2010 Rutherford medal and prize
Professor Martin Freer
University of Birmingham
For establishing the existence of nuclear configurations analogous to molecules and demonstrating the existence of nucleon-clustering in key light nuclei, a long-standing issue in the field.
Whether nuclei and nuclear matter should be thought of as being composed of alpha particles (or larger stable shell-structure entities) is a long-standing fundamental issue in nuclear research. Prof Freer has resolved this issue by providing the definitive evidence that there is such a cluster structure visible in excited states of key light nuclei. Indeed, one of his most recent results is that the celebrated “Hoyle State”, upon which the carbon production in stars depends, is a state with a cluster structure. Such cluster structure is not in general synonymous with alpha-clustering; for example some excited states of 24Mg have two discernable 12C clusters, where again he has performed some of the key studies.
The fact that clustering is observable in excited states may be interpreted as a dilute nucleonic gas phase-separating into high and low density components, with the former determined by the “magic numbers” of shell structure. He performed the decisive experiments in establishing the existence of nuclear “molecules” in neutron-rich cluster systems. There he established the nature of the states in Beryllium isotopes where neutrons are covalently exchanged between clusters which are alpha-particles – just as electrons are exchanged in atomic covalent molecules, with an associated “nuclear orbital” model analogous to a tight-binding approximation in a molecule.
In the limiting case of highly neutron-rich nuclei near the neutron drip-line, Prof Freer has shown that the “valence neutrons” become delocalised in haloes surrounding stable cluster cores. This form of binding is more like “metallic” binding in analogy with condensed matter. His work is characterised by the development of ingenious experimental techniques (e.g. coincident resonant scattering studies) and an ability to pioneer ideas outside of the mainstream. He has performed the first studies of cluster structure at the very extremes of nuclear existence, the neutron drip-line – as yet unrivalled involving the measurement of all reaction channels. This an area which will be of great importance as new experimental facilities allow the boundaries to be more resolved in more detail. He has, in addition, made an important contribution to the study of halo-type nuclei.