Monday, March 11, 2002 - DPG-Tagungen

Nuclear Physics Tuesday
HK 17.3 Tue 17:45 B
Study of the oblate deformed g9/2 band in the N=Z+1 nucleus
69 Se — •I. Stefanescu 1 , J. Eberth 1 , G. Gersch 1 , T. Steinhardt
1 , O. Thelen 1 , N. Warr 1 , D. Weisshaar 1 , G. de Angelis
2 , T. Martinez 2 , D. Curien 3 , K.P. Lieb 4 , A. Jungclaus 4 , R.
Schwengner 5 ,andE. Stefanova 5 for the Euroball collaboration —
1 Institut für Kernphysik, Zülpicherstr. 77, D-50937 Köln, Germany —
2 Laboratori Nazionali INFN, I-35020 Legnaro, Italy — 3 CRN Strasbourg,
F-67037 Strasbourg, France — 4 II. Phys. Institut, Universität Göttingen,
D-37073 Göttingen, Germany — 5 Inst. für Kern- und Hadronenphysik ,
Forschungszentrum Rossendorf, Germany
The γ-ray transitions in the 69 Se nucleus have been investigated in the
40 Ca( 32 S,2pn) 69 Se reaction at 105 MeV using the Euroball array coupled
with ancillary detectors. The target consisted of a 800 µg/cm 2 99.965 %
enriched self-suporting 40 Ca foil.
The level scheme of 69 Se has been established by means of particlegated
γ-γ and γ-γ-γ coincidences up to 13.7 MeV and J π =53/2 + for the
prolate-deformed band which crosses the oblate band built on the g9/2
orbital. Some new lines have also been added to the structure resulting
from the coupling of one octupole phonon to the 9/2 + state in the prolate
band.
The spins of the levels were deduced, whenever possible, from the analysis
of the directional correlation ratios from oriented states (DCO) using
the γ-γ and the neutron-γ-γ events.
Funded by German BMBF under Contract No. 06OK958.
HK 17.4 Tue 18:00 B
Nature of the Scissors Mode near Shell Closure ⋆ — •P. von
Neumann-Cosel 1 , E. Guliyev 2 , F. Hofmann 1 , A.A. Kuliev 3 ,and
A. Richter 1 — 1 Institut für Kernphysik, Technische Universität Darmstadt
— 2 Department of Engineering Physics, Ankara University, Turkey
— 3 Department of Physics, Sarkara University, Turkey
While the global features of the scissors mode in heavy nuclei are quite
well understood, peculiarities remain to be solved in nuclei near shell
closure. There, the simple geometrical picture of a scissors-like motion
of deformed proton and neutron bodies breaks down. Two examples
are discussed. QRPA calculations of the low-energy dipole strength in
122−130 Te are reported which provide new insight into the complex distributions
which cannot be understood in spherical models. The extracted
deformation dependence of the scissors mode strength is discussed with
respect to various models. Another interesting case are the 194,196 Pt isotopes
in the region of γ-softness near the N = 126 shell gap. Photon
scattering experiments surprisingly suggest an upward shift of the scissors
mode centroid in 194 Pt by about 500 keV with respect to the sys-
HK18 Nuclear and Particle Astrophysics II
tematics in rare-earth nuclei. First microscopic calculations of the M1
and E1 strengths in 194,196 Pt are presented using the above model.
⋆ Supported by the DFG under contract FOR 272/2-1.
HK 17.5 Tue 18:15 B
Phase transitions and two-neutron separation energies in
algebraic models — •Ruben Fossion 1 , Kris Heyde 1 , and
Jose-Enrique Garcia-Ramos 2 — 1 Department of Subatomic
Physics, University of Gent, Proeftuinstraat,86 B-9000 Gent(Belgium)
— 2 Departamento de Fiscia Aplicada. EPSLa Rabida, Universidad de
Huelvam 21819 Palos de la Frontera (Spain)
In the last few years, interest for the study of phase transitions and
phase coexistence in atomic nuclei has been revived, in particular making
use of algebraic methods such as the Interacting Boson Model (IBM).
In the present study we consider the three transitional regions in which
one observes rapid structural changes i.e. (a) the Nd-Sm-Gd region, (b)
the Ru-Pd region and, (c) the Os-Pt region. Although these regions
have been studied extensively emphasizing excited state properties, not
so much attention has been given, up to now, concerning possible phase
transitions in the nuclear ground-state properties.
In this work, a new type of plot is presented that allows the study of
phase transitions in finite systems, such as atomic nuclei. This approach
allows to establish a connection between excited-state properties and
binding energies. It can be shown that the binding energy and in particular
the 2-neutron separation energy presents a very sensitive observable
that allows to discriminate between apparently equivalent Hamiltonians.
