J. H. Kelley, NC State U. and Triangle Universities Nuclear Laboratories (TUNL),
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Transcript of J. H. Kelley, NC State U. and Triangle Universities Nuclear Laboratories (TUNL),
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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
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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
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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
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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
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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
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186Re Decay Schemes of interest
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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)
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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
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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
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Data: Planar 5
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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
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TUNL Low Background Counting Area:Target showed appreciable 187Re(n,2n)186Reg.s.(3.7 Day) activation
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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
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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
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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
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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)
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187Re half-life
Bosch et al. PRL 77(1996)5190.
32.9 +/- 2.0 yr
43.5 +/- 1.3 Gyr
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186,187,188Os(n,) cross sections
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187Os*9.5 keV(n,) cross sections
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Planar 6
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Planar 6
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Data/Results: Planar 5