Systematics of Magnetic Dipole Strength in the Stable Even

Systematics of Magnetic Dipole Strength in the Stable Even-Mass Mo Isotopes D
G. Rusev, R. Schwengner, F. Dönau, M. Erhard, S. Frauendorf,1 E. Grosse,2 A.R. Junghans, L. Käubler,
K. Kosev, L. Kostov,3 S. Mallion, K.D. Schilling, A. Wagner, H. von Garrel,4 U. Kneissl,4 C. Kohstall,4
M. Kreutz,4 H. H. Pitz,4 M. Scheck,4 F. Stedile,4 P. von Brentano,5 J. Jolie,5 A. Linnemann,5
N. Pietralla,6 V. Werner7
The origin and strength of magnetic-dipole (M 1) radiation in nuclei has been the subject of various experimental and theoretical investigations. Generally,
large M 1 strength is generated by spin-flip transitions
or by a recoupling of the spins in few-particle multiplets. In nearly spherical nuclei, 1+ states with large
B(M 1, 0+ → 1+ ) values were described as two-phonon
states with mixed proton-neutron symmetry [1]. In
deformed nuclei, an isovector mode representing rotational oscillations of protons against neutrons was
predicted [2] and observed [3] as a group of 1+ states
with energies around 3 MeV.
The isotopic chain of stable even molybdenum isotopes offers the possibility to study the behavior of M 1
strength simultaneously with increasing neutron number and the onset of deformation. We have studied the
nuclides 92 Mo, 98 Mo and 100 Mo in photon-scattering
experiments. The nuclide 92 Mo was investigated using
the bremsstrahlung facility at the ELBE accelerator [4]
at an electron energy of 6 MeV. The nuclides 98 Mo and
100
Mo were studied at the Dynamitron accelerator of
the University of Stuttgart at various energies up to
3.8 MeV [5]. Transition strengths were deduced from
intensities of ground-state and branching transitions.
The cumulative M 1 strengths of the even Mo isotopes
from 92 Mo to 100 Mo up to an excitation energy of
4 MeV are shown in Fig. 1. The data included for 94 Mo
and 96 Mo were taken from Refs. [6] and [7], respectively. Positive parity was assumed for all J = 1 states
found up to 4 MeV in 98 Mo and 100 Mo.
The experimental M 1 strengths have been compared with predictions of quasiparticle-random-phaseapproximation (QRPA) calculations in a Nilsson-like
deformed basis [8]. The hamiltonian used for the description of 1+ states includes the Nilsson mean-field
plus monopole pairing at the particular deformation
and interaction terms composed of the isoscalar and
isovector parts of the total angular momentum operator and the spin operator. The results of the calculations are compared with the experimental values in
Fig. 1. The calculated total strengths up to 4 MeV are
somewhat below the experimental values, but the increase of the total strength and its fragmentation with
increasing neutron number is in agreement with the
experiment. This tendency is caused by the increasing
deformation. The large strength observed in 94 Mo is
mainly caused by a two-phonon state which cannot be
treated in the present QRPA calculations.
Fig. 1 Cumulative M 1-strengths in stable even-mass Mo
isotopes. Experimental values are given as circles with dotted lines. The results of the QRPA calculations are shown
as solid lines.
[1] N. Pietralla, C. Fransen et al., Phys. Rev. Lett. 83
(1999) 1303
[2] N. Lo Iudice and F. Palumbo, Phys. Rev. Lett. 41
(1978) 1532
[3] A. Richter, Prog. Part. Nucl. Phys. 34 (1995) 261
[4] R. Schwengner, R. Beyer et al., Nucl. Instr. Meth. A
555 (2005) 211
[5] G. Rusev, R. Schwengner et al., Phys. Rev. Lett. 95
(2005) 062501
[6] C. Fransen, N. Pietralla et al., Phys. Rev. C 67 (2003)
024307
[7] C. Fransen, N. Pietralla et al., Phys. Rev. C 70 (2004)
044317
[8] F. Dönau, Phys. Rev. Lett. 94 (2005) 092503
1 also
University of Notre Dame, USA
TU Dresden
3 INRNE Sofia, Bulgaria
4 Institut für Strahlenphysik, Universität Stuttgart, 70569 Stuttgart, Germany
5 Institut für Kernphysik, Universität Köln, 50937 Köln, Germany
6 Nuclear Structure Laboratory, Dept. of Physics & Astronomy, SUNY, Stony Brook, NY 11794-3800, USA
7 Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06520-8124, USA
2 also
10