Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft Neutron capture cross sections on light nuclei M. Heil, F. Käppeler, E. Uberseder Torino workshop, Granada, February 2006 Outline: Motivation: neutron poisons Results of (n,  ) 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 Conclusions and outlook

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: 12 C(n,  ), 14 N(n,p), 16 O(n,  ), 20,21,22 Ne(n,  ), 23 Na(n,  ), 24,25,26 Mg(n,  )… Motivation 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. Michael Heil Torino workshop, Granada, February 2006 For stellar models we need Maxwellian averaged neutron capture cross sections for thermal energies of kT = 5 – 90 keV. Methods: TOF: measure  (E n ) 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

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

Activation technique at kT=25 keV Neutron production via 7 Li(p,n) reaction at a proton energy of 1991 keV. Michael Heil Torino workshop, Granada, February 2006 HPGe detector Lead shield 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 Induced activity can be measured after irradiation with HPGe detectors.

Results - neutron capture cross sections at kT=25 keV kT=25 keV in mbarn Bao et kT=25 keV in mbarn 23 Na1.8 ± ± Al3.3 ± ± K22.1 ± ± Sc64 ± 380 ± 7 Preliminary results! Michael Heil Torino workshop, Granada, February 2006

Comparison Activation - TOF Michael Heil Torino workshop, Granada, February 2006 Mass Number “old” TOF measurements seem to overestimate the cross sections of light nuclei. Larger uncertainties then quoted.

Background due to elastic scattering Old measurements possibly suffer from underestimation of background from scattered neutrons. PM neutrons C6D6C6D6 Michael Heil Torino workshop, Granada, February 2006

To be checked !!! 20 Ne, 24 Mg, 25 Mg, 31 P, 32 S, 33 S, 39 K, 41 K, 40 Ca, 42 Ca, 43 Ca, 44 Ca, 46 Ti, 47 Ti, 48 Ti, 49 Ti, 51 V, 50 Cr, 52 Cr, 53 Cr, 54 Cr, 55 Mn 64 Zn, 66 Zn, 67 Zn, 68 Zn, 86 Sr, 87 Sr, 90 Zr, 91 Zr, 92 Zr, 93 Zr, 92 Mo, 94 Mo, 95 Mo, 96 Mo, 97 Mo, 99 Tc, 100 Ru, 101 Ru, 102 Ru, 104 Ru, 104 Pd, 105 Pd, 106 Pd, 107 Pd, 108 Pd, 110 Pd, 127 I, 129 I, 182 W, 183 W, 184 W, 186 W, 198 Hg, 199 Hg, 200 Hg, 201 Hg, 202 Hg, 204 Hg, 203 Tl, 205 Tl Nuclei for which only “old” TOF measurements are available: 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 Results at kT=5 keV for 23 Na: 9.8 ± 0.5 mbarn Using the 18 O(p,n) reaction at E p =2582 keV gives a Maxwellian neutron distribution at kT=5 keV. Michael Heil Torino workshop, Granada, February 2006

23 Na(n,  ) kT (keV) This work (mbarn) Bao et al. (mbarn) 59.8 ± ± ± E res =2.78 keV,  n =361±4.7 eV,   =0.35 eV E res =35.3 keV,  n =1.37±0.16 eV,   =0.85±0.14 eV E res =53.0 keV,  n =1045±2.2 eV,   =0.96±0.10 eV Now: E res =2.78 keV,  n =361±4.7 eV,   =0.25 eV Michael Heil Torino workshop, Granada, February 2006 E. Uberseder

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: - 30 Si, 37 Cl, 51 V, 54 Cr, 55 Mn, … TOF measurements are important to determine the temperature dependence. Conclusions and outlook Michael Heil Torino workshop, Granada, February 2006

Kadonis Karlsruhe Astrophysical Database of Nucleosynthesis in Stars Michael Heil Torino workshop, Granada, February Online data base for neutron capture cross sections (I. Dillmann, R. Plag) Example 1

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

Comparison TOF 4  - TOF Michael Heil Torino workshop, Granada, February 2006 “old” TOF measurements with C 6 D 6 detectors seem to overestimate the cross sections of light nuclei. Examples: 134 Ba 1976: 225 ± : 221 ± : 179 ± 6 93 Nb 1976: 310 ± : 260 ±16