j. h. kelley, nc state u. and triangle universities nuclear laboratories (tunl),

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

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

Small 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

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

(187Re(n,2n)186mRe) is unknown

187Re(n,2n)186mRe

Activation Cross sections

Yamamuro, Nuclear Science and Engineering, 118 (1994)249.INDC(NDS)-288

43.5 +/- 1.3 Gyr

Depend heavily on lifetime

186Re Decay Schemes of interest

Experimental Approach:

Exploratory Measurement

TUNL 187Re(n, 2n)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

-ray flash

2H(d,n)3He neutrons

2H(d,n+p)2H

neutronsId

=1A7.8 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

Neutron Beam

ReFe

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 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,)188Re (observed) 185Re(n,)186Re (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 (104 n/s) Activation measurements with Cd screen (108 n/s)

TUNLNNSA 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 Approx. 44 hours

Run 728-744: Target Out Approx. 17 hours

En=12MeV

Average current: 1.5 micro-amps Neutron flux ~104 /s

Issues for the 187Re/187Os

Cosmochronometer

187Re (-)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,) rate Mosconi Prog.Part.Nucl.Phys. 59 (2007) 165

187Os*9.5 keV(n,) Mosconi ibid 187Re Destruction Reactions

187Re(n,) 187Re(n,2n)186Re 187Re(n,2n)186mRe (2.0 x105 y)

187Re half-life

Bosch et al. PRL 77(1996)5190.

32.9 +/- 2.0 yr

43.5 +/- 1.3 Gyr

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

187Os*9.5 keV(n,) cross sections

Planar 6

Planar 6

Data/Results: Planar 5