45 research outputs found
Scissors Modes and Spin Excitations in Light Nuclei including =2 excitations: Behaviour of and
Shell model calculations are performed for magnetic dipole excitations in
and in which all valence configurations plus
excitations are allowed (large space). We study both the orbital
and spin excitations. The results are compared with the `valence space only'
calculations (small space). The cumulative energy weighted sums are calculated
and compared for the =0 to =1 excitations in and
for =1 to both =1 and = =2 excitations in
. We find for the =1 to =1 isovector
{\underline {spin}} transitions in that the summed strength in the
{\underline {large}} space is less than in the {\underline {small}} space. We
find that the high energy energy-weighted isovector orbital strength is smaller
than the low energy strength for transitions in which the isospin is changed,
but for =1 to =1 in the high energy strength
is larger. We find that the low lying orbital strength in is
anomalously small, when an attempt is made to correlate it with the
strength to the lowest states. On the other hand a sum rule of Zheng and
Zamick which concerns the total strength is reasonably satisfied in
both and . The Wigner supermultiplet scheme is a useful
guide in analyzing shell model results. In and with a
interaction the T=1 and T=2 scissors modes are degenerate, with the latter
carrying 5/3 of the T=1 strength.Comment: 51 pages, latex, 9 figures available upon reques
The Question of Low-Lying Intruder States in and Neighboring Nuclei
The presence of not yet detected intruder states in e.g. a
intruder at 9 excitation would affect the shape of the -delayed alpha spectra of and . In order to test the
plausibility of this assumption, shell model calculations with up to excitations in (and up to excitations in
) were performed. With the above restrictions on the model spaces, the
calculations did not yield any low-lying intruder state in . Another
approach -the simple deformed oscillator model with self-consistent frequencies
and volume conservation gives an intruder state in which is lower in
energy than the above shell model results, but its energy is still considerably
higher than 9 .Comment: 16 pages (RevTeX), 1 PS figure. To appear in Phys. Rev.