We also present results on the systematics of 2-neutron separation energies
in long chains of isotopes.
HK 17.6 Tue 18:30 B
Cluster interpretation of the properties of the alternating parity
bands in actinides — •T.M. Shneidman 1,2 , G.G. Adamian 1,2,3 ,
N.V. Antonenko 1,2 , R.V. Jolos 1,2 , and W. Scheid 1 — 1 Institut
für Theoretische Physik der Justus-Liebig-Universität, D-35392 Giessen,
Germany — 2 Joint Institute for Nuclear Research, 141980 Dubna, Russia
— 3 Institute of Nucear Physics, Tashkent 702132, Uzbekistan
A cluster model interpretation is suggested for the properties of the
alternating parity bands in Ra, Th and U isotopes, which includes a
description of the parity splitting [1] and the Eλ (λ=1,2,3) transition
probabilities. The mass asymmetry and relative distance coordinates are
the most important variables of the model. The characteristics of the
Hamiltonian used are determined in investigations of heavy ion reactions
at low energies.
[1] T.M.Shneidman et al., Phys.Lett. B(2002), in press.
Time: Tuesday 16:45–18:45 Room: C
Group Report HK 18.1 Tue 16:45 C
Nuclear Input Data for the r-Process: Cosmochronometer for
Old Halo Stars — •B. Pfeiffer 1 , K.-L. Kratz 1 , H. Schatz 2 ,and
J.J. Cowan 3 — 1 Inst. f. Kernchemie, Univ. Mainz — 2 NSCL, Michigan
State Univ., East Lansing, USA — 3 Dep. of Phys. and Astr., Univ.
Oklahoma, Norman, USA
The description of the rapid neutron-capture process (r-process) requires
environments with a high neutron density, where neutron captures
are faster than β-decays, even for unstable nuclei up to 15 - 30 units from
stability near the neutron drip-line. These exotic nuclei with a large neutron
excess are of considerable interest not only for astrophysics but for
nuclear structure studies. They experience dramatic changes giving rise
to new shell structures characterized by vanishing gaps [1].
The updated nuclear physics input is applied for classical r-process
models. The calculated abundances are compared to new observations
of extremely metal-poor, old stars in the Galactic halo. The observed
abundances for heavier neutron-capture elements (including the third rprocess
peak elements) are consistent wth a scaled solar system distribution.
Our theoretical production value for Th/U ratio combined with the
new observations make more reliable chronometric age estimates possible
[2]. These ages are lower limits for the age of the Universe.
[1] B. Pfeiffer et al., Nucl. Phys. A693, 282 (2001)
[2] R. Cayrel et al., Nature 409, 691 (2001); J.J. Cowan et al., submitted
to Ap. J.
HK 18.2 Tue 17:15 C
Measurement of the 197 Au(γ,n) 196 Au cross section close above
the reaction threshold ∗ — •P. Mohr, K. Vogt, M. Babilon,
W. Bayer, T. Hartmann, C. Hutter, K. Lindenberg, K.
Sonnabend, S . Volz, and A. Zilges — Institut für Kernphysik,
Technische Universität Darmstadt, D-64289 Darmstadt, Germany
Recently, the interest in (γ,n) reactions has been revived because of
their astrophysical relevance for the nucleosynthesis of neutron-deficient
p nuclei [1]. It has been shown that the astrophysical (γ,n) reaction rate
depends on the (γ,n) cross section only in narrow energy window – a
Gamow-like window – above the (γ,n) threshold [1,2]. We have measured
(γ,n) reaction rates for several nuclei in a quasi-thermal photon
bath at typical temperatures of a supernova explosion [1,3].
The cross section of the reaction 197 Au(γ,n) 196 Au has been measured at
the S-DALINAC close above the reaction threshold at Ethr =8.071 MeV
using the method of photoactivation. From a combination of our result
and data from literature [4] we derive the 197 Au(γ,n) 196 Au cross section
from threshold to the giant dipole resonance with very small uncertainties.
This cross section can be used as standard in future experiments.
[1] P. Mohr et al., Phys. Lett. B 488, 127 (2000).
[2] P. Mohr et al., Nucl.Phys.A688, 82c (2001).
[3] K. Vogt et al., Phys.Rev.C63, 055802 (2001).
[4] B. L. Berman et al., Phys.Rev.C36, 1286 (1987); A. Veyssiere et
al., Nucl.Phys.A159, 561 (1970); G. M. Gurevich et al., Nucl.Phys.
A351, 257 (1981); H. Utsunomiya et al., to be published.