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Neutron capture cross sections on light nuclei

Neutron capture cross sections on light nuclei. Outline: Motivation: neutron poisons Results of (n, g ) cross section measurements (activation method) on 23 Na, 27 Al, 45 Sc, 41 K , 58 Fe, 59 Co, 63 Cu, 65 Cu, 79 Br, 81 Br, 87 Br Comparison with previous TOF measurements

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Neutron capture cross sections on light nuclei

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  1. 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 M. Heil, F. Käppeler, E. Uberseder Torino workshop, Granada, February 2006

  2. 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 kT=25 keV in laboratory, • measure directly MACS Michael Heil Torino workshop, Granada, February 2006

  4. TOF method g Pulsed neutron source sample neutron production Flight path: s g-ray detector g Heavy nuclei Light nuclei Michael Heil Torino workshop, Granada, February 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. HPGe detector • Only possible when product nucleus is radioactive • 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 Lead shield Michael Heil Torino workshop, Granada, February 2006

  6. Results - neutron capture cross sections at kT=25 keV Preliminary results! Michael Heil Torino workshop, Granada, February 2006

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 23Na(n,g) 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 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

  14. 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

  15. 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

  16. Comparison TOF 4p - TOF Examples: 134Ba 1976: 225 ± 35 1978: 221 ± 35 1996: 179 ± 6 93Nb 1976: 310 ± 17 1980: 260 ±16 “old” 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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