate at s-process temperatures. We compare the currently recommended rate to our new results and discuss their astrophysical impact.

Authors:
 [1];  [1];  [1];  [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Oak Ridge Electron Linear Accelerator
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
951925
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Conference
Resource Relation:
Conference: 10th International Symposium on Nuclei in the Cosmos (NIC-X), Mackinac Island, MI, USA, 20080727, 20080801
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; CAPTURE; ELECTRONS; ISOTOPES; LINEAR ACCELERATORS; METEORITES; MODIFICATIONS; NEUTRON REACTIONS; NUCLEI; PARITY; R MATRIX; REACTION KINETICS; RESONANCE; S PROCESS; SPIN; TOTAL CROSS SECTIONS; UNIVERSE

Citation Formats

Koehler, Paul Edward, Guber, Klaus H, Harvey, John A, and Wiarda, Dorothea. High-Resolution Neutron Capture and Total Cross-Section Measurements, and the Astrophysical 95Mo(n,gamma) Reaction Rate at s-process Temperatures. United States: N. p., 2008. Web.
Koehler, Paul Edward, Guber, Klaus H, Harvey, John A, & Wiarda, Dorothea. High-Resolution Neutron Capture and Total Cross-Section Measurements, and the Astrophysical 95Mo(n,gamma) Reaction Rate at s-process Temperatures. United States.
Koehler, Paul Edward, Guber, Klaus H, Harvey, John A, and Wiarda, Dorothea. 2008. "High-Resolution Neutron Capture and Total Cross-Section Measurements, and the Astrophysical 95Mo(n,gamma) Reaction Rate at s-process Temperatures". United States. doi:.
@article{osti_951925,
title = {High-Resolution Neutron Capture and Total Cross-Section Measurements, and the Astrophysical 95Mo(n,gamma) Reaction Rate at s-process Temperatures},
author = {Koehler, Paul Edward and Guber, Klaus H and Harvey, John A and Wiarda, Dorothea},
abstractNote = {Abundances of Mo isotopes predicted by stellar models of the s process are, except for {sup 95}Mo, in good agreement with data from single grains of mainstream presolar SiC. Because the meteorite data seemed sound and no reasonable modification to stellar theory resulted in good agreement for {sup 95}Mo, it has been suggested that the recommended neutron capture reaction rate for this nuclide is 30% too low. Therefore, we have made a new determination of the {sup 95}Mo(n,{gamma}) reaction rate via high-resolution measurements of the neutron-capture and total cross sections of {sup 95}Mo at the Oak Ridge Electron Linear Accelerator. These data were analyzed with the R-matrix code SAMMY to obtain parameters for resonances up to E{sub n} = 10 keV. Also, a small change to our capture apparatus allowed us to employ a new technique to vastly improve resonance spin and parity assignments. These new resonance parameters, together with our data in the unresolved range, were used to calculate the {sup 95}Mo(n,{gamma}) reaction rate at s-process temperatures. We compare the currently recommended rate to our new results and discuss their astrophysical impact.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2008,
month = 1
}

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:
  • Because of its small cross section, the 88Sr(n,g) reaction is an important bottleneck during s-process nucleosynthesis. Hence, an accurate determination of this rate is needed to better constrain the neutron exposure in s-process models and to more fully exploit the recently discovered isotopic anomalies in certain meteorites. We have completed the resonance analysis of our new and improved measurements of the neutron capture and total cross sections for 88Sr made at the Oak Ridge Electron Linear Accelerator (ORELA). We describe our experimental procedures and resonance analysis, compare our results to previous data, and discuss their astrophysical impact.
  • New neutron capture cross section of 58,60Ni were measured in the energy range from 100 eV to 600 keV using the Oak Ridge Electron Linear Accelerator (ORELA). The combination of these new neutron capture data with previous transmission data allowed a resonance analysis up to 900 keV using R-matrix theory. The theoretically determined direct capture (DC) cross sections were included in the analyses. From these resonance parameters and the DC contribution, new (n,y) astrophysical reaction rates were determined over the entire energy range needed by the lastest stellar models describing the so-called weak s process. PACS numbers: 25.40.Lw, 26.20Kn, 27.40.+z,more » 27.50.+e, 97.10.Cv« less
  • New neutron capture cross sections of {sup 58,60}Ni were measured in the energy range from 100 eV to 600 keV using the Oak Ridge Electron Linear Accelerator. The combination of these new neutron capture data with previous transmission data allowed a resonance analysis up to 900 keV using R-matrix theory. The theoretically determined direct capture cross sections were included in the analyses. From these resonance parameters and the direct capture contribution, new (n,{gamma}) astrophysical reaction rates were determined over the entire energy range needed by the latest stellar models describing the so-called weak s process.
  • High-resolution neutron total and capture cross-section measurements have been performed on a 99.82% enriched 206Pb metallic sample. The transmission and capture measurements were carried out at the 25- and 60-m stations, respectively, of the Time-Of-Flight facility GELINA of the IRMM in Geel (B). The small amount of material allowed us to detect 13 resonances below 80 keV in the transmission measurements and 70 were seen in the capture measurements below 150 keV. The resonance parameters for the resonances seen in transmission agree within the uncertainties of the parameters determined by Horen et al. at ORELA. The capture yield was measuredmore » up to 600 keV and the capture areas for resonances up to 150 keV were compared with published data. This comparison reveals systematic differences, which are due to the detection geometry, the different neutron sensitivity in the detection systems, the applied weighting function, and normalisation.« less
  • Because of the low-energy threshold and relatively large cross section of the {sup 9}Be(n,2n) reaction, beryllium is a very attractive neutron multiplier for some fast-neutron systems, such as those used in the production of fusion energy. But, tritium is also produced when beryllium is irradiated with 14-MeV neutrons emitted from the fusion of deuterium and tritium ions. Among the two exit channels of the {sup 9}Be(n,t){sup 7}Li reaction of {approx}14-MeV incident-neutron energy, the {sup 9}Be(n,t{sub 1}{gamma}){sup 7}Li channel also emits a 0.478-MeV gamma ray. The purpose of the present study is to measure the cross section for the {sup 9}Be(n,t{submore » 1}{gamma}){sup 7}Li reaction and also that of the more general {sup 9}Be(n,x{gamma}){sup 7}Li reaction with the production of the same 0.478-MeV gamma ray for incident-neutron energies from the 12-MeV threshold to 200 MeV. Because the {sup 7}Li levels excited above 0.478 MeV are unstable against particle emission, the study of the {sup 9}Be(n,t{sub 1}{gamma}){sup 7}Li reaction gives direct access to the cross section for the formation of {sup 7}Li* in its 0.478-MeV excited state. The few previous experimental data for this reaction are restricted to incident-neutron energies of {approx}14 MeV with large discrepancies between the results. The present data are obtained with a BeO sample, using the pulsed source of high-energy neutrons of the Weapons Neutron Research Facility (WNR) at the Los Alamos Neutron Science Center (LANSCE). The 478-keV gamma rays emitted in {sup 9}Be(n,x{gamma}){sup 7}Li reactions are detected with two high-resolution Ge detectors. The data thus obtained are presented and compared with previous data on the {sup 9}Be(n,t{sub 1}{gamma}){sup 7}Li and the {sup 9}Be(n,t){sup 7}Li reactions. Examination of the present data also provides insight into the {sup 10}Be level scheme.« less