STUDIES OF OCTUPOLE NUCLEI

Introduction

The region of the periodic chart which has shown the best evidence for octupole instability in the nuclear ground state is around Z = 88-90 and N around 134. These nuclei have low-lying negative parity states and relatively strong B(E1) values for the transitions between the bands of opposite parity; in the case of ²²⁶Ra large B(E3) values have been measured consistent with its interpretation as a rotating pear shape. The very inaccessibility of these far-from-stability nuclei has, however, meant that there are large gaps in our knowledge of octupole effects in heavy nuclei. Complete measurements of the high-spin behaviour of the yrast octupole band only exist for the isotopes of Th. For the Ra isotopes such measurements are available for the weakly quadrupole coupled ^218,220Ra and the strongly coupled ²²⁶Ra. There is only a limited amount of data on ²²⁴Ra and virtually no information exists for ²²²Ra. The U isotopes with N around 132 are predicted to have the largest octupole correlations, but there is no information about the low lying structure of these nuclei.

Preliminary study of octupole U nuclei

The nuclei predicted to have the deepest minimum in potential energy at a non-zero value of octupole deformation are uranium isotopes with N around 132. These fissile nuclei are accessible via compound nucleus reactions but the survival cross sections are very small. In order to determine the low-lying structure of these nuclei using in-beam spectroscopy, it is necessary to detect and identify the associated evaporation residue nuclei. We carried out a test experiment to determine whether spectroscopic measurements could be made of nuclei in this charge and mass region. We used the technique of alpha decay tagging, in which the recoiling heavy nucleus is separated from the beam and allowed to implant into a position-sensitive detector. The measurement of the subsequent alpha decay is associated with the recoil by position and slow timing, and the recoil is associated with the prompt gamma-ray emission by fast timing. The experiments were carried out at the University of Jyväskylä and employed 8 TESSA Compton-suppressed spectrometers to detect the prompt gamma-ray emission and a silicon strip detector placed at the focal plane of the RITU gas-filled separator to detect the recoil and subsequent alpha decay. We employed the reaction ²⁰⁸Pb(²²Ne, 4n)²²⁶U at a bombarding energy of 112 MeV; this has a measured cross-section of approximated 5 microbarns. Data were collected for approximately 12 hours; in this time 250 events were collected in which a 2 alpha particle chain corresponding to the ²²⁶U decaying to ²²²Th and then to ²¹⁸Ra was detected. The prompt gamma-ray spectrum associated with ²²⁶U decaying to ²²²Th contained 60 counts. It is clear that although these statistics are insufficient, the lowest transitions will be identified if a more efficient germanium detector array is provided (a factor of 5 is easily achievable) and the experiment run for a longer time (another factor of 10).

Population of Octupole Nuclei Using Transfer Reactions

The development of efficient Compton-suppressed germanium-detector arrays has allowed multi-nucleon transfer reactions to become an effective method of accessing many nuclei inaccessible by compound-nucleus reactions. In these reactions many excited nuclei around the projectile and target are produced although the resulting complexity of the spectra demands high statistics and good energy resolution. This can be achieved by the use of efficient germanium-detector arrays in conjunction with the use of thick targets so that the decays from low-lying yrast states are emitted from nuclei at rest. This approach, in which gamma-gamma coincidence techniques are employed, can allow precise identification of the two reaction partners and measurement of their yield.

Initial studies were performed at the Accelerator Laboratory in Jyväskylä, Finland. A thick (30 mg/cm**2) ²³²Th target was bombarded by a ⁵⁶Fe beam from the K-130 accelerator at an energy of 362 MeV. The gamma-gamma in-beam and out-of-beam coincidence data were collected with an array of twelve Compton-suppressed TESSA-type germanium detectors. In these experiments we observed that population is dictated by the mass and charge equilibration processes so that the N/Z value of the populated nuclei tends to that of the composite system. It is also evident that many nuclei are produced via fission of the target-like transfer products. We have made measurements of the mass distributions of the nuclei populated in the ⁵⁶Fe + ²³²Th reaction. The yields are from quantitative in-beam and out-of-beam gamma-gamma coincidence analyses, where the intensities are corrected for efficiency and internal conversion. Near the projectile and target we can see a significant yield of nuclei produced by transfer of neutrons from quasi-elastic processes. The yield of target-like nuclei corresponding to multi-nucleon (deep inelastic) transfer is substantially reduced compared to the corresponding projectile-like partner. The minimum yield appears to occur near A = 224. Two peaks in the fission distribution were observed at around A = 90 and A = 140, these presumably arise from asymmetric fission of near-target products of quasi-elastic processes, with an additional component arising from fission of the compound nucleus.

