The Astrophysical 187Re/187Os Ratio: First Direct Measurement of the 187Re(n, 2nγ)186mRe Cross Section J. H. Kelley, NC State U. and Triangle Universities Nuclear Laboratories (TUNL), D.B. Masters, Samford University (TUNL REU), S. Hammond and H.J. Karwowski UNC-Chapel Hill and TUNL, E. Kwan, A. Hutcheson, , A.P. Tonchev, W. Tornow, Duke and TUNL, J.P. Green, F.G. Kondev, S. Zhu, Argonne National Laboratory

Nucleosynthesis: background r-process (produces 187Re) High neutron flux environment: supernovae Time between captures is less than decay time s-process (produces 187Os) Low neutron flux environment Time between captures is greater than decay time 186,187Os are shielded from r-process by 186W &187R

The 187Re/187Os Cosmochronometer: r-process nucleosynthesis S-process mechanism is reasonably well understood aSmall uncertainty in 187Os abundance related to 186Os abundance 187Re beta decays to 187Os 187Re(t½) on the order of galactic lifetimes (1010 years) 187Os abundance vs. expected s-process abundance r-process uncertainties are irrelevant –(if 187Os abundance understood) Ratio must be corrected for other reactions that destroy 187Re

Issues for the 187Re/187Os Cosmochronometer 187Re (b-)187Os rate Atomic 43.5 +/- 1.3 Gyr Ion 32.9 +/- 2.0 yr Bosch et al PRL 77(1996)5190. 187Os Abundance s-process ~186Os abundance 186,187Os(n,g) rate Mosconi Prog.Part.Nucl.Phys. 59 (2007) 165 187Os*9.5 keV(n,g) Mosconi ibid 187Re Destruction Reactions 187Re(n,g) 187Re(n,2n)186Re 187Re(n,2n)186mRe (2.0 x105 y)

We focus on 187Re destruction reactions 187Re(n,2n)186Re (Q=7.4 MeV) Motivation: Improve nuclear physics inputs for the 187Re/187Os Cosmochronometer We focus on 187Re destruction reactions 187Re(n,2n)186Re (Q=7.4 MeV) 1 data point ~1.5 b @14.8 MeV ENDF gives 2.2 b @14.8 MeV 187Re(n,2n)186mRe Complicated by 186mRe( t ½ ~ 2.0x105 years) structure above the isomer is poorly known so s(187Re(n,2n)186mRe) is unknown

Motivation: Improve nuclear physics inputs for the 187Re/187Os Cosmochronometer: 187Re(n,2n)186mRe Cross sections Depend heavily on lifetime 43.5 +/- 1.3 Gyr Yamamuro, Nuclear Science and Engineering, 118 (1994)249. INDC(NDS)-288

186Re Decay Schemes of interest

Experimental Approach: Exploratory Measurement TUNL 187Re(n, 2ng)186mRe measurement Natural Re cylindrical target in hand Array of HPGe detectors (2 clovers and 2 planars) 12 MeV neutrons 2 days of beamtime Objectives: Find gamma rays populating the isomer (singles) Build decay scheme (coincidence)

TUNL Shielded Neutron Source g-ray flash 2H(d,n)3He neutrons 2H(d,n+p)2H neutrons Id =1mA 7.8 atm beam pickoff 2H(d,n)3He

Target Re cylinder (2.35 g natural Re)(187Re~63%) Iron endcaps 2 layers of iron on each end Total Fe mass≈125 mg Circular surface of cylinder perpendicular to beam Fe Re Neutron Beam

Data: Planar 5

Planar 5 Results Sum at 144 keV: 789 ±153 counts Complications with target composition Complications with target geometry: high X-ray attenuation (thin foil is preferred)

TUNL Low Background Counting Area: Target showed appreciable 187Re(n,2n)186Reg.s.(3.7 Day) activation

Activation products from neutrons on natural Rhenium Not as simple as we’d hoped Natural Target (n,2n) 185Re(n,2n)184Re (observed) 187Re(n,2n)186Re (observed) 187Re(n,2n)186mRe (not observed T1/2~2.0e5 y) Also (n,g) 187Re(n,g)188Re (observed) 185Re(n,g)186Re (assumed) aProblems

Discussion/Conclusion Our experience has taught us Planar detectors have better prospects than clovers detectors for these low energy X-rays Use Re foil for simpler geometry and fewer systematic assumptions- coincidences Two phases for future measurements Online prompt g-ray measurements (104 n/s) Activation measurements with Cd screen (108 n/s) TUNL NNSA Grant # DE-FG52-06NA26155, U.S. Dept. of Energy DE-FG02-97FR41033 and NSF-05-52723 ANL U.S. Dept. of Energy DE-AC02-06CH11357

In Beam Information Run 683-727: Target In Run 728-744: Target Out Approx. 44 hours Run 728-744: Target Out Approx. 17 hours En=12MeV Average current: 1.5 micro-amps Neutron flux ~104 /s

187Re half-life 32.9 +/- 2.0 yr 43.5 +/- 1.3 Gyr Bosch et al. PRL 77(1996)5190.

186,187,188Os(n,g) cross sections

187Os*9.5 keV(n,g) cross sections

Planar 6

Planar 6

Data/Results: Planar 5