How to Transmute Elements with Laser Light
A technique for triggering nuclear reactions with photons could revolutionise the production of medical radioisotopes, say physicists
Every year, doctors around the world carry out tens of millions of procedures involving nuclear medicine. The most common medical radioisotope is technetium-99 which is used in some 30 million procedures per year; that’s 80 per cent of the total.
Technetium-99 is short-lived with a half life of only 6 hours. So hospitals get it from the decay of the longer-lived molybdenum-99. This, in turn, must be made by bombarding uranium-235 with neutrons and separating mo-99 from the numerous fission products.
This is a difficult and dangerous procedure which is possible in only a handful of nuclear facilities around the world. That makes the supply of these essential medicines hugely expensive and extremely fragile.
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Indeed, when the Chalk River nuclear reactor in Ontario, Canada, shut down for emergency repairs in 2009, it turned out to be producing a large fraction of the world’s technetium-99 supply. The result was a global shortage that lasted for months. Clearly new suppliers are needed.
Today, Hiroyasu Ejiri at Osaka University and S. Daté at the Japan Synchrotron Radiation Research Institute say there is an entirely new way to make nuclear medicine.
The idea is to stimulate nuclear reactions using powerful laser beams. At a specific frequency, these beams cause a nucleus to resonate violently, triggering the nuclear reaction and effectively shaking it apart. And since almost all the photons trigger a reaction, this process can be close to 100 per cent efficient.
So for example, Ejiri and Daté say this method transmutes iodine-127 into the medical isotope iodine -126 with an abundance of 100 per cent. And it can do it at a rate of up to 10^13 nuclei per second.
This has big advantages over the current techniques. First, it is possible to tune the frequency of the light so that it triggers specific reactions, allowing physicists to choose exactly what they want to make. Second, the resulting samples are relatively pure. And finally, this technique creates few, if any, nasty radioactive byproducts and so is more environmentally friendly.
There are a couple of caveats, of course: making the right kind of laser light is tricky. It can only be done by bouncing photons off a high energy beam of electrons circulating in a particle accelerator. And the required intensity of such an electron beam would only be possible with an expensive, bespoke, yet-to-be-built facility.
And while photonuclear reactions are good for making all kinds of medical isotopes–for example, PET tracers such as carbon-11, nitrogen-13, oxygen-15 and so on–they are not so good for making technetium-99, by far the most important radioisotope. In this case, the abundance is less than 10 per cent.
Nevertheless, Ejiri and Daté argue that their method “provides exclusively various kinds of specific/desired isotopes with the large production rate and the high density for basic and applied science”.
And with the supply for medical radioisotopes so fragile, it seems likely that ideas like this will be given greater consideration in the coming years.
Ref: arxiv.org/abs/1102.4451: Coherent Photonuclear Reactions For Isotope Transmutation
