First excited state of the s-process branching nucleus
95
Zr
K. Sonnabend,
*
P. Mohr, and A. Zilges
Institut für Kernphysik, Technische Universität Darmstadt, Schlossgartenstraße 9, D-64289 Darmstadt, Germany
R. Hertenberger, H.-F. Wirth, and G. Graw
Sektion Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, D-85748 Garching, Germany
T. Faestermann
Physik Department, Technische Universität München, James-Franck-Straße, D-85748 Garching, Germany
(Received 30 July 2003; published 10 October 2003)
The transfer reaction
94
Zrd , p
95
Zr has been studied to check the existence of a low-lying first excited level
at 23 keV in
95
Zr. This level—if it exists—might have significant influence on the branching of the astrophysi-
cal s process at the branching nucleus
95
Zr. Our experimental spectra, measured with very good energy
resolution, do not show any evidence for the existence of such a low-lying level. This is in agreement with
experimental results from several reactions and contradicts the only claim for evidence of such a low-lying
level which was also derived from
94
Zrd , p
95
Zr data, however with moderate resolution, but still the best of
the previous experiments.
DOI: 10.1103/PhysRevC.68.048802 PACS number(s): 25.40.Hs, 27.60.+j, 26.20.+f
The nucleus
95
Zr is a branching point in the astrophysical
s process. Depending on the
-
-decay half-life, neutron cap-
ture cross section, and the neutron density, the nucleosynthe-
sis path proceeds either via
94
Zrn , y
95
Zr
-
95
Nb
-
95
Mon , y
96
Mon , y
97
Mo on the neutron-deficient branch or
via
94
Zrn , y
95
Zrn , y
96
Zrn , y
97
Zr
-
97
Nb
-
97
Mo on the
neutron-rich branch. As a consequence, characteristic isoto-
pic patterns have been found for various molybdenum iso-
topes in meteorites [1–3] which are thought to be formed
during thermal pulses of so-called AGB (asymptotic giant
branch) stars. These stars are the astrophysical sites of the
main component of the s process [4–6].
The stellar model of Ref. [7] is able to provide the tem-
perature and neutron density profiles during the s process in
AGB stars, and is able to predict the correct branching ratios
for practically all branching points in the s process path.
However, there are significant problems with the branching
point at
95
Zr.
A direct measurement of the neutron capture cross section
of the unstable nucleus
95
Zr is extremely difficult because of
its short half-life of T
1/2
=64 days. Indirect approaches using
the inverse
96
Zr , n
95
Zr reaction are presently under analy-
sis [8]. Theoretical predictions of the Maxwellian averaged
neutron capture cross section (MACS) at a typical tempera-
ture of kT =30 keV are based on statistical model calcula-
tions. The results vary significantly: values of 23 mb [9],
50 mb [10], 72 mb [11], and 126 mb [12] have been re-
ported. The value of Ref. [9] has been derived from the given
reaction rates; the other values are compiled in Ref. [13].
Besides the very discrepant values for the MACS, one
more essential problem appears for the nucleus
95
Zr. The
adopted level scheme [14] which is based on Ref. [15] lists
the following levels of
95
Zr. The ground state with J
=5/2
+
has a half-life of T
1/2
=64.032±0.006 days. The next level at
E
x
=23 keV is shown with a question mark because it has
been seen only in one d , p transfer experiment [16]. Be-
cause of the L =2 transfer it is tentatively assigned to J
=3/2
+
,5/2
+
. The next level is located at E
x
=953.95 keV with
J
=1/2
+
. It is obvious that the 23 keV level—if it exists—
will be significantly populated at typical s process tempera-
tures of kT =30 keV whereas the level at 954 keV is practi-
cally not populated and can be neglected in the following
discussion. Such a thermal population of the 23 keV level
may lead to two consequences. First, the effective half-life of
95
Zr will depend on temperature, because the
-
-decay half-
life of the 23 keV level may differ from the
-
-decay half-
life of the ground state. The difference will be large espe-
cially in the case when J
of the excited state differs from J
of the ground state. Second, the neutron capture cross section
may be influenced because the capture cross sections of the
23 keV level may differ from the capture cross section of the
ground state. Again, the difference will be large especially in
the case of different J
. Consequently, there are huge uncer-
tainties from nuclear physics for this important s process
branching nucleus. To our knowledge, this 23 keV level was
taken into account neither in the above predictions of the
MACS [9–12] nor in a systematic study of -decay half-lives
under stellar conditions [17].
It is the motivation of the present experiment to clarify
this situation using the same d , p transfer reaction as in Ref.
[16] and a high-resolution magnetic spectrometer. Earlier
d , p transfer experiments were limited in energy resolution.
In Ref. [18] the
94
Zrd , p
95
Zr reaction was measured at
15 keV; the resolution was between 75 and 100 keV which
is not sufficient to detect the 23 keV level. Reference [19]
measured the
94
Zrd , p
95
Zr reaction at 33 MeV and the
94
Zr ,
3
He
95
Zr reaction at 66 MeV with energy resolutions
of about 25 keV for the d , p and about 100 keV for the
,
3
He reactions. They do not report the 23 keV level, but *Electronic address: kerstin@ikp.tu-darmstadt.de
PHYSICAL REVIEW C 68, 048802 (2003)
0556-2813/2003/68(4)/048802(3)/$20.00 ©2003 The American Physical Society 68 048802-1