Do nuclei go pear-shaped? Coulomb excitation of 220Rn and 224Ra at REX-ISOLDE \(CERN\)

The IS475 collaboration conducted Coulomb-excitation experiments with post-accelerated radioactive Rn and Ra beams at the REX-ISOLDE facility. The beam particles (Ebeam: 2.83 MeV/u) were Coulomb excited using Ni, Cd, and Sn scattering targets. De-excitation -rays were detected employing the Miniball array and scattered particles were detected in a silicon detector. Exploiting the Coulomb-excitation code GOSIA for each nucleus several matrix elements could be obtained from the measured -ray yields. The extracted ⟨3||E3||0⟩ matrix element allows for the conclusion that, while Rn represents an octupole vibrational system, Ra has already substantial octupole correlations in its ground state. This finding has implications for the search of CP-violating Schiff moments in the atomic systems of the adjacent odd-mass nuclei. DOI: 10.1051/ C © Owned by the authors, published by EDP Sciences, 2015 / 010 ( 2015) 201 epjconf EPJ Web of Conferences , 010 59 93 3 38 38


Introduction
There is manifold experimental evidence for nuclei within specific regions of the nuclear landscape to have strong octupole correlations [1].Possibly, some nuclei possess these correlations even in their ground state.In a geometrical picture these correlations lead to a pear-shape distortion of the nuclear surface.The experimental evidence comprises of low-lying 3 -states in nearspherical and 1 -band heads of the K=0 octupole band in deformed nuclei, odd-even staggering of positive and negative parity yrast bands at comparably low spins, parity doublets in the neighbouring odd-mass systems and most important enhanced B(E3) strength for stretched E3 transitions.Strong octupole correlations occur in nuclei for which the Fermi level for protons as well as for neutrons is situated between the unique-parity subshell and the subshell having an orbital (l) as well as total (j) angular momentum difference of Δl = 3ħ and Δj = 3ħ.These particular subshell combinations are realized near the neutron (N) and proton (Z) numbers N or Z = 36, 56, 88, and 136.Supported by theoretical investigations using various approaches (e.g., see Refs.[1,2] and references therein), especially 224 Ra is a promising candidate for a nucleus with strong octupole correlations.Prior to this work a Coulomb excitation measurement on 226 Ra already exhibited strong B(E3, J + → (J + 3) -) transition probabilities in this nucleus [3].The reflection asymmetric pear-shape associated with strong octupole correlations leads to an asymmetric charge distribution in the nucleus.This asymmetric charge distribution would result in an enhancement of a CP-violating nuclear Schiff moment [4,5,6] in the neighbouring odd-mass nuclei, which would induce an electric-dipole moment (EDM) in the atomic system.Upper limits on EDMs have been measured to date constraining models, which propose physics beyond the standard model.

Experiments
The experiments were performed at ISOLDE using the Radioactive ion beam EXperiment (REX) accelerator [8].The REX-ISOLDE facility has the world-wide unique capability to produce and post-accelerate the radon and radium nuclei of interest with a sufficient intensity and energy to perform sub-barrier Coulomb-excitation measurements.Since the latter are sensitive to E2-and E3-matrix elements they represent a tool to measure E3transition strength which otherwise cannot be observed due the presence of competing fast E1 transitions predominantly depopulating the level of interest.In order to produce the nuclei of interest in a spallation reaction a primary UC x was irradiated with 1.4 GeV protons.The radioactive isotopes were mass separated, charge bred (A/q ~4.1), post-accelerated to 2.82 and 2.83 MeV/u, respectively, using REX and delivered to the secondary Coulomb-excitation target inside the Miniball setup [9]

Results and Conclusion
The Coulomb-excitation code GOSIA was used to extract the respective matrix elements from the observed J-ray yields.This analysis resulted amongst others in matrix elements corresponding to a B(E3, 0 + → 3 -) value of 33(4) W.u. for 220 Rn and 42(3) W.u. for 224 Ra [11].The corresponding values are shown as red points in Fig. 4. Here, it is worthwhile to point out, that the experimental trend of the two radium isotopes opposes the predictions of mean-field based calculations (e.g., see Ref. [2]) as well as cluster models [12].Assuming axial symmetry, the nuclear shape can be parameterised in terms of deformation parameters, E O and the corresponding Legendre Polynomials, P O0 (cos T).The deformation parameters can be extracted from the measured matrix elements [10].The shape of 220 Rn and 224 Ra as resulting from the current experiment are shown in Fig. 6. 224 Ra exhibits the pear-shape associated with an octupole correlated nucleus.In conclusion the presented Coulomb-excitation experiment provided strong evidence that 220 Rn is rather an octupole vibrational system, while 224 Ra exhibits behaviour associated with an octupole deformed ground state.The associated nuclear Schiff moment [4,5,6] is predicted to result in an enhanced atomic EDM.Consequently, atoms of the odd-mass radium isotopes are favourable cases in the search for CP-violating physics.
In order to answer the question, whether Rn isotopes heavier than the investigated 220 Rn are also favourable cases in the search for atomic EDMs more spectroscopic data are needed.The collaboration has already further approved experiments at HIE-ISOLDE, which aim to investigate even-even and odd-mass isotopes near A=224 [14] and isotopes in the octupole-soft mass region near 144 Ba [15].

Figure 1 .
Figure 1.Excitation energy of the lowest-lying 3 -state of nuclei situated in the mass region north-east of 208 Pb

Figure 2 .
Figure 2. Low-energy level scheme of 224 Ra as it is relevant for sub-barrier Coulomb excitation.Data are taken from Ref. [7].
. The Miniball setup consists of 24 high-purity germanium (HPGe) detectors grouped into eight triple clusters for J-ray detection and a double-sided silicon strip detector (DSSD) for particle detection and identification.The high granularity of the DSSD and the six-fold segmentation of the individual HPGe detectors guarantee a good angular resolution of the detected J-rays and particles.Particle-J coincidences allow for a Doppler correction and background suppression.Examples of spectra of 220 Rn Coulomb excited on 60 Ni and 120 Sn secondary targets are shown in Fig. 3. Additionally, 114 Cd was used as secondary target.The well-known Coulombexcitation cross sections of 114 Cd [10] in combination with known lifetimes of 220 Rn and 224 Ra were used for the normalisation of the observed Coulomb-excitation cross sections.The use of several secondary targets with a variety of Z was necessary in order to disentangle onestep and multi-step Coulomb excitation paths.

Figure 3 .
Figure 3. Particle-J coincidence gated and Doppler-corrected J-ray spectra of 220 Rn Coulomb excited using secondary 60 Ni (bottom) and 120 Sn (top) targets.Peaks are labelled corresponding to the transition they represent.

Figure 6 .
Figure 6.Surface contour plots showing 220 Rn (left) and 224 Ra (right) The deformation parameters are given in the upper right corner.Figure is taken from Ref. [11].