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J. H. Kelley, NC State U. and Triangle Universities Nuclear Laboratories (TUNL),

The Astrophysical 187 Re/ 187 Os Ratio: First Direct Measurement of the 187 Re(n, 2n γ ) 186m Re Destruction Cross Section. J. H. Kelley, NC State U. and Triangle Universities Nuclear Laboratories (TUNL), D.B. Masters, Samford University (TUNL REU),

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J. H. Kelley, NC State U. and Triangle Universities Nuclear Laboratories (TUNL),

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  1. The Astrophysical 187Re/187Os Ratio:First Direct Measurement of the 187Re(n, 2nγ)186mRe Destruction 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

  2. 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

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

  4. 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

  5. 187Re(n,2n)186mReActivation Cross sections Yamamuro, Nuclear Science and Engineering, 118 (1994)249. INDC(NDS)-288 Depend heavily on lifetime 43.5 +/- 1.3 Gyr

  6. 186Re Decay Schemes of interest

  7. 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)

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

  9. 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 Re Fe Neutron Beam

  10. Data: Planar 5

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

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

  13. 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

  14. 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 • 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

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

  16. Issues for the187Re/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)

  17. 187Re half-life Bosch et al. PRL 77(1996)5190. 32.9 +/- 2.0 yr 43.5 +/- 1.3 Gyr

  18. 186,187,188Os(n,g) cross sections

  19. 187Os*9.5 keV(n,g) cross sections

  20. Planar 6

  21. Planar 6

  22. Data/Results: Planar 5

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