Neutron capture cross section of
18
O and its astrophysical implications
J. Meissner, H. Schatz, J. Go ¨rres, H. Herndl,
*
and M. Wiescher
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556
H. Beer and F. Ka ¨ppeler
Forschungszentrum Karlsruhe, Institut fu ¨r Kernphysik III, P.O. Box 3640, D-76021 Karlsruhe, Germany
Received 30 June 1995
The neutron capture rate on
18
O is of considerable interest in the interpretation of nucleosynthesis in
inhomogeneous big bang scenarios and for stellar helium burning in massive red giant stars as well as AGB
stars. We measured the reaction cross section of
18
O( n , )
19
O at the Forschungszentrum Karlsruhe with a fast
cyclic neutron activation technique at laboratory neutron energies of 25, 129, 152, 250, and 370 keV. Direct
capture and shell model calculations were performed to interpret the results as well as previous unpublished
data. Contributions to the reaction cross section and to the new stellar reaction rate will be discussed.
PACS numbers: 25.40.Lw, 21.10.Jx, 25.40.Ny, 26.35.+c
I. INTRODUCTION
Neutron capture on
18
O is of considerable interest for the
interpretation of nucleosynthesis in inhomogeneous big bang
scenarios 1,2. In these scenarios the nucleus
18
O represents
a bottleneck being mainly produced by the two dominant
reactions sequences, via
14
C( , ) 1,3,4 and via
17
O( n , ) 2,5. Further nucleosynthesis towards higher
masses is controlled by the reaction rate of
18
O( n , )
19
O.
Only if this reaction is stronger than the
18
O( p , )
15
N reac-
tion can material be processed out of the CNO range to the
region above A =20 2.
The reaction rate of
18
O( n , )
19
O is also of interest for
stellar helium burning. Helium core burning in massive red
giant stars as well as He-shell burning in low mass asymp-
totic giant branch AGB stars is considered to be the main
site for the s process 6–9. In both scenarios,
18
O is pro-
duced abundantly via capture,
14
N( , )
18
F(
+
, )
18
O,
from
14
N, the main reaction product of the preceding
CNO hydrogen burning. This triggers the neutron pro-
duction for the s process via the capture sequence,
18
O( , )
22
Ne( , n ). For a recent discussion see 10. With
an enhanced rate of
18
O( n , ), however, the reaction would
be a neutron poison and would limit the neutron production
triggered by
18
O( , )
22
Ne.
Previous estimates of the
18
O( n , ) reaction rate 11,2
are based on the thermal cross section
th
=0.16 mb, which
represents the s -wave component in the reaction rate as well
as direct capture p -wave contributions. Also estimated 2
was the possible contribution of a resonance at E
R
=152 keV,
which was proposed to dominate the reaction rate at higher
temperatures. Higher energy resonances at E
R
= 371, 625,
and 746 keV have been observed previously but no experi-
mental details are given 12. These resonances seem to cor-
respond to neutron unbound states in
19
O 21, which have
been observed in transfer reaction studies. No detailed infor-
mation is available about the level parameters, and their pos-
sible contributions to the reaction rate have been neglected
in the previous estimates. The reaction cross section of
18
O( n , ) has been measured for a wide range of neutron
energies to study the possible resonance at E
R
=152 keV and
the low energy tails of the previously observed resonances
above 370 keV as well as the predicted p -wave direct cap-
ture contributions.
In the following sections we describe the experimental
technique and the new results. We will discuss the data in
comparison with previous, unpublished results 12 and ex-
tract resonance parameters.
A detailed interpretation of the data is necessary to make
a reliable extrapolation of the cross section beyond the in-
vestigated energy range. Therefore extensive shell model cal-
culations have been performed to understand the various
resonant and nonresonant reaction contributions. In particu-
lar the partial widths of the higher energy resonances have to
be calculated to determine possible low energy tail contribu-
tions to the reaction rate. In Sec. IV we present the experi-
mental and theoretical excitation energies, the spectroscopic
factors, and the partial widths of the neutron unbound
states, which were taken into account.
In the last section the experimental reaction rate will be
discussed and compared with previous predictions. The im-
plications of the new rate for the reaction flow will be dis-
cussed.
Throughout this work all energies are given in the center
of mass system, except for the neutron energies E
n
.
II. EXPERIMENTAL METHOD
The experiments were performed at the 3.75 MV Van de
Graaff accelerator at the Forschungszentrum Karlsruhe
FZKA. Because of the short half-life of the reaction prod-
uct
19
O( t
1/2
=26.91 s, a fast cyclic neutron activation tech-
nique was applied 13 to measure the total ( n , ) cross sec-
tion of the reaction at different neutron energies.
A. Neutron spectra
The neutrons were produced via the
7
Li( p , n ) reaction at
different proton energies. The Li targets were made by
*
Present address: Technische Universita ¨t Wien, A-1040 Wien,
Austria.
PHYSICAL REVIEW C JANUARY 1996 VOLUME 53, NUMBER 1
53 0556-2813/96/531/45910/$06.00 459 © 1996 The American Physical Society