ELEMENTARY PROGRESS

Heavy ion research sounds dull, but its implications for cancer treatments should get us all excited, says Norman Miller

Adding 30 and 82 to make 112 was not easy for scientists at the "element factory". But far from being a mark of abysmal schooling, getting elements 30 and 82 to fuse to become element 112 has been one of the latest triumphs in a field which blurs the boundaries between physics, chemistry and biology - heavy ion research.

The element factory, officially known as the Society for Heavy Ion Research (GSI) is in Darmstadt, south-west Germany. It leads the world in the creation of new chemical elements and is also at the forefront of a strand of research whose impact is stretching from new treatments for cancer to explaining the basic nature of matter.

Involving much of the paraphernalia commonly associated with smashing atoms in search of the most fundamental building blocks, work carried out at GSI and similar institutions turns this process on its head. It is a cutting-edge discipline using enormous energies not to smash atoms to pieces but rather to fuse atomic nuclei together to create "exotic nuclei" (new, super-heavy elements that do not exist naturally on Earth due to their instability). As well as the sheer "for the hell of it" kick of adding to chemistry's periodic table, studying these exotic elements sheds light on the origin and basic nature of the more prosaic matter around us, since it is now known that the synthesis of elements in stars (from which all matter on Earth ultimately derived) takes place via the decay of just such exotic nuclei through various stages until the stable elements known to us on Earth are left.

GSI's chemical cooks specialise in what is called "soft fusion", nothing to do with progressive rock music but a reference to the delicate process of inducing heavy atoms to merge. High energies are needed to bring different elements close enough together to fuse due to their electrical repulsion of each other. But by giving the beam just a little bit more oomph, the aim is to make them fuse with what is called a low excitation energy, a gentle embrace rather than a fierce clinch. This oomph comes from the high-speed bombardment of ions of one of the elements on to the other. Ions are used in this process rather than atoms as, due to their charge, ions can be accelerated with magnetic and electric fields to the required speed.

To create element 112 last year, scientists at GSI accelerated zinc ions (atomic number 30) to around 80 per cent of the speed of light, then fired them on to a target made of lead atoms (atomic number 82). The nuclei of the zinc ions reacting with the nuclei of the lead atoms in the target produced various exotic nuclei which were categorised according to nuclear charge and mass. This nuclear panhandling turned up a few atoms of the heaviest element achieved on Earth, with mass number 277 (112 protons, 165 neutrons). Hey presto for nuclear cookery, though the few atomic nuggets of this particular dish vanished faster than any of Delia Smith's creations, decaying in a matter of milliseconds, their brief existence detectable only by the presence of daughter isotopes - the decayed remains of the heavy ions. GSI's six previous successes in the field - from the 1980 synthesis of element 104 to element 111 in late 1994 - had equally short lives, but their creation and passing has improved our understanding of basic atomic processes and structures.

Studying the way heavy ions fuse and then fall apart has developed the theory that the protons and neutrons in atomic nuclei are bound into groupings or "shells". The arrangements of particles within the shells is now understood to have a key impact on the chemical nature of each element. In particular, the most stable elements are now known to have "magic nuclein" in which shells are filled to capacity with either protons or neutrons, akin to the way the amount and arrangement of ballast in a ship has a bearing on how stable it is. The main isotopes of elements such as calcium and lead are doubly magic in that their nuclei contain full shells of both protons and neutrons. Like any model, this one makes predictions, not only about particular arrangements of protons and neutrons within the nuclear shells of new elements (something which can be tested by studying how they decay after their brief instants upon the stage) but also on the stability of various isotopes of new elements. In particular, the goal is to create a specific isotope of element 114 (with mass number 298) that theory predicts should be relatively stable. The creation of this island in the transuranic elements' sea of instability is now one of the major goals in heavy ion research.

While playing around with exotic nuclei previously only found in the atomic pressure cookers of stars has a high gee whizz factor, heavy ion research also has very real practical applications, most notably in medicine and material technology.

