Neutron capture measurements for the weak s-process

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Neutron capture measurements for the weak
s-process

• Michael Heil

• 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

Hirschegg workshop, January 2006
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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 gt10 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
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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
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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 kT25 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 kT25
  keV in laboratory,
• measure directly MACS

Michael Heil


Hirschegg workshop, January 2006
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Activation technique at kT25 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 kT25 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 -gt 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
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Results - neutron capture cross sections
Isotope MACS _at_ kT30 keV in mbarn Bao et al. _at_ kT30keV in mbarn
45Sc 57 2 69 5
58Fe 13.1 0.6 13 1.3
59Co 41 2 38 4
63Cu 53 2 94 10
65Cu 29 2 41 5
79Br 626 19 627 42
81Br 241 9 313 16
87Rb 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,g), 12C(n,g), 23Na(n,g),
Isotope MACS _at_ kT25 keV in mbarn Bao et al. _at_ kT25 keV in mbarn
23Na 1.81 0.1 2.2 0.2
27Al 3.3 0.2 4.1 0.3
Michael Heil


Hirschegg workshop, January 2006
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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
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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
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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 kT90 keV

Michael Heil


Hirschegg workshop, January 2006