Evolution of Octupole Deformation in Radium Nuclei from Coulomb Excitation of Radioactive Ra222 and Ra

Abstract

There is sparse direct experimental evidence that atomic nuclei can exhibit stable 'pear' shapes arising from strong octupole correlations. In order to investigate the nature of octupole collectivity in radium isotopes, electric octupole (E3) matrix elements have been determined for transitions in 222,228 Ra nuclei using the method of sub-barrier, multi-step Coulomb excitation. Beams of the radioactive radium isotopes were provided by the HIE-ISOLDE facility at CERN. The observed pattern of E3 matrix elements for different nuclear transitions is explained by describing 222 Ra as pear-shaped with stable octupole deformation, while 228 Ra behaves like an octupole vibrator.

Pear-shaped atomic nuclei

“…The measured quadrupole moment Q 2 and octupole moment Q 3 for nuclei in the Z ≈ 88, N ≈ 134 mass region are compared with those measured for adjacent heavy nuclei in figure 4. In this figure, the intrinsic moments Q λ are derived [35] Figure 4. The systematics of measured E2 and E3 intrinsic moments Q λ for 0 + → 2 + and 0 + → 3 − transitions, respectively, in the heavy mass region (A ≥ 208).…”

“…The systematics of measured E2 and E3 intrinsic moments Q λ for 0 + → 2 + and 0 + → 3 − transitions, respectively, in the heavy mass region (A ≥ 208). For the source of the data shown here, see fig. 14 in [12], and see [35]. (Online version in colour.)…”

“…To explore further required the development of accelerated radioactive ion beams (RIBs), which enables a much larger range of isotopes to be studied using the method of Coulomb excitation. In this manner, measurements of E 3 matrix elements have been performed in recent years for 220 Rn [ 32 ], 224 Ra [ 32 ], 144 Ba [ 33 ], 146 Ba [ 34 ] and 222,228 Ra [ 35 ], using the RIB accelerators REX- and HIE-ISOLDE, CERN, Geneva, Switzerland, and CARIBU, Argonne National Laboratory, Lemont, IL, USA. (It should be noted that the measurements [ 7 , 17 ] of energy levels of 224,226 Rn presented in figure 2 also exploited Coulomb excitation of accelerated radon ions, but at higher energies where the interaction is not purely electromagnetic.)…”

“…For the source of the data shown here, see fig. 14 in [ 12 ], and see [ 35 ]. (Online version in colour.)…”

Spectroscopy of short-lived radioactive molecules

“…Radium monofluoride, RaF, is of particular interest because it is predicted to have an electronic structure appropriate for laser cooling 6 , thus paving the way for its use in high-precision spectroscopic studies. Furthermore, the effects of symmetry-violating nuclear moments are strongly enhanced 5 , 7 – 9 in molecules containing octupole-deformed radium isotopes 10 , 11 . However, the study of RaF has been impeded by the lack of stable isotopes of radium.…”