1. 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

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
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

4. Issues for the 187Re/187Os Cosmochronometer
187Re (b-)187Os rate
Atomic /- 1.3 Gyr
Ion /- 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)

5. 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 MeV
ENDF gives 2.2 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

6. 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

7. 186Re Decay Schemes of interest

8. 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)

9. 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

11. 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

12. Data: Planar 5

13. 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)

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

15. 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

16. 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
ANL
U.S. Dept. of Energy DE-AC02-06CH11357

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

19. 187Re half-life 32.9 +/- 2.0 yr 43.5 +/- 1.3 Gyr
Bosch et al. PRL 77(1996)5190.

20. 186,187,188Os(n,g) cross sections

22. 187Os*9.5 keV(n,g) cross sections

23. Planar 6

24. Planar 6

25. Data/Results: Planar 5