1. ObservedChannel181Hf180Hf(n,γ)181Hf180mHf180Hf(n,n')180mHf179m2Hf180Hf(n,2n)179m2Hf179Hf(n,n')179m2Hf175Hf176Hf(n,2n)175Hf173Hf174Hf(n,2n)173Hf The Astrophysical 187 Re/ 187 Os Ratio: Measurement of the 187 Re(n,2n) 186m Re Destruction Cross Section Experimental Setup The REU program is funded by National Science Foundation (NSF) NSF-05-52723. 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. Future plans: At present the decay spectrum is dominated by the Compton spectrum from 185 Re(n,2n) 184 Re/ 184m Re decay  -rays. We are actively seeking solutions that can lower the impact of the Compton spectrum, with active suppression shields on a HPGe planar detector. We aim to obtain 187 Re destruction cross sections with an emphasis on the production of 186m Re. We have continued a program to measure (n,2n) reaction cross sections on 187 Re with an emphasis on population of the 186mRe isomer with half-life 2x10 5 y. We produced 12 MeV quasi- monoenergetic neutrons with the TUNL Tandem Van de Graff accelerator and a deuterium gas cell, and we measured the 187 Re(n,b)Y cross sections using activation techniques. While various reaction channels were populated in the activation, our emphasis is on the (n,2n) reaction channels, where 186 Re is populated. Of specific interest is the population of 186m Re, which has an extremely long lifetime that complicates the measurement. This data is intended to reduce uncertainties in the 187 Re/ 187 Os cosmochronometer. 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 241 Am(n,2n) cross section for any possible contribution from low- energy breakup neutrons. ] Irradiation TUNL Low Background Counting Facility. After irradiation with neutrons, the activity of the Re samples and the Au and Al monitor foils were measured at the TUNL Low Background Counting Area using lead shielded 60% efficient HPGe detectors. The neutron beam was produced via the 2 H(d,n) 3 He reaction using a deuterium gas cell pressurized to 3.0 atm. The 0.2 g nat Re samples were placed together with a set of gold and aluminum monitor foils encased within an Cd screen to reduce the interaction of thermal neutrons. The holder was placed 2.5 cm from the front face of a deuterium gas cell. The neutron flux on target at this distance was approximately 3  10 7 n (cm 2 s) -1. Typical neutron energy distribution spectra measured at TUNL showing the monoenergetic peak and low energy breakup neutrons. The activation setup consisted of a beam monitor NE- 213 liquid scintillator detector positioned at 0˚ relative to the incident deuterium beam. The detector was located at 4.7 m from the deuterium gas cell. Eric Pooser 1, J. Kelley 2, A. Hutcheson 3, H. Karwowski 4, E. Kwan 3, C. Huibregtse 2, A. Tonchev 3, W. Tornow 3, F. Kondev 5, S. Zhu 5 1 North Georgia College and State University, 2 Triangle Universities Nuclear Laboratory and North Carolina State University, 3 TUNL and Duke University, 4 TUNL and UNC Chapel Hill, 5 Argonne National Laboratory s-process (produces 187 Os) Yamamuro, Nuclear Science and Engineering, 118 (1994)249. INDC(NDS)-288 I d =1  A 7.8 atm beam picko ff 2 H(d,n) 3 He For long-term measurements the irradiated samples were arranged in a 3 x 3 matrix and will be placed at fixed distances from our HPGe detector. We have observed transitions from several reactions (see Table) and we are tracking the evolution of their decays so that reaction cross sections for the various channels can be determined. Experimental Results Measurements Motivation ENDF/B-VII.0 Library, Druzhinin 1967, Khurana 1961Evaluated Nuclear Data File, www.nndc.bnl.gov 187 Re(n,2n) 186 Re Cross sections We intend to decrease the systematic uncertainties concerning the 187 Re/ 187 Os cosmochronometer. Since the s-process abundance from 187 Os can be deduced from the observed abundance of 186 Os. Excess in the abundance of 187 Os, implies contribution from 187 Re decay. Using the life time of 187 Re ( 4.35 x 10 10 y) the ratio of 187 Os (from 187 Re decay) to the abundance of 187 Re dates the age of formation in a given sample. Thus we are able to determine the age of formation of galaxies 3.7d 8.7 h 17 h 23.7 h 38.0 d 187 Re(n,2n) 186 Re 187 Re(n,α) 184 Ta 187 Re(n,γ) 188 Re(thermals) 187 Re(n,p) 187 W 185 Re(n,2n) 184 Re Reaction T 1/2 Observed Reaction Channels 187Re(n,2n)186Re y = -0.0076x + 11.384 11.22 11.24 11.26 11.28 11.3 11.32 11.34 11.36 11.38 11.4 0510152025 Time (hours) Ln(Activity) One foil: t~3 days:  t=24 hrs Nine foils: t~week 10:  t=24 hrs 184 Re # 186 Re @ 188 Re % # # # # # # # # # # @ @ @ @ % % % Nucleosynthesis of elements in the 187 Re/ 187 Os region occurs by the r-process and s-process. r-process:High neutron flux environment: supernovae Time between captures is less than decay time s-process: Low neutron flux environment Time between captures is greater than decay time 186,187 Os are shielded from r-process by 186 W & 187 R creating the cosmochronometer.