Evidence for Resonances in the 7$\alpha $ Disassembly of $^{28}$Si

Abstract

The excitation function for the 7 alpha de-excitation of 28 Si nuclei excited to high excitation energies in the collisions of 35 MeV/nucleon 28 Si with 12 C reveals resonance structures that may indicate the population of toroidal high-spin isomers such as those predicted by a number of recent theoretical calculations. This interpretation is supported by extended theoretical analyses.

Shells in a toroidal nucleus in the intermediate-mass region

“…Although the basic ideas on the conditions favorable for toroidal configurations were presented many decades ago [2][3][4][5][6][7][8][9][10], the subject matter gains renewed interest recently because powerful theoretical and experimental tools are now readily available . The interest is heightened by recent experimental evidence for the presence of resonances at high excitation energies in the 7α disassembly of 28 Si which may suggest the production of toroidal high-spin isomers predicted in many theoretical calculations [28]. Should these experimental results be confirmed by further studies, toroidal nuclei would potentially be interesting objects of study because of their new form of geometry, new toroidal shells and magic numbers, new types of yrast high-spin states, new toroidal nuclei species in different mass regions, new probes of nuclear energy density functional and nuclear equation of state in a new density regime, and new possible doorways to energy-producing mechanisms.…”

“…The extra stability associated with toroidal shells [40] leads to toroidal local energy minima for many light nuclei. Toroidal excited (diabatic) states have been predicted for Ca to Ge, with mass numbers 40 A 70 using the Strutinsky shell correction method [2,3], and for 24 Mg [14] and 28 Si [28] using a self-consistent relativistic mean-field theory. Relative to a toroidal core, spin-aligning Bohr-Mottelson particle-hole excitations occupying the lowest Routhian single-particle energies [41] can be constructed to yield a toroidal nucleus with a spin as an yrast state, by promoting nucleons with angular momentum aligned opposite to a chosen symmetry z-axis to populate orbitals with angular momentum aligned along the symmetry zaxis [15][16][17][18][19][20].…”

“…The balance between the two energies gives rise to a local toroidal energy minimum [4]. Self-consistent calculations have been carried out to locate toroidal high-spin isomers as yrast states in the light mass region [15][16][17][18][19][20]28].…”

“…Along with theoretical predictions [2, 3, 11-20, 25-27, 29] and experimental investigations [28] in the light and heavy mass regions, it is natural to ask whether toroidal nuclei may also be possible in the intermediate mass region. In this regard, our knowledge of toroidal nuclei in this mass region is scanty.…”

“…We would like to know whether there are some regularities in the shell structure, how frequent do the toroidal shells occur, what are their nucleon numbers, and where their corresponding toroidal deformations are located. Furthermore, Bohr-Mottelson spin-aligning particle hole excitations relative to a toroidal core can lead to highspin toroidal isomers and these high-spin isomers as yrast states have longer lifetimes and a better chance of being detected [28]. We would like to examine the possibilities of toroidal high-spin isomers in this mass region by studying the shell structure of single-particle Routhian energies as a function of the cranking frequency.…”