鈥淲E DID an experiment the other day that turned gold into mercury,鈥� says Ken Ledingham, a laser specialist at the University of Strathclyde in Glasgow, UK. The achievement might not have impressed medieval alchemists, whose goal was precisely the reverse 鈥� turning base metals into gold. But it could still be the start of a revolution. Using lasers to transmute elements means that within a few years scientists could be doing alchemy on their desktops, with huge implications for applications as diverse as medicine and nuclear power.
The alchemists of old never got close to creating gold. Their experiments were mere chemistry, which affects only the electrons on the surface of atoms. True alchemy requires changes to the atom鈥檚 nucleus: either embedding more protons and neutrons in it or tearing them out. Altering the number of protons changes one element into another, while adjusting the number of neutrons tunes the atom鈥檚 stability, transforming an unstable isotope into a stable one, or vice versa.
Since Ernest Rutherford 鈥渟plit the atom鈥� in 1919, we have known that bombarding atoms with particles such as neutrons or protons can convert one element into another. This generally requires nuclear reactors or particle accelerators with kilometres of tunnels and huge superconducting magnets, but Ledingham and colleagues have used a laser to do the job. True, the laser is a huge one. Called Vulcan, and housed at the Rutherford Appleton Laboratory in Oxfordshire, it is the most powerful laser in the world and the size of a small hotel. But laser technology is progressing fast, and within 5 years lasers nearly as powerful as Vulcan could be small enough to fit on a table top. And this could bring the power of transmutation to the masses.
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Ledingham and his colleagues have used Vulcan to add protons to gold nuclei to create mercury. But there is more to the new alchemy than turning one heavy metal into another. In a paper accepted by Journal of Physics D: Applied Physics, the team holds out the tantalising possibility of neutralising dangerous radioactive waste. They used Vulcan to convert iodine-129, an isotope that remains active for millions of years, into iodine-128, which decays in minutes.
To carry out the transmutation, the researchers fired a picosecond laser pulse at a gold target. The intense energy of the laser beam blasts the gold atoms into a plasma of free nuclei and electrons, which then emit gamma rays as they pass through the rest of the target. These intense gamma rays collide with the atoms of iodine-129, shaking the nuclei so violently that a neutron is squeezed out.
Transmuting nuclear waste has long been considered an attractive way of dealing with the ugly by-products of nuclear power. Researchers in France, which uses nuclear energy to supply 80 per cent of its electricity, are obligated by law to investigate transmutation. The US also has an active research programme into this kind of alchemy (快猫短视频, 16 January 1999, p 30) and the British government is considering whether to start one. Until now, the only options have been modified versions of nuclear reactors, in which neutrons released during fission collide with the unwanted isotopes and break them apart. But many anti-nuclear groups see this as a ploy for reviving nuclear power.
Laser transmutation might provoke less hostility, say its advocates, as well as potentially being able to clean up waste that already exists, so it has long been a goal of laser researchers. 鈥淚n the early 1990s we were thinking 鈥榳ouldn鈥檛 it be great if we could transmute waste with high intensity lasers?'鈥� recalls Scott Wilks at the Lawrence Livermore National Laboratory in California, who was part of the team that demonstrated laser-induced fission for the first time in 2000. Ledingham says that lasers should now be considered as a serious alternative to reactor transmutation.
But it won鈥檛 happen any time soon. Because the laser light has to be converted to gamma rays 鈥� only a small fraction of which collide with the target atoms 鈥� the process Ledingham has demonstrated is extremely inefficient. His recent experiments converted only 3 million atoms of iodine-129 into iodine-128 鈥� less than a billionth of a microgram. To convert the entire test sample, which measured just a couple of centimetres across, the laser would have had to fire more than 1017 times, swallowing an enormous amount of energy in the process. 鈥淵ou might need to build a power station to do it,鈥� says Karl Krushelnick, a laser physicist at Imperial College London and a member of Ledingham鈥檚 team. Besides, the laser can currently only fire once an hour.
As well as destroying unwanted isotopes, alchemists can also make new elements. 鈥淣uclear physicists can make everything that is in nature, and more,鈥� says Jim Al-Khalili, a nuclear expert at the University of Surrey. For example, the element with 110 protons has just been officially named Darmstadtium by the International Union of Pure and Applied Chemistry. This element is not found in nature, but was created in a particle accelerator in Darmstadt, Germany. Other unnaturally heavy nuclei have also been created (快猫短视频, 2 October 1999, p 38) and scientists continue to search for more.
But the first practical benefits of desktop alchemy are more likely to appear in medical physics, which might have pleased the original alchemists, who believed that transmutation would lead to cures for disease. 鈥淎lchemy is really useful in making radioisotopes for medicine,鈥� says Philip Walker, head of the nuclear and particle physics division at Institute of Physics in the UK. These isotopes are used in medical imaging: for example, fluorine-18 decays by emitting an antimatter particle called a positron, which annihilates in a burst of energy as soon as it hits an electron. If this happens within the body, detectors arranged outside can capture the photons emitted and pinpoint the location of the fluorine isotope. The technique is called positron emission tomography, or PET scanning, and it is often used to look for tumours.
Fluorine-18 and other radioisotopes used in medicine have to decay rapidly so that they are picked up during scanning and don鈥檛 persist for long in the body, which means that they have to be made just hours before they are used. But the small particle accelerators that are currently needed to make them are available in very few hospitals, and have to be housed in concrete bunkers to shield patients and staff from the radiation they produce. Ledingham believes that within 5 years fluorine-18 could be produced by lasers housed in back rooms. 鈥淚t could have a huge impact,鈥� says Walker.
To prove the point, the team has just used Vulcan to make fluorine-18 from oxygen. This work has been submitted to Nature. The new atoms were rushed in a taxi to the Patterson Institute for Cancer Research in Manchester, where they were incorporated into the sugary compound that is used to treat patients. One laser shot created one-tenth of the amount of fluorine-18 needed for a single treatment, says Ledingham.
