Experimental approaches to s-process branchings

Experimental approaches to s-process branchings René Reifarth Los Alamos National Laboratory Astrophysics and Nuclear Structure International Workshop XXXIV on Gross Properties of Nuclei and Nuclear Excitations Hirschegg, Kleinwalsertal, Austria, January 15 - 21, 2006

s-process path ~ t1/2-1(T) s-process path – branched A A+1 ~ scapturefn s-process path

Presolar grains: Left-overs from stellar events (E. Zinner, WUSTL)

What’s needed? • Reaction rates • Half-lives

Classical s-process Modern s-process models (AGB stars) 95Zr 93Zr Classical s-process 93Zr new n-facility sample production DANCE @ LANL DANCE @ LANL Connection between theory and experiment

1. neutrons neutrons sample 2. Ge Ge prompt g-rays g-rays (n,γ) @ radioctive isotopes Activation technique - DELAYED ( AX(n,g)A+1X(b-)A+1Y ) PROMPTg-detection (4p - scintillators)  TOF experiments  sensitivity to background  very high sensitivity (~ µg)  only average neutron energies  only if A+1X is “reasonable“ radioactive sample

Activation-Method AX(n,g)A+1X reaction detected via A+1X(b-) A+1Y decay Au copper proton beam Determination of neutron flux via 197Au(n,g)198Au neutron cone lithium AX Neutron source: 7Li(p,n)7Be

Experiment vs. previous estimates <s> = 22.6 + 2.4 mb MACS (mb) kT (keV)

LANSCE @ LANL

g-Detector: • 160 BaF2 crystals • 4 different shapes • Ri=17 cm, Ra=32 cm • 7 cm 6LiH inside • eg 90 % • ecasc 98 % neutrons: • spallation source • thermal .. 500 keV • 20 m flight path • 3 105 n/s/cm2/decade sample t1/2 > 100 d m ~ 1 mg Detector for Advanced Neutron Capture Experiments collimated neutrons beam 34 cm »Nuclear Astrophysics with Neutron Facilities and LANL and RIA«

155 156 153 0.7 a 152 154 Received funding 151 152 13 a 153 154 9 a 155 4.8 a 151 93 a 152 This year 151Sm combining to decoupled branching regions Gd Eu 148 150 149 Sm 148 5.4 d 147 2.6 a 149 50 h Pm Branch Point stable s-only 148 146 147 10 d Nd

A specific example: branchpoint 154Eu Branching ratio: f=  / ( + n) -decay rate:  = (ln 2) / t1/2 n-capture rate: n =n vT Nn f(154Eu) ~ 154Gd/152Gd

0.5 mg of 151Sm (t1/2 = 100 yr) with DANCE Multiplicity & Energy cuts allow optimization of the signal/background ratio Q = 8.2 MeV Esum (MeV)

Neutron Capture Cross Section (barn) Neutron Energy (eV) 0.5 mg of 151Sm(n,g) – TOF, t1/2 = 100 yr

What could be done in the future • Optimize neutron source • Increased source brightness • Improved sample production • FAIR, RIA, ISOLDE • NCAP • Short flight path • 7Li(p,n) • intense proton source • n-TOF2 • 20 m flight path • 20 GeV protons, spallation • DANCE upgrade • 10 m flight path • 0.8 MeV protons, spallation

Summary • (n,g) data on radioactive isotopes are extremely important for modern astrophysics • s-process branching analysis allows knowledge about stellar convections • DANCE contributes in the half live time range above a few hundred days • some important isotopes can be measured now, more will have to wait for future facilities • 152Eu, 154Eu, 153Gd is planned and funded

Experimental approaches to s-process branchings