Critical insights into nuclear collectivity from complementary nuclear spectroscopic methods

Abstract

Low-energy collectivity of nuclei has been, and is being, characterized in a critical manner using data from a variety of spectroscopic methods, including Coulomb excitation, β decay, inelastic scattering of charged and uncharged particles, transfer reactions, etc. In addition to level energies and spins, transition multipolarities and intensities, lifetimes, and nuclear moments are available. The totality of information from these probes must be considered in achieving an accurate vision of the excitations in nuclei and determining the applicability of nuclear models. From these data, major changes in our view of low-energy collectivity in nuclei have emerged; most notable is the demise of the long-held view of low-energy quadrupole collectivity near closed shells as due to vibrations about a spherical equilibrium shape. In this contribution, we focus on the basic predictions of the spherical harmonic vibrator limit of the Bohr Hamiltonian. Properties such as B(E2) values, quadrupole moments, E0 strengths, etc are outlined. Using the predicted properties as a guide, evidence is cited for and against the existence of vibrational states, and especially multi-phonon states, in nuclei that are, or historically were considered to be, spherical or have a nearly spherical shape in their ground state. It is found that very few of the nuclei that were identified in the last major survey seeking nearly spherical harmonic vibrators satisfy the more stringent guidelines presented herein. Details of these fundamental shifts in our view of low-energy collectivity in nuclei are presented.

Description of the critical point symmetry in 124 Te by IBM-2

“…[12] and denoted as CPST), the experimental data of 124 Te (taken from Refs. [1,3,[44][45][46] and labeled as 124 Te) and the calculated results (labeled as IBM-2).…”

“…How to characterize the underlying structure of 124 Te is a difficult task and has attracted considerable interests in variety of theoretical and experimental studies [1][2][3][4][5][6]. The 124 Te lies outside the major shell Z = 50 with two protons, and its ratio E(4 + 1 /2 + 1 ) = 2.07 is very close to the spherical vibrator value of 2.00.…”

“…are compared with the calculated results. The experimental data are taken from Refs [1,3,10,[44][45][46]. .…”

γ -ray spectroscopy of Sm150 through the β decay of PmSchmelzenbach1, Krane2, Wood3 et al. 2018Phys. Rev. C2000View full textAdd to dashboardCite

“…Formerly, it would have been natural to consider them in a multi-phonon picture, either associated with spherical quadrupole vibrations or deformed β and γ vibrations. However, serious doubt has been cast on spherical quadrupole vibrations at the multi-phonon level [25], and β vibrations likewise are in serious doubt [26][27][28]. The most natural interpretation would involve nucleon-pair degrees of freedom, such as manifest in shape coexistence [29] and in "pairing vibrations" [30][31][32].…”

Collective states and shape coexistence in the Z = 50 region

“…transitions that arise from the contribution of the large intruder in-band B(E2; 2 + → 0 + ) value and the 2I + 1 spin factor involved [23].…”