1. Measurement of the 187Re(n,2n)186mRe Destruction Cross Section
ObservedChannel181Hf180Hf(n,γ)181Hf180mHf180Hf(n,n')180mHf179m2Hf180Hf(n,2n)179m2Hf179Hf(n,n')179m2Hf175Hf176Hf(n,2n)175Hf173Hf174Hf(n,2n)173Hf
Measurement of the 187Re(n,2n)186mRe Destruction Cross Section
Clark Casarella1, J. Kelley2, R. Raut3, C. Howell3, G. Rusev3, A. Tonchev3, H. Karwowski4,
E. Kwan3, W. Tornow3, S.L. Hammond3, F. Kondev5, S. Zhu5 1North Georgia College and State University, 2Triangle Universities Nuclear Laboratory and
North Carolina State University, 3TUNL and Duke University, 4TUNL and UNC Chapel Hill,
5Argonne National Laboratory
We are continuing a program to measure cross sections for 187Re(n, 2n γ) reactions with particular interest in confirming a transition that has tentatively been identified as a doorway transition feeding 186mRe at Ex=149(7) keV. The cross sections are being measured using pulsed, nearly mono-energetic neutron beams, as well as an array of planar HPGe γ-ray detectors. At present, the reaction cross sections for the 187Re(n, 2n γ) reaction are poorly known, so measuring the cross sections have positive implications, for example, on reactor physics where Re is used as a cladding for Uranium fuel rods. Furthermore, refining the cross section measurements may reduce uncertainties in the Re/Os cosmochronometer.
Motivation
187Re(n,2n)186Re Cross sections
We are looking to improve the measurement of the 187Re(n,2n)186Re Cross sections with an emphasis on confirming the γ-ray transition that feeds the isomer, 186mRe. Currently, the cross section measurements for the 187Re(n,2n)186,186mRe reactions are poorly known, as shown in the figures on the right. Recently, F. Kondev reported on a 186W(d,2n) experiment that suggests that this isomer is fed via a series of Eγ=144.2 keV and 30 keV transitions out of the Ex=324 keV level. However, these transitions have not been observed or confirmed to populate the isomer.
ENDF/B-VII.0 Library, Druzhinin 1967, Khurana 1961
Evaluated Nuclear Data File,
Yamamuro, Nuclear Science and Engineering, 118 (1994)249. INDC(NDS)-288 ]
Experimental Setup
Irradiation
Id =1mA
7.8 atm
beam
pickoff
2H(d,n)3He
Pulsed deuterons from the DENIS source at TUNL were used to produced the neutron beam at the Shielded Source Area via the 2H(d,n)3He reaction using a deuterium gas cell pressurized to 3.0 atm. Our 5 cm x 5 cm x 0.01 cm natRe target (187Re 63%/185Re 37%) was then placed in between two planar HPGe detectors with passive shielding (see figures).
The neutron profile was carefully measured at each energy and convoluted with the cross section curves for the monitor foils in order to correct the 241Am(n,2n) cross section for any possible contribution from low-energy breakup neutrons.
Typical neutron energy distribution spectra measured at TUNL showing the “monoenergetic” peak and low energy breakup neutrons.
Measurements
As one can see, the region of interest around from 100 keV to 150 keV is heavily populated, so in order to identify the
144 keV transition of interest, we needed as high energy resolution as possible to resolve the peak.
Data was collected using the VM-USB version of SpectroDAQ/SpecTCL. The background subtracted g-ray energy spectrum (shown on the right) is generated by subtracting events collected during the “beam off” period in the time-of flight spectrum from events that are correlated with the “quasi-monoenergetic” prompt neutron peak in the tof spectrum
Remarks
Experimental Results
1: 187Re(n,n’g) Inelastic scattering transition
2: 187Re(n,2n g) 144 keV transition of interest
3: 185Re(n,2n g) 132 keV transition for comparison
Verification of participation of the Eg=144 keV transition for populating the 186mRe isomeric state, and measurement of the 187Re(n,2n)186mRe cross section remain elusive experimental pursuits. Further work on unraveling the decay scheme will require greater solid angle coverage in an experiment aimed at determining g-g correlations.
Determination of the 187Re(n,2n)186mRe cross section, via activation techniques, is similarly complicated. In 2008 a natRe target was irradiated and counting on the sample continues (searching for a Eg=137 keV transition from 186Re b-decay ). Observation of the transition is hindered by a huge Compton background from transitions populated in T1/2=170 day 184mRe decay (from 185Re(n,2n)184mRe). See figure below.
186Re 1 2 3
At present, there is a great deal of ambiguity for interpreting the observed spectra that prevents a verification of involvement of the Eg=144 keV transition in populating the isomer, and hence prevents a determination of the 187Re(n,2n)186mRe cross section. Particularly, in 186Re, there are two transitions with Eγ=144 keV (shown in the above figure). It was suggested that the long life-time (17 ns) of the Ex=324 keV state could provide an additional observable that could be used to interpret the decay scheme; that is, a significantly broad peak in the time-of-flight spectrum (above) could be associated with a 144 keV transition out of the Ex=324 keV T1/2=17 ns level. Using timing cuts on our TOF spectra, we see a slight broadening, but it is not so profound to be associated with decay from a T1/2=17 ns level.
The REU program is funded by National Science Foundation (NSF) NSF Other support from the NNSA under the Stewardship Science Academic Alliances Program DE-FG52-06NA26155, and Office of Science of the US Department of Energy DE-FG02-97ER41033 (Duke), DE-FG02-97ER41042 (NC-State), DE-FG02-97ER41041 (UNC), DE-AC02-06CH11357 (ANL) is acknowledged.