Transcript Kein Folientitel

Forschungszentrum Karlsruhe
in der Helmholtz-Gemeinschaft
M. Heil, F. Käppeler, E. Uberseder
Neutron capture cross sections on light nuclei
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
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
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:
• Activation:
Michael Heil
measure s(En) between 0.1 and 500 keV by time of flight,
determine MACS for stellar spectrum
produce stellar spectrum at kT=25 keV in laboratory,
measure directly MACS
Torino workshop, Granada, February 2006
TOF method
g
Pulsed neutron source
neutron
production
sample
Flight path: s
g
g-ray
detector
Heavy nuclei
Light nuclei
Michael Heil
Torino workshop, Granada, February 2006
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.
HPGe
detector
• Only possible when product nucleus is radioactive
Lead shield
• Only MACS at 25 keV
• High sensitivity -> small sample masses or small cross sections
• Use of natural samples possible, no enriched sample necessary
• Direct capture component included
Michael Heil
Torino workshop, Granada, February 2006
Results - neutron capture cross sections at kT=25 keV
Preliminary results!
Isotope
23Na
27Al
41K
45Sc
Michael Heil
MACS @ kT=25 keV
in mbarn
1.8 ± 0.1
Bao et al. @ kT=25 keV
in mbarn
2.2 ± 0.2
3.3 ± 0.2
22.1 ± 0.2
64 ± 3
4.1 ± 0.3
24.7 ± 0.9
80 ± 7
Torino workshop, Granada, February 2006
Cross Section Ratio Activation/TOF
Comparison Activation - TOF
2
1.5
1
0.5
0
Mass 0Number 50
100
150
200
250
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
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
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
Energy dependent MACS’s
We have measured the MACS at kT=25 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
Activation measurement at 5 keV
Using the 18O(p,n) reaction at Ep=2582 keV gives a Maxwellian neutron
distribution at kT=5 keV.
Results at kT=5 keV for 23Na: 9.8 ± 0.5 mbarn
Michael Heil
Torino workshop, Granada, February 2006
23Na(n,g)
kT This work Bao et al.
(keV) (mbarn) (mbarn)
5
9.8 ± 0.5 1.4
Eres=2.78 keV, Gn=361±4.7 eV,
Gg=0.35 eV
Eres=35.3 keV, Gn=1.37±0.16 eV, Gg=0.85±0.14 eV
Eres=53.0 keV, Gn=1045±2.2 eV, Gg=0.96±0.10 eV
Now:
Eres=2.78 keV, Gn=361±4.7 eV, Gg=0.25 eV
10
15
20
3.9
2.6
2.1
5.2
3.4
2.7
25
30
40
50
1.8 ± 0.1
1.6
1.4
1.2
2.2 ± 0.2
2.1
1.7
1.5
60
80
1.1
1.0
1.4
1.3
100
0.9
1.2
E. Uberseder
Michael Heil
Torino workshop, Granada, February 2006
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
Kadonis
Karlsruhe 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
Cross Section Ratio TOF 4Pi / TOF C6D6
Comparison TOF 4p - TOF
2
1.5
1
0.5
0
0
50
100
150
200
Mass Number
TOF measurements with C6D6 detectors seem to overestimate the cross
sections of light nuclei.
Michael Heil
Torino workshop, Granada, February 2006
Comparison TOF 4p - TOF
Cross Section Ratio TOF 4Pi / TOF C6D6
2
Examples:
134Ba
1976: 225 ± 35
1978: 221 ± 35
1996: 179 ± 6
1.5
1
93Nb
1976: 310 ± 17
1980: 260 ±16
0.5
0
1970
1975
1980
1985
1990
1995
2000
Year
“old” TOF measurements with C6D6 detectors seem to overestimate the cross
sections of light nuclei.
Michael Heil
Torino workshop, Granada, February 2006