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

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

What’s needed? Reaction rates Half-lives

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

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

Activation-Method AX(n,g)A+1X reaction detected via A+1X(b-) A+1Y decay copper Au 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

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

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

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)

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

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