Visions 3 - Exotic nuclear beams

High intensity beams of short-lived nuclei will be an invaluable tool in physics, chemistry, materials science and the biosciences

An image of the
brain obtained using
positron emission
tomography

Exotic nuclear beams High intensity beams of short-lived nuclei will be an invaluable tool in physics, chemistry, materials science and the biosciences During the past 70 years, physicists have developed ever more sophisticated machines for accelerating beams of subatomic particles such as electrons and nuclei. These are species which are generally stable and do not spontaneously disintegrate into other particles. Now, a new generation of accelerators is under consideration designed to produce intense beams of a wide range of less stable, radioactive nuclei. Such beams could be used not only to explore the structure of exotic nuclei never made before but also as an extremely useful resource in industry, medicine and the environment.

Extreme nuclei
Atomic nuclei, which make up most of the matter we can see in the Universe, consist of protons and neutrons in many combinations. The 283 stable or long-lived nuclear species that exist are, however, only a fraction of a possible 7000 that could be created with radioactive beams. Very little is known about most of these nuclei so the possibility of making them is very exciting. Certainly some of them possess unusual structures and unexpected properties. One type recently discovered are neutron-rich species which contain a mantle of neutrons or an extended neutron 'halo' surrounding a dense neutron-proton core; another group are some curious nuclei that remain trapped in a high-energy state for a long period of time (see Box on spin traps below).

Beams of exotic, less stable nuclei will allow physicists to investigate the limits of nuclear stability: how many protons relative to neutrons - or vice versa - can a nucleus hold before they start to 'drip' out? Researchers expect to find fascinating phenomena along these borders of stability - the so-called proton and neutron drip-lines. There is also keen interest in making the heaviest atoms possible. At the moment, uranium, with atomic (proton) number 92, is the last element in the Periodic Table to have a lifetime comparable with the age of the Earth. However, researchers around the world are now trying to make new 'superheavy' elements with atomic numbers above 114, which theory and recent experiments suggest could be fairly stable and have unusual chemical properties.

Another intriguing scientific question is how the existing elements were made in the first place. Most of them from lithium as far as iron (atomic number 26) in the Periodic Table are created in the stars, which behave as giant astrophysical pressure cookers fusing light nuclei into progressively heavier ones. Elements with higher atomic numbers are made in the explosive death throes of novae and supernovae. Such processes involve a complex network of reactions between nuclei, often at the limits of stability, which could be studied with exotic nuclear beams.

Silicon 'doped' with erbium can
be used in optoelectronic circuits,
so it is important to know where
the erbium atoms end up in the
crystal lattice. The images
compare simulations and
experiments for this process

A new analytical tool
Beams of radioactive ions provide a versatile probe of all kinds of structures from electronic devices to living tissue. Just one of the many techniques involves monitoring radioactive ions implanted into solids as 'secret agents', which can report back on their local environment. The decay pattern of these nuclear spies parachuted onto a surface or interface is perturbed by the electric and magnetic fields of nearby atoms thus revealing their position and the nature of their surroundings. It is easy to see that with the broad range of atoms and energies obtainable with radioactive beams, such techniques could be invaluable in studying technologically important magnetic and semiconductor materials, as well as wear and tear in engineering parts.

The production of radioactive beams also generates positrons (positively-charged electrons) whose energies can be controlled within a wide range. Positrons are often used to investigate surfaces, interfaces and thin films. When they encounter electrons, they annihilate to release a burst of characteristic gamma-rays which can be detected. An intense pulsed positron beam will enable the introduction of novel spectroscopic techniques previously inaccessible, and also extend the use of positron imaging techniques in studies of, for example, catalysts and biological samples.

Medical applications
One established imaging technique that will gain from a radioactive beam facility is positron emission tomography (PET), in which positrons emitted by radionuclides reveal their locations when they annihilate. PET is applied in medical research to study blood flow, glucose metabolism and neurological problems; it can also be employed clinically to image organs such as the brain and heart.

Radio-isotopes are themselves already used extensively in medicine as tracers, diagnostic tools and in cancer treatments. Radioactive beam technology will make available many more radionuclides for specific therapies than are currently on the market. For example, radio-lanthanides such as samarium-153 could be used in cancer therapy. When these isotopes are combined with compounds that selectively carry them to the tumour site, the radiation they emit during their decay kills the cells. Another radio-pharmaceutical, gallium-67, is often used to image fast-growing soft tissue. However, the use of a shorter-lived isotope, gallium-73, would greatly reduce the radiation dose to the patient.

Dealing with nuclear waste
Beams of short-lived nuclei will have environmental benefits too. One method for storing nuclear waste is to vitrify it into glass blocks. The effectiveness of such storage materials can be tested by implanting in them shorter-lived isotopes of the same elements and then tracing them from the gamma-rays produced.

On a more speculative level, radioactive beams might find a use in experiments exploring how to destroy nuclear waste by transmutation, or even in seeking more environmentally-friendly forms of nuclear power (see the Energy Amplifier below).

How to make radioactive beams
Beams of a wide range of radioactive nuclei with low energies are produced at several laboratories including the European particle physics laboratory, CERN, in Geneva and a small facility at Louvain-La-Neuve in Belgium. In the UK there is a proposal to build a radioactive beam facility called Sirius at the Rutherford Appleton Laboratory. The basic requirement is a beam of high-energy protons which impinge on a target, creating radioactive ions which are then extracted and separated according to mass so as to generate pure beams of single nuclear species. A second accelerator then raises the energies of the radionuclei to that required for a particular experiment or application.

Some unusual isotopes have recently been discovered which possess stable excited states with high angular momentum, or spin. They cannot lose energy but remain trapped in their high-energy spin state and are thus dubbed 'spin traps'. The most remarkable spin trap is the naturally occurring isotope tantalum-180 which has a half life of more than 100 million million years! Another spin trap, hafnium-178, has 30 times the energy of the tantalum trap and survives for 31 years. Similar species with even more energy are expected to be created with radioactive beams. Such nuclei present an intriguing technological challenge: if a large number of these nuclei could be de-excited on command they would offer nuclear energy on tap, perhaps in the form of a gamma-ray laser or as an energy storage device for space propulsion.

Supernova events like this
one build up the heavier
elements in the Universe

Physicists including the former director of CERN and Nobel Laureate Carlo Rubbia have produced an alternative scheme for producing nuclear energy in which a beam of high-energy protons interacts with thorium to yield neutrons which then cause the thorium to fission. The energy produced is very much more than is needed to accelerate the proton beam - hence its name 'Energy Amplifier'. Its advantage is that it is sub-critical and would result in less long-lived and toxic radioactive waste. The underlying technology could also be adapted to transmute waste products from conventional reactors, and is most likely to be used in this mode first.

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Thanks go to Bill Gelletly and Phil Walker of Surrey University and Jean-Pierre Revol of CERN for their help with this paper.