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The Missing Link

Article Illustration
Team members with the ANU gamma detector array - much of this work was also performed on gammasphere in the USA.
Article Illustration
The nuclear potential energy surface is a function of the shape of the nucleus. The minimum energy and thus the energetically preferred shape is spherical. There is however, a second stable energy minimum for an excited axially symmetrical state. The situation is analogous to a gravitational potential where boulders can site stably in either of two hollows in a hillside.

Gamma ray spectroscopy is shedding light on the mysterious process of nuclear superdeformation

Anna Wilson, George Dracoulis, Aidan Byrne, Paul Davidson and Greg Lane.

What is superdeformation?

It came as something of a surprise when experiments in the mid-80s showed that some rapidly spinning nuclei can adopt extremely deformed shapes, rather like rugby balls, without splitting into two (fissioning) because of the associated stresses. These exotic nuclear states are stabilized by nuclear shell effects which give extra binding energy at large deformations. When nuclei take on such shapes they are described as superdeformed. As can be seen in the figure, the excited superdeformed state occupies a real quantum well, separated from the ground state quantum well by a potential energy barrier. The problem of how the nucleus escapes from the superdeformed minimum is one with parallels in many fields - direct analogies can be made with the nuclear fusion process, or transport between coupled quantum dots. In fact, it is an example of the more general phenomenon of quantum tunnelling.

What is the missing link?

Unless we know the excitatin energy of these states, it is impossible to extract detailed information concerning either their underlying nucleon structure or the mechanism which allows the escape from the superdeformed well to take place. Despite the fact that more than 100 superdeformed states have been identified in nuclei with mass numbers around 150 and 190, it has only been possible to precisely measure their excitation energy in four cases: 194Pb, 194Hg, 152Dy and, in ANU-led work, 192Pb. One of the reasons that it has proved difficult to make these measurements is that, for every superdeformed nucleus that we make in a fusion reaction, we make a few hundred others in other states. Even worse, when we do make a superdeformed nucleus, it can decay out of the superdeformed well and down to the nucelar ground-state by any one of thousands of different paths. If we want to measure the excitation energy, we must be able to trace one or more of these paths - that is, we have to find the missing link between the superdeformed and ground states. Because of this, it is essential that we apply the most efficient data selection methods possible.

Time-correlations the key to the search

The gamma-ray spectroscopy group in the Department of Nuclear Physics has pioneered a technique which turns out to be ideally suited to filtering the data in certain superdeformed nuclei. The method exploits the fact that some special excited nuclear states survive for much longer than most others. Typical lifetimes of superdeformed states is of the order of a few hundred femtoseconds (10-13 seconds). In contrast, some of the less excited states associated with the normal nuclear shape persist for hundreds of nanoseconds, or even microseconds, before decaying down to the ground state. With modern spectroscopy equipment, it is posible to record nuclear decays with time resolution significantly smaller than the lifetimes of these isomers, allowing us to correlate decays that feed them with decays that de-excite them. This is a powerful selection tool: by selecting the subset of the nuclear levels whose decays feed an isomer, we throw away an otherwise overwhelming background of unwanted information.

Successful measurement using state-of-the-art equipment

While the facilities at the ANU are ideal for the essential step of accurately characterising the long-lived nuclear states, a more efficient detection system is required to detect the extremely weak gamma-ray transitions which are the object of the search. The ANU team, led by Anna Wilson, won approval for access to the Gammasphere array at Lawrence Berkeley National Laboratory to apply their technique to the nucleus 192Pb in collaboration with USA groups and others. During the experiment, around 100 Gigabytes of energy and time data were collected over a period of about three days, as hundreds of billions of 192Pb nuclei were created in fusion-evaporaiton reactions. Without the time-correlation filter, it proved impossible to identify the missing linking transitions which connect the superdeformed and ground-state minima. But when the filter was applied, the extra selectivity provided meant that some linking transitions stood out clearly above the drastically-reduced background.

The Gammasphere measurement established the excitation energy of the superdeformed well in 192Pb as only about 2 MeV. This is extremely low, and has important implications for current theories of the tunnelling process between superdeformed and ground-state minima. The results have already prompted new theoretical investigations into the problem of the decay-out.

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