neutron capture measurements for the weak s-process

Forschungszentrum Karlsruhein der Helmholtz-Gemeinschaft

Neutron capture measurements for the weak s-process

Michael Heil

Hirschegg workshop, January 2006

Outline:• Motivation• Nuclear data needs for the weak s-process• Results of (n,) cross section measurements (activation method) on

23Na, 27Al, 45Sc, 58Fe, 59Co, 63Cu, 65Cu, 79Br, 81Br, 87Br• Conclusions and outlook

Motivation

Michael Heil Hirschegg workshop, January 2006

• 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.

• 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.

• 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(,n)25Mg) - but also carbon shell burning could contribute (22Ne(,n)25Mg, 13C(,n)16O, 17O(,n)20Ne)

Effect of changing the cross section from 12.5 mb to 28.4 mb

62Ni(n,)

Motivation

ssolarr NNN

12N

Nlogε(A) log

H

A


• 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) 93612

N

Nlogε(A) log

H

A

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.

Methods:• TOF: measure (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

Problems: • small cross sections• resonance dominated • contributions from direct capture


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


Results - neutron capture cross sections

Isotope MACS @ kT=30 keV in mbarn Bao et al. @ kT=30keV in mbarn45Sc 57 ± 2 69 ± 558Fe 13.1 ± 0.6 13 ± 1.359Co 41 ± 2 38 ± 463Cu 53 ± 2 94 ± 1065Cu 29 ± 2 41 ± 579Br 626 ± 19 627 ± 4281Br 241 ± 9 313 ± 1687Rb 16.1 ± 2.0 15.5 ± 1.5


Neutron poisons can have a large effect on the neutron balance during the s-process e.g. 16O(n,), 12C(n,), 23Na(n,), …

Isotope MACS @ kT=25 keV in mbarn Bao et al. @ kT=25 keV in mbarn

23Na 1.81 ± 0.1 2.2 ± 0.227Al 3.3 ± 0.2 4.1 ± 0.3

Results – weak s-process abundances

combined effect of 59Co, 63Cu, 65Cu, and 81Br

Stellar model calculations performed by Marco Pignatari

25 M⊙ star at the end of carbon shell burning


Background due to elastic scattering

• Old measurements possibly suffer from underestimation of background from scattered neutrons.


PM

neutrons

C6D6

• 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

Conclusions and outlook


neutron capture measurements for the weak s-process

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