Forschungszentrum Karlsruhein 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

23Na, 27Al, 45Sc, 41K, 58Fe, 59Co, 63Cu, 65Cu, 79Br, 81Br, 87Br• 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:12C(n,), 14N(n,p), 16O(n,), 20,21,22Ne(n,), 23Na(n,), 24,25,26Mg(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 (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

TOF method

Michael Heil Torino workshop, Granada, February 2006

Pulsed neutron sourceneutron

production

sample

Flight path: s

Heavy nuclei

Light nuclei

-raydetector

Activation technique at kT=25 keV

Neutron production via 7Li(p,n) reaction at a proton energy of 1991 keV.

Michael Heil Torino workshop, Granada, February 2006

HPGedetector

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

Isotope MACS @ kT=25 keV

in mbarn

Bao et al. @ kT=25 keV

in mbarn23Na 1.8 ± 0.1 2.2 ± 0.227Al 3.3 ± 0.2 4.1 ± 0.341K 22.1 ± 0.2 24.7 ± 0.9

45Sc 64 ± 3 80 ± 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.

0

0.5

1

1.5

2

0 50 100 150 200 250

Mass Number

Cro

ss

Se

cti

on

Ra

tio

Ac

tiv

ati

on

/TO

F

Background due to elastic scattering

• Old measurements possibly suffer from underestimation of background from scattered neutrons.

PM

neutrons

C6D6

Michael Heil Torino workshop, Granada, February 2006

To be checked !!!

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

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 23Na: 9.8 ± 0.5 mbarn

Using the 18O(p,n) reaction at Ep=2582 keV gives a Maxwellian neutron distribution at kT=5 keV.

Michael Heil Torino workshop, Granada, February 2006

23Na(n,)

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

Eres=2.78 keV, n=361±4.7 eV, =0.35 eVEres=35.3 keV, n=1.37±0.16 eV, =0.85±0.14 eVEres=53.0 keV, n=1045±2.2 eV, =0.96±0.10 eV

Now:Eres=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:- 30Si, 37Cl, 51V, 54Cr, 55Mn, …

• TOF measurements are important to determine the temperature dependence.

Conclusions and outlook

Michael Heil Torino workshop, Granada, February 2006

KadonisKarlsruhe Astrophysical Database of Nucleosynthesis in Stars

Michael Heil Torino workshop, Granada, February 2006

http://nuclear-astrophysics.fzk.de/kadonis

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.

0

0.5

1

1.5

2

0 50 100 150 200

Mass Number

Cro

ss S

ecti

on

Rat

io T

OF

4P

i /

TO

F C

6D6

Comparison TOF 4 - TOF

Michael Heil Torino workshop, Granada, February 2006

“old” TOF measurements with C6D6 detectors seem to overestimate the cross sections of light nuclei.

Examples: 134Ba1976: 225 ± 351978: 221 ± 351996: 179 ± 6

93Nb1976: 310 ± 171980: 260 ±16

0

0.5

1

1.5

2

1970 1975 1980 1985 1990 1995 2000

Year

Cro

ss

Se

cti

on

Ra

tio

TO

F 4

Pi

/ T

OF

C6

D6