Neutron capture measurements for the weak s-process

1. Neutron capture measurements for the weak s-process Outline: • Motivation • Nuclear data needs for the weak s-process • Results of (n,g) cross section measurements (activation method) on 23Na, 27Al, 45Sc, 58Fe, 59Co, 63Cu, 65Cu, 79Br, 81Br, 87Br • Conclusions and outlook Michael Heil Hirschegg workshop, January 2006

2. Motivation • The weak s-component is mainly responsible for the production of the elements between Fe and Y. • Contrary to the main component of the s-process, the weak s-process is not so well understood. 62Ni(n,g) • Experiments: • - Neutron capture cross sections are most • important quantities but show large uncertainties. • Modeling: • - most promising scenario is core He burning of • massive stars M >10 M⊙ (22Ne(a,n)25Mg) • - but also carbon shell burning could contribute • (22Ne(a,n)25Mg, 13C(a,n)16O, 17O(a,n)20Ne) Effect of changing the cross section from 12.5 mb to 28.4 mb • Since the local equilibrium does not hold for the weak s-process, propagation effects are • expected. Therefore, one needs to know ALL neutron capture cross sections. Michael Heil Hirschegg workshop, January 2006

3. Motivation • The weak s-process takes place in massive stars and is therefore also important for the r-process since it determines the composition before the supernova explosion. • Reliable s-abundances are indispensable for determining the r-abundances • via the r-residual method: Sneden et al., Ap. J. 591 (2003) 936 Michael Heil Hirschegg workshop, January 2006

4. Nuclear data needs for the weak s-process s-process abundances are determined mainly by Maxwellian averaged neutron capture cross sections for thermal energies of kT=25 – 90 keV. • Problems: • small cross sections • resonance dominated • contributions from direct capture • Methods: • TOF: measure s(En) between 0.1 and 500 keV by time of flight, • determine MACS for stellar spectrum • Activation: produce stellar spectrum at kT=25 keV in laboratory, • measure directly MACS Michael Heil Hirschegg workshop, January 2006

5. Activation technique at kT=25 keV • Neutron production via 7Li(p,n) reaction at a proton energy of 1991 keV. • Induced activity can be measured after irradiation with HPGe detectors. • Result: MACS at kT=25 keV • Only possible when product nucleus is radioactive • Only MACS at 5 keV [18O(p,n)], 25 keV [7Li(p,n)], and 52 keV [3H(p,n)] • High sensitivity -> small sample masses or small cross sections • Use of natural samples possible, no enriched sample necessary • Direct capture component included Michael Heil Hirschegg workshop, January 2006

6. Results - neutron capture cross sections Neutron poisons can have a large effect on the neutron balance during the s-process e.g. 16O(n,g), 12C(n,g), 23Na(n,g), … Michael Heil Hirschegg workshop, January 2006

7. Results – weak s-process abundances 25 M⊙ star at the end of carbon shell burning combined effect of 59Co, 63Cu, 65Cu, and 81Br Stellar model calculations performed by Marco Pignatari Michael Heil Hirschegg workshop, January 2006

8. Background due to elastic scattering • Old measurements possibly suffer from • underestimation of background from • scattered neutrons. PM C6D6 neutrons Michael Heil Hirschegg workshop, January 2006

9. Conclusions and outlook • We have measured the MACS of several light and medium mass nuclei. • Old TOF measurements seem to systematically overestimate the cross sections. • Many neutron capture cross sections are not known with sufficient accuracy. • Neutron capture cross sections of all involved isotopes are necessary. • Neutron capture cross sections of neutron poisons are also important. • Future measurements: • - Zn isotopes • - Ga isotopes • - Ge isotopes (no existing data) • - Se isotopes (no existing data) • - also TOF measurements for MACS up to kT=90 keV Michael Heil Hirschegg workshop, January 2006