Neutron capture cross sections on light nuclei

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Neutron capture cross sections on light nuclei

• M. Heil, F. Käppeler, E. Uberseder

• Outline
• Motivation neutron poisons
• Results of (n,g) cross section measurements
  (activation method) on
• 23Na, 27Al, 45Sc, 41K, 58Fe, 59Co, 63Cu, 65Cu,
  79Br, 81Br, 87Br
• Comparison with previous TOF measurements
• Conclusions and outlook

Torino workshop, Granada, February 2006
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Motivation
Neutron poisons Although the cross sections of
light nuclei are small, they can have large
effects on the neutron balance during the
s-process since they are very abundant in stars.
Examples 12C(n,g), 14N(n,p),
16O(n,g), 20,21,22Ne(n,g), 23Na(n,g),
24,25,26Mg(n,g)
Michael Heil

Torino
workshop, Granada, February 2006
3
Experimental challenges

• The neutron capture cross sections of light
  nuclei
• are small
• are resonance dominated
• have Direct Capture contributions

Therefore, these measurements are difficult to
perform.
For stellar models we need Maxwellian averaged
neutron capture cross sections for thermal
energies of kT 5 90 keV.

• 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

Torino
workshop, Granada, February 2006
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TOF method
g
Pulsed neutron source
sample
neutron production
Flight path s
g
g-ray detector
Heavy nuclei
Light nuclei
Michael Heil

Torino
workshop, Granada, February 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.
HPGe detector

• Only possible when product nucleus is
  radioactive
• Only MACS at 25 keV
• High sensitivity -gt small sample masses or small
  cross sections
• Use of natural samples possible, no enriched
  sample necessary
• Direct capture component included

Lead shield
Michael Heil

Torino
workshop, Granada, February 2006
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Results - neutron capture cross sections at kT25
keV
Preliminary results!
Isotope MACS _at_ kT25 keV in mbarn Bao et al. _at_ kT25 keV in mbarn
23Na 1.8 0.1 2.2 0.2
27Al 3.3 0.2 4.1 0.3
41K 22.1 0.2 24.7 0.9
45Sc 64 3 80 7
Michael Heil

Torino
workshop, Granada, February 2006
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Comparison Activation - TOF
Mass Number

• old TOF measurements seem to overestimate the
  cross sections of light nuclei.
• Larger uncertainties then quoted.

Michael Heil

Torino
workshop, Granada, February 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

Torino
workshop, Granada, February 2006
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To be checked !!!
Nuclei for which only old TOF measurements are
available
20Ne, 24Mg, 25Mg, 31P, 32S, 33S, 39K, 41K, 40Ca,
42Ca, 43Ca, 44Ca, 46Ti, 47Ti, 48Ti, 49Ti, 51V,
50Cr, 52Cr, 53Cr, 54Cr, 55Mn
64Zn, 66Zn, 67Zn, 68Zn, 86Sr, 87Sr, 90Zr, 91Zr,
92Zr, 93Zr, 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 99Tc,
100Ru, 101Ru, 102Ru, 104Ru, 104Pd, 105Pd, 106Pd,
107Pd, 108Pd, 110Pd, 127I, 129I, 182W, 183W,
184W, 186W, 198Hg, 199Hg, 200Hg, 201Hg, 202Hg,
204Hg, 203Tl, 205Tl
Michael Heil

Torino
workshop, Granada, February 2006
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Energy dependent MACSs
We have measured the MACS at kT25 keV 1.81
0.1 mbarn How to get MACS at thermal energies
from 5 keV 100 keV ? Calculation based on JEFF
2.0 mbarn
Michael Heil

Torino
workshop, Granada, February 2006
11
Activation measurement at 5 keV
Using the 18O(p,n) reaction at Ep2582 keV gives
a Maxwellian neutron distribution at kT5 keV.
Results at kT5 keV for 23Na 9.8 0.5 mbarn
Michael Heil

Torino
workshop, Granada, February 2006
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23Na(n,g)
kT (keV) This work (mbarn) Bao et al. (mbarn)
5 9.8 0.5 1.4
10 3.9 5.2
15 2.6 3.4
20 2.1 2.7
25 1.8 0.1 2.2 0.2
30 1.6 2.1
40 1.4 1.7
50 1.2 1.5
60 1.1 1.4
80 1.0 1.3
100 0.9 1.2
Eres2.78 keV, Gn3614.7 eV, Gg0.35
eV Eres35.3 keV, Gn1.370.16 eV,
Gg0.850.14 eV Eres53.0 keV, Gn10452.2 eV,
Gg0.960.10 eV Now Eres2.78 keV, Gn3614.7
eV, Gg0.25 eV
E. Uberseder
Michael Heil

Torino
workshop, Granada, February 2006
13
Conclusions and outlook

• We have measured the MACS of several light
  nuclei.
• Old TOF measurements seem to systematically
  overestimate the cross sections.
• Many neutron capture cross sections are not
  known with sufficient accuracy.
• Future measurements
• - 30Si, 37Cl, 51V, 54Cr, 55Mn,
• TOF measurements are important to determine the
  temperature dependence.

Michael Heil

Torino
workshop, Granada, February 2006
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KadonisKarlsruhe Astrophysical Database of
Nucleosynthesis in Stars
Online data base for neutron capture cross
sections (I. Dillmann, R. Plag)
http//nuclear-astrophysics.fzk.de/kadonis
Example 1
Michael Heil

Torino
workshop, Granada, February 2006
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Comparison TOF 4p - TOF
TOF measurements with C6D6 detectors seem to
overestimate the cross sections of light nuclei.
Michael Heil

Torino
workshop, Granada, February 2006
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Comparison TOF 4p - TOF
Examples 134Ba 1976 225 35 1978 221
35 1996 179 6 93Nb 1976 310 17 1980 260
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old TOF measurements with C6D6 detectors seem
to overestimate the cross sections of light
nuclei.
Michael Heil

Torino
workshop, Granada, February 2006