A distinctive feature of heavy ion radiation in comparison with other types such as X-rays or gamma rays is the way its energy is distributed when it hits human tissue. When used against tumours, X-rays and gamma rays are weapons, spreading their energies wastefully over a wide area and damaging many healthy cells in the attempt to destroy malignant ones. In sharp contrast, ion beams remain sharply focused, drifting off line by less then a millimetre for a tissue penetration depth of 10cm in the case of carbon ions, making them a scalpel compared to X-ray's shovel.

More remarkably, ion beams exhibit what is called an inverted depth dose curve in the effect of their radiation. This means that the zapping power of their energy increases the further they penetrate into tissue, reaching maximum intensity a few tenths of a millimetre before the ion beam comes to a complete halt at the end of its range. And since the range of the beam can be fixed precisely by varying its energy, this allows the maximum radiation dose to be delivered to a point deep within tissue (on to a deep-seated tumour, for example) while causing relatively little damage to intervening cells - again, in sharp contrast to therapies using X-rays or gamma radiation. Different energy ion beams can also be deployed in sequence to target various parts of a tumour at different depths, making it possible to destroy irregularly shaped tumours with some precision.

Biophysicists at Darmstadt and elsewhere have now built up a model of the "radiation quality" of ion beams using different ions and energies on over 100,000 biological samples, from bacteria to human cells. Around 15,000 people worldwide have taken part in successful trials of therapy using proton beams, and heavy ion beam therapy will take this a step further. However, the heavy ion synchrotron at GSI is the only accelerator in Europe capable of producing beams of sufficient range and intensity for clinical use, so an experimental radiation therapy unit has been set up at GSI which is now in the middle of a major clinical study. The next problem to be solved is an engineering one - how to build particle accelerators at hospitals suitable for clinical use but at a reasonable cost. GSI has produced a model for a 17m diameter machine capable of treating 2,000 patients a year for the same cost as conventional tumour therapy, while the Tera project in Italy is trying to establish a number of cyclotrons (similar to the synchrotron). Now, as ever, all that's needed is funding.

High energy ion beams also work their magic when they penetrate other materials, and this has sparked great interest among solid-state physicists and materials engineers with the promise of new manufacturing techniques and materials.

As with biological samples, when an ion beam is directed into innate matter, atoms along the line of the beam will be altered in a way which depends on the type and energy of beam, as well as the structure and chemical composition of the material. With high enough energy, the material will undergo changes at a macroscopic (rather than just atomic) level significant enough to change its structure and properties, paving the way for limitless application.

Picture a typical result. Where the beam has cut through the material there is a cylindrical track measuring about a 100,000th of a millimetre across and several millimetres long, a superb example of what material experts call a microstructure. Taking this, with the right combination of acids and base materials, scientists and engineers can etch out the tiny trails left by the ion beams to form materials full of tiny, precisely cut channels or, by using the tiny hollows as a casting mould, produce tiny needle structures on a metal base, like a microscopic bed of nails - or, as one suggestion already has it, an array of tiny antennae for 21st century communication devices. The tiny tunnels produced by ion etching have also been used to create microscopic tubes with possible applications in micro-engineering or medicine.

Another medical use for ion beams is ion implantation. This process allows a metal's properties to be tweeked by the insertion of atoms of a particular element. Already, ion implantation promises to extend the life of artificial hip joints. Presently made from a titanium-based alloy ball and shaft with a polymer in the artificial socket, they suffer from the formation of titanium oxide particles between the ball of the joint and the polymer socket which affects the joint's resistance to wear and tear. By implanting ions of certain elements into the surface layer of the titanium alloy, the abrasion caused by the titanium oxide has been greatly reduced.

Aviation and space safety are also benefitting. On-board computer systems in spacecraft and high-flying planes are vulnerable to the destruction of data by cosmic rays. Ion beams can be used to test electronic circuits by simulating the destruction process - scanning an ion beam across a circuit can detect when and where data has been changed by an ion strike. These changes to previously stored information are called "bit flips", a source of increasing worry to aviation experts as aircraft become increasingly reliant on computers. Using ion beams to simulate cosmic rays reveals sensitive areas on the circuit, providing valuable information to help find a form of protection.

Heavy ion research provides an answer to those who look at the cost of the paraphernalia used in high energy disciplines and question the point of it all. Heavy ion beams are not only aiding our understanding of the elements, they are stamping their imprint on many aspects of our future. Their fine points are far from pointless.