In two additional experiments of this nature a ²³²Th target was bombarded by a 511 MeV ⁸⁶Kr beam from the Jyväskylä facility and a similar target was bombarded by a ¹³⁶Xe beam from the ATLAS accelerator of the Argonne National Laboratory at a beam energy of 830 MeV. In each experiment both in-beam and out-of-beam gamma-gamma coincidences were stored. As with the ⁵⁶Fe + ²³²Th reaction it was observed that the mass and charge equilibration processes control the population of nuclei in this system. A comparison of the target-like products produced in the three reactions indicates that the ¹³⁶Xe + ²³²Th reaction can populate the nuclei of interest in the octupole-deformed light-actinide region.

High spin studies of octupole Rn and Ra nuclei

As the reaction ¹³⁶Xe + ²³²Th offered the largest yield for Rn and Ra isotopes with N around 134 (see previous section), we chose this reaction in order the make spectroscopic measurements of ^218,220,222Rn and ^222,224,226Ra. The ¹³⁶Xe projectile was accelerated to an energy of 833 MeV by the 88" cyclotron at the Lawrence Berkeley National Laboratory. This bombarded a ²³²Th target of thickness 36 mg/cm**2. De-excitation gamma rays emitted from reaction products were collected with the GAMMASPHERE spectrometer which consisted of 73 large volume Compton-suppressed germanium detectors.

In the case of ²²⁴Ra fourfold coicidences were analysed as this nucleus was populated with the greatest intensity. This number of coincidences were not used to study the other 5 nuclei as no significant improvement in peak-to-background over threefold coincidences was observed in these cases.

Level schemes of ²¹⁸Rn, ²²⁰Rn and ²²²Rn were determined based on previous spin and parity assignments to the low-spin states and for higher spins, energy sums and intensity balance arguments. The level schemes have been significantly extended in the present work, and in each nucleus two interleaving rotational bands of opposite parity have been observed for the first time. The level schemes of ²²²Ra, ²²⁴Ra and ²²⁶Ra have also been considerably extended in this work and interleaving positive- and negative-parity states have been observed for the first time in ²²²Ra.

We observe that interband E1 transitions depopulate states up to J = 17 in ²²²Ra and J = 20 in ²²⁶Ra but although the yrast band in ²²⁴Ra has been observed up to J = 28, no interband E1 transitions have been observed above J = 9 in this nucleus. For each state that is depopulated by both E1 and E2 transitions in the three Ra isotopes, intrinsic electric dipole to quadrupole (D₀/Q₀) ratios were extracted from B(E1)/B(E2) branching ratios. The D₀/Q₀ values measured for the excited states in ²²⁴Ra are much lower than those in ²²²Ra and ²²⁶Ra. At low spin, calculations by Butler and Nazarewicz reproduce the anomalously low D₀/Q₀ for ²²⁴Ra by treating the intrinsic electric dipole moment, D₀, as the sum of a macroscopic (liquid drop) component and a microscopic (shell-correction) term, which cancel for ²²⁴Ra. Our upper limits of D₀/Q₀ for high-spin states in ²²⁴Ra indicate that the cancellation persists to high angular momenta in this nucleus. For ²²²Ra and ²²⁶Ra the D₀/Q₀ values are constant over the full spin ranges, which suggests that the reflection- asymmetric charge distribution in all three isotopes does not change with increasing spin. Values of D₀/Q₀ were extracted for excited states in the three Rn isotopes in a similar way.

Using weighted mean values of D₀/Q₀ and published values of Q₀, or using Grodzins' principle, D₀ values were determined for the Rn and Ra isotopes. The values extracted for the Rn isotopes appear to be in good agreement with theoretical values. The theoretically predicted minimum in D₀ as one moves from ²²²Ra to ²²⁴Ra is observed experimentally. However, the values of D₀ measured in this work for ²²²Ra and ²²⁶Ra are significantly larger than the theoretical values.

Pronounced alignment effects are observed for the positive parity states in ²¹⁸Rn and ²²⁰Rn at rotational frequencies of about 0.22 MeV. Cranked shell model calculations performed with the Woods-Saxon deformed shell model potential with ''universal'' parameterisation predict alignments of pairs of i_13/2 protons at a rotational frequency of 0.25 MeV. In contrast the Ra isotopes and Th isotopes maintain regular structures to the highest spins observed. One explanation for this behaviour is that strong alignment effects are expected at Z=86 because the mixing of the i_13/2 proton orbital with lower spin orbitals, responsible for washing out the alignment effects in Ra and Th isotopes, is weaker for this proton number and quadrupole deformation. An alternative explanation is that octupole correlations for both protons and neutrons are weaker for the Rn isotopes.

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Last modified: 15 October 1996

Maintained by P.A. Butler, pab@ns.ph.liv.ac.uk