The Astrophysical 187 Re/ 187 Os Ratio: First Direct Measurement of the 187 Re(n, 2nγ) 186m Re Destruction Cross Section J. H. Kelley, NC State U. and.

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Measurement of the 187Re(n,2n)186mRe Destruction Cross Section

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

Nucleosynthesis: background r-process (produces 187 Re) High neutron flux environment: supernovae Time between captures is less than decay time s-process (produces 187 Os) 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

The 187 Re/ 187 Os Cosmochronometer: r-process nucleosynthesis S-process mechanism is reasonably well understood  Small uncertainty in 187 Os abundance related to 186 Os abundance r-process uncertainties are irrelevant –(if 187 Os abundance understood) 187 Re beta decays to 187 Os  187 Re(t ½ ) on the order of galactic lifetimes (10 10 years)  187 Os abundance vs. expected s-process abundance Ratio must be corrected for other reactions that destroy 187 Re

Motivation: Improve nuclear physics inputs for the 187 Re/ 187 Os Cosmochronometer We focus on 187 Re destruction reactions 187 Re(n,2n) 186 Re (Q=7.4 MeV)  1 data point ~1.5 MeV  ENDF gives 2.2 MeV 187 Re(n,2n) 186m Re  Complicated by 186m Re( t ½ ~ 2.0x10 5 years)  structure above the isomer is poorly known so  ( 187 Re(n,2n) 186m Re) is unknown

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

186 Re Decay Schemes of interest

Experimental Approach: Exploratory Measurement TUNL 187 Re(n, 2n  ) 186m Re 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  -ray flash 2 H(d,n) 3 He neutrons 2 H(d,n+p) 2 H neutrons I d =1  A 7.8 atm beam picko ff 2 H(d,n) 3 He

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

Data: Planar 5

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

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

Activation Products from neutrons on natural Rhenium Not as simple as we’d hoped Natural Target (n,2n)  185 Re(n,2n) 184 Re (observed)  187 Re(n,2n) 186 Re (observed)  187 Re(n,2n) 186m Re (not observed T 1/2 ~2.0e5 y) Also (n,g)  187 Re(n,  ) 188 Re (observed)  185 Re(n,  ) 186 Re (assumed)  Problems

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  -ray measurements (10 4 n/s)  Activation measurements with Cd screen (10 8 n/s) TUNL NNSA Grant # DE-FG52-06NA26155, U.S. Dept. of Energy DE-FG02-97FR41033 and NSF ANL U.S. Dept. of Energy DE-AC02-06CH11357

In Beam Information Run : Target In  Approx. 44 hours Run : Target Out  Approx. 17 hours E n =12MeV Average current: 1.5 micro-amps Neutron flux ~10 4 /s

Issues for the 187 Re/ 187 Os Cosmochronometer 187 Re (  -) 187 Os rate  Atomic /- 1.3 Gyr  Ion /- 2.0 yr Bosch et al PRL 77(1996) Os Abundance s-process ~ 186 Os abundance  186,187 Os(n,  ) rate Mosconi Prog.Part.Nucl.Phys. 59 (2007) 165  187 Os* 9.5 keV (n,  ) Mosconi ibid 187 Re Destruction Reactions  187 Re(n,  )  187 Re(n,2n) 186 Re  187 Re(n,2n) 186m Re (2.0 x10 5 y)

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

186,187,188 Os(n,  ) cross sections

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

Planar 6

Data/Results: Planar 5