Coulomb excitation studies of shape coexistence in atomic nuclei

Abstract

Low-energy Coulomb excitation provides a well-understood means of exciting atomic nuclei and allows measuring electromagnetic moments that can be directly related to the nuclear shape. The availability of radioactive ion beams (RIBs) at energies near the Coulomb barrier has made it possible to study shape coexistence in a variety of short-lived exotic nuclei. This review presents a short overview of the methods related to multi-step Coulomb excitation experiments, followed by a discussion of several examples. The focus is on two mass regions where recent Coulomb excitation experiments have contributed to the quantitative understanding of shape coexistence: nuclei with mass A 70 » near the N = Z line and nuclei with A 100 » near neutron number N = 60. Experimental results are summarized and their significance for understanding shape coexistence is discussed. Experimental observables such as quadrupole moments and electromagnetic transition strengths represent furthermore important benchmarks for advancing theoretical nuclear structure models. With several new RIB facilities planned and under construction, Coulomb excitation will remain to be an important tool to extend the studies of nuclear shapes toward more exotic systems, and to obtain a more comprehensive and quantitative understanding of shape coexistence.

Identification of a dipole band above theIπ=31/2−isomeric state inPb189

“…As reported in Ref. [10], this band-head spin was predicted to be higher than those of the most intense shears-mode bands in the heavier Pb nuclei [2][3][4][5][6]. This increase in spin was thought to be due to the decreased residual interaction between the two proton πs −2 1/2 h 9/2 i 13/2 excitation and the i 13/2 neutron-hole excitation.…”

“…These range from single-particle configurations associated with the coupling of a single-unpaired neutron to the evenmass nuclear core at low excitation energies [2][3][4][5][6], through triple-shape coexisting states at intermediate excitation energies [7,8], to highly collective superdeformed structures at the highest excitation energies [9]. The particular behavior depends on the competition between collective effects from the many neutrons in the middle of the N = 82-126 shell and noncollective effects associated with the protons at the fully paired Z = 82 shell closure [10].…”

“…A common feature of this region is the existence of dipole bands with strong M1 and weak E2 transitions in weakly deformed nuclei [2][3][4][5][6]8,[11][12][13]. These dipole bands originate from the coupling of protons in the h 9/2 orbital to neutron holes in the i 13/2 orbital [10,12,13].…”

“…This mechanism is known as the shears mode, due to the alignment of the proton and neutron orbits resembling the closing of a pair of shears [12,13]. For the odd-mass, Pb nuclei, with A = 191-199, the most intense shears-mode dipole bands are based upon a π [s [2][3][4][5][6]. The next lightest isotope in this chain, 189 Pb, might also be expected to form a dipole band built upon these orbits.…”

Shell-model-based deformation analysis of light cadmium isotopes

“…These methods have been used in a large number of recent papers (in particular in the region of the Ge, Kr, Mo, Ru and Pd nuclei [90][91][92][93][94][95], as well as in the much heavier Pb mass region (W, Os, Pt, Hg, Po isotopes) [96][97][98][99][100][101].…”