Introduction

   Actinide nuclei possess significant Octupole deformations.

   Predicted to possess Low lying SUPERDEFORMED (SD) and HYPERDEFORMED (HD) minima.

   TRS calculations predict the HD minima in Uranium isotopes to have a large OCTUPOLE component.

S. Cwiok et al, Phys. Lett. B322 (1994) 304.

    ²³²Uranium has 92 protons and 140 neutrons and is predicted to have a "dinuclear" shape.

    nearly spherical heavier component (around doubly-magic ¹³²Sn)

    well-deformed lighter component (from the neutron-rich A ~ 100 region)

Experimental signatures of hyperdeformation

Octupole deformed interleaved levels of positive and negative parity which will decay strongly through E1 emission

low energy gamma-rays (Energies from 50 to 150keV)

E2's strongly converted

look for E1's regularly spaced gamma-rays, energy spacing ~ 5keV

Problem: Contaminants caused by fission....

	HD states	Fission
Mean Gamma Multiplicity	15	10
Gamma Energy	100 keV	1 MeV
Detection Efficiency	11%	1%
Cross section	10 microbarns	1 barn

Need a gamma-ray Array with high efficiency at low energy .... AFRODITE

Experimental details

²³²Th(,4n)²³²U with a beam energy of 40 MeV

Target thickness 2mg/cm².

Collected 5 x 10⁸ four fold and higher events.

Method of Analysis

Data was sorted into a -- cube.

Using the predicted -ray energies as a starting point, a large number of gate lists were produced by slightly varying the moment of inertia of the deformed nucleus.

Each of these gate lists were applied to the cube.

However, no clear evidence for the existence of a hyperdeformed band was seen.

Monte Carlo Simulation

In order to test the sensitivity of the experiment a Monte Carlo simulation was carried out.

An assumed hyperdeformed band was populated at a varied fraction ranging from 0.5 to 5 percent of the ground state (yrast) band population.

The resulting gamma cascades were corrected for internal conversion and the spectrometer response was folded in. The background was modelled empirically to reproduce the experimental background in the -- cube.

Results

    The top figure below shows the experimental ground state band brought back from a sum of gates on the labelled transitions.