Transcript The Astrophysical 187Re/187Os Ratio: First Measurement of
The Astrophysical 187 Re/ 187 Os Ratio: First Direct Measurement of the 187 Re(n, 2n γ) 186m Re 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 a Small uncertainty in 187 Os abundance related to 186 Os abundance 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 r-process uncertainties are irrelevant –(if 187 Os abundance understood) Ratio must be corrected for other reactions that destroy 187 Re
Issues for the 187 Re/ 187 Os Cosmochronometer
187 Re ( b -) 187 Os rate Atomic 43.5 +/- 1.3 Gyr Ion 32.9 +/- 2.0 yr Bosch
et al
PRL
77
(1996)5190.
187 Os Abundance s-process ~ 186 Os abundance 186,187 Os(n, g ) rate Mosconi Prog.Part.Nucl.Phys.
59
(2007) 165 187 Os* 9.5 keV (n, g ) Mosconi ibid 187 Re Destruction Reactions 187 Re(n, g ) 187 Re(n,2n) 186 Re 187 Re(n,2n) 186m Re (2.0 x10 5 y)
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 b @14.8 MeV ENDF gives 2.2 b @14.8 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 s ( 187 Re(n,2n) 186m Re) is unknown
Motivation: Improve nuclear physics inputs for the 187 Re/ 187 Os Cosmochronometer: 187
Re(n,2n)
186m
Re Cross sections
Yamamuro, Nuclear Science and Engineering, 118 (1994)249.
INDC(NDS)-288 Depend heavily on lifetime 43.5 +/- 1.3 Gyr
186 Re Decay Schemes of interest
Experimental Approach: Exploratory Measurement
TUNL 187 Re(n, 2n g ) 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
I d =1
m
A beam picko ff 2 H(d,n) 3 He 7.8 atm
2 H(d,n) 3 He neutrons 2 H(d,n+p) 2 H neutrons g -ray flash
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 Fe Re Neutron Beam
Data: Planar 5
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)
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, g ) 188 Re (observed) 185 Re(n, g ) 186 Re (assumed) a 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- coincidences Two phases for future measurements Online prompt g -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-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 E n =12MeV Average current: 1.5 micro-amps Neutron flux ~10 4 /s
187
Re half-life
Bosch
et al.
PRL 77(1996)5190.
32.9 +/- 2.0 yr 43.5 +/- 1.3 Gyr
186,187,188
Os(n,
g
) cross sections
187
Os*
9.5 keV
(n,
g