EVEN the best laid plans can go awry. Laurent Schwartz dreamt of revolutionising a type of cancer treatment that kills malignant cells by bombarding them with highly energetic beams of protons. Creating these beams normally requires huge particle accelerators that cost tens of millions of pounds. 鈥淔ar more than our hospital can afford,鈥� he says. The idea was to build a cheap, desktop proton accelerator . . . but it didn鈥檛 quite work out like that.
Schwartz is a doctor at the St Louis hospital in the heart of Paris. Last year, with a team of physicists from the Ecole Polytechnique, he started work on the new accelerator. But the pint-sized proton machine proved difficult to perfect. Then, by a strange quirk of fate, Schwartz and the team discovered something far more exciting. In the course of routine experiments, they found that their device produced X-rays. To their astonishment, these X-rays turned out to have potential for medical imaging, and in December last year they broke the news to the world in Nature.
Field X-rays
Their discovery could revolutionise X-ray imaging and treatment. If they are right, the new machine would be cheaper and more portable than conventional devices, more efficient and capable of running off ordinary power supplies. The possible applications are many and varied. Truly portable machines could assess the seriousness of soldiers鈥� wounds on the battlefield, or be used to check for concealed weapons at airports. They could even be used to examine the innards of astronauts in space. And in two completely different fields, X-rays from these machines may one day be used to carve circuits onto microchips or build atomic clocks.
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But how can a device designed to produce one type of beam end up producing another? The machine behind the French work is called an electron cyclotron resonance device or ECR. It consists of a cylindrical chamber about the size of a cake tin with magnets at either end. Electrons are created by the ionisation of a low-pressure gas inside the chamber, and the magnets create a 鈥渕agnetic bottle鈥� that traps them and sends them in a circular path around the chamber.
The electrons are then accelerated by an electric field. Of course, on each side of the circular path they must be accelerated in opposite directions. This is done using the oscilalting electric field component of a microwave beam, which changes direction as it passes through the chamber. The optimum acceleration occurs when the electrons are in step with the field and this resonance occurs only when the electrons are circulating at a certain speed, which in turn depends on the size of chamber and the strength of the magnetic field within it. When this happens the field accelerates the electrons in one direction on one side of the chamber and in the opposite direction on the other. Above and below this resonance frequency, the microwaves tend to slow them down.
As the electrons are accelerated, they smash into other gas atoms, ionising them and generating more electrons. This mixture of ionised atoms and electrons-a plasma-is what most physicists use ECRs to generate. Ionised oxygen, for example, is highly reactive and can be used to clean or etch the surfaces of microchips. And by ionising hydrogen, researchers can produce protons.
But when Schwartz and his team discovered that the device produced X-rays, their plans changed. X-rays are created when electrons smash into atoms. They either decelerate violently, emitting an X-ray photon, or dislodge an inner electron which itself emits an X-ray photon when it recombines with the atom. They discovered that they could maximise the output of X-rays by placing a small metal target just outside the circular path that resonant electrons would trace out. The idea was that any electrons with higher energies would veer off and hit the metal target, generating X-rays.
The energy of an X-ray can be no greater than the energy of the electron that creates it. The big surprise was that these X-rays had energies of up to 100 kiloelectronvolts-enough to penetrate the human body. Until then nobody had thought that electrons inside an ECR as small as the French team鈥檚 could reach this energy. In theory, the electrons would no longer be in step with the microwave field and so would be decelerated by it. Even the French team admits to being slightly flummoxed.
Cheap and cheerful
Nevertheless, the work has huge potential. Conventional X-ray machines work by boiling electrons off a piece of metal and then accelerating them in a straight line down a vacuum tube and onto a metal target. These machines are large and require special power supplies to create the huge electric fields needed for acceleration. They also cost the Earth. By comparison, the microwaves in an ECR are produced by a magnetron of the type used in microwave ovens. These cost around 拢60 and run off the mains.
With smaller, cheaper machines all sorts of things become possible. Elliot Fishman, professor of radiology and oncology at the Johns Hopkins School of Medicine in Baltimore, says hospitals desperately need smaller X-ray sources, provided the quality of the images can be preserved. 鈥淭he biggest advantage is that anything that becomes smaller becomes more portable,鈥� says Fishman. 鈥淚t would be easier to do intensive care and emergency patients. It would be ideal in clinics, physicians鈥� offices, any place where space becomes an issue.鈥� Even though conventional X-ray machines are gradually shrinking, even portable models are still about the size of a refrigerator. Making them truly portable 鈥渨ould be a tremendous advantage鈥�, says Fishman.
The big question is whether image quality can be preserved. The energy of the X-rays is not the only important factor when it comes to medical imaging-the intensity of the source is also important. For the moment, Schwartz admits that intensities from his ECR are at least ten times less than is needed for imaging. However, he points out that this is only a lab prototype and there is huge potential for improving performance.
Tabletop physics
Other scientists are also getting in on the act. Vincent Needham, an atomic physicist at Kansas State University, currently relies on vacuum ultraviolet or 鈥渟oft X-rays鈥� to study atomic collisions. At the moment, he uses sources for soft X-rays such as the $100-million Advanced Light Source at the Lawrence Berkeley National Laboratory in California. But the ALS is expensive to operate and takes up a space the size of a football field. 鈥淚f an ECR can be coaxed into providing sharp, intense spectral lines, many atomic collision problems become tabletop rather than national lab type experiments,鈥� says Needham.
And he envisages other uses for ECRs in atomic physics. 鈥淚f you have a stable source of particular spectral lines, then you may be able to use it as a frequency standard for creating or calibrating new atomic clocks or other instruments,鈥� he says.
Computer engineers, he adds, could use ECRs to burn patterns on silicon. 鈥淒own the road, the semiconductor manufacturers are going to be needing a new source of short-wavelength light for creating ever-smaller features on chips. I doubt that they鈥檙e going to want to build a $100-million synchrotron at every fabrication plant.鈥�
But much work remains to be done. For example, physicists have no idea how the electrons reach energies of 100 kiloelectronvolts. 鈥淚n our experience we never get electrons of that energy,鈥� says Stephen Kuo, a physicist at the Brookhaven National Laboratory on Long Island, New York, and an expert on ECRs. 鈥淚t sounds like a good way to generate X-rays, but the results need to be repeated.鈥�
And although X-rays of this energy are known to be created by larger ECR machines, in theory they should be more difficult to create in small chambers, says Leon Shohet, an electrical engineer at the University of Wisconsin and one of the leading ECR researchers in the US. Pumping more energy into the chamber simply drives the electrons into the walls.
The French team are still working to understand the process. One day they may even get round to building their desktop proton accelerator.
