杏吧原创

FOR ALL its eureka moments, science has taught us many unpalatable lessons about what we are powerless to do. We can鈥檛 dim the sun to remedy droughts or global warming. We can鈥檛 stave off the ravages of time to live for thousands of years. And there鈥檚 little we can do about radioactive waste from nuclear reactors that will be a health hazard for generations to come. Radioactivity cannot be tamed; all we can do is bundle the waste somewhere safe and wait for it to decay away. So it takes some nerve to say otherwise, and suggest that there are, after all, ways to speed up radioactive decay.

Yet that is exactly what Claus Rolfs, a physicist at the Ruhr University in Bochum, Germany, is doing. His dramatic 鈥� and controversial 鈥� claim is that by encasing certain radioisotopes in metal and chilling them close to absolute zero, it ought to be possible to slash their half-lives from millennia to just a few years. He says it鈥檚 time to rewrite defeatist textbooks that insist we cannot alter the pace of radioactivity. 鈥淲hen I was studying physics, my teachers said nuclear properties are independent of the environment 鈥� you can put nuclei in the oven or the freezer, or any chemical environment, and the nuclear properties will stay the same,鈥� says Rolfs. 鈥淭hat is not true any more.鈥�

鈥淓ncasing radioisotopes in metals and chilling them could slash millennia off their half-lives鈥�

If Rolfs is right, it could have profound implications not just for nuclear waste management, but also for understanding the Earth鈥檚 interior and measuring the age of the universe. So far, other physicists have reacted to his claim with an equal mix of scepticism and intrigue. Yet everyone is keen to see his idea put to the test properly, which could happen within the next year at CERN, the European centre for particle physics near Geneva, Switzerland.

Rolfs has spent most of his career studying the nuclear reactions that rumble in the hearts of stars. He heads the Laboratory for Underground Nuclear Astrophysics beneath Gran Sasso mountain in Italy, where researchers use accelerators to trigger low-energy nuclear fusion reactions similar to those in stars. His work has helped to establish precise values for the rate at which the nuclei of light elements fuse. In a typical experiment at Gran Sasso, researchers might fire a beam of helium-4 ions into a little cloud of helium-3 gas and measure how often the nuclei fuse to form beryllium-7 鈥� a key reaction in the sun.

Then, around five years ago, Rolfs heard about some curious results from a lab in Berlin, where an international team of researchers was studying fusion of deuterons 鈥� a proton bound to a neutron 鈥� implanted inside different materials, including the metal tantalum. The team reported that when they fired deuterons into a deuteron-soaked sliver of tantalum, the deuterons fused with each other unexpectedly often (Europhysics Letters, vol 54, p 449). It was a startling result because conventional wisdom says that, as with radioactive decay, nuclear fusion rates should not depend on the environment.

Baffled, Rolfs asked his students to repeat the experiment at his lab in Bochum. Sure enough, they found the same effect. Rolfs went on to test the fusion rates of deuterons in more than 50 materials, from metals to semiconductors and insulators. Compared to fusion rates in a gas, rates in a metal were consistently higher, especially at low temperatures. 鈥淯ntil then, no one thought a nuclear property like the fusion rate could be enhanced by the environment,鈥� Rolfs says. 鈥淏ut it was 鈥� this was the first surprising news.鈥�

To explain these findings, Rolfs adapted a model developed by the Dutch physicist Peter Debye in the 1920s for the energetic ionised gases known as plasmas. Rolfs reasoned that like electrons in plasmas, the loosely bound conduction electrons in metals might cluster around positively charged deuterons. As the temperature drops, he argues, the electrons would cluster ever closer, generating electric fields that would accelerate other deuterons towards them, increasing the probability of deuteron-deuteron fusion. Rolfs has shown that this picture can be extended to heavier elements as well. His experiments verified that protons fuse more easily with lutetium-176, for instance, if the lutetium is inside a metal rather than an insulator (Journal of Physics G, vol 32, p 489).

Nuclear freeze

Radioactive decay can sometimes be like fusion in reverse, and this set Rolfs wondering: could a cold metallic environment influence decay just as it influences fusion? If a radioactive nucleus was inside a metal, perhaps the metal鈥檚 conduction electrons might help accelerate a positively charged particle out of it, Rolfs reasoned; that should enhance alpha decay, in which a nucleus emits an alpha particle (a helium nucleus). The same would apply to a radioactive process called beta-plus decay, where a proton converts to a neutron and emits a positron. Conversely, being inside metals should hinder electrons leaving or entering a nucleus, which is what happens in conventional beta-minus decay and another process called electron capture, which involves a proton in the nucleus combining with an electron to form a neutron.

Using his simple model, Rolfs has come up with ballpark estimates for how much the half-lives of different radioactive nuclei should change inside metals. Some changes are subtle, he calculates, while others are sky-high 鈥� certainly large enough to make a real difference to the longevity of radioactive isotopes. That, at least, is the theory. Now Rolfs is starting to run lab experiments to see whether these changes occur in practice.

In one of these tests, he and his colleagues prepared samples of beryllium-7 implanted inside the metals palladium and indium. Beryllium-7 naturally decays via electron capture with a half-life of 53 days. Rolfs and his team did not directly measure the half-life of beryllium inside metals but measured the decay rate, which counts the number of electron captures per second.

From measurements of the decay rates of each sample made at room temperature, and then at 12 kelvin, the team estimates that beryllium-7鈥檚 half-life in chilled metal increases by about 0.8 per cent. This is slightly less than the model鈥檚 prediction of 1.1 per cent, but definitely in the expected direction (European Physical Journal A, vol 28, p 375). The team didn鈥檛 stop there. Similar tests with sodium-22, which undergoes beta-plus decay, showed that the normal half-life of 2.6 years shrank by about 1.2 per cent in metal at 12 kelvin. This was far less the model鈥檚 prediction of 14 per cent. But again, Rolfs is encouraged that the effect is in the direction that his model predicts (European Physical Journal A, vol 28, p 251).

Perhaps the most striking result to date comes from an experiment this year in which Rolfs bombarded a sliver of highly purified gold-197 鈥� the metal鈥檚 normal stable isotope 鈥� with neutrons. Some of the gold atoms absorb a neutron to create radioactive gold-198, which undergoes beta-minus decay with a half-life of 2.7 days. With the gold-198 atoms surrounded by ordinary gold, Rolfs鈥檚 model predicts that their half-life should increase by about 1.7 hours, even at room temperature. Conveniently, he could now measure the half-life directly, by counting the gamma rays released during the beta decay, and timing how long the count took to halve. The results, which are still to be published, amazed him. It turned out that the half-life actually increased by a whopping 5 hours. 鈥淚鈥檓 astonished myself 鈥� I didn鈥檛 expect such a large effect,鈥� says Rolfs.

Now his team is starting experiments with alpha decay, which interests Rolfs most of all. That鈥檚 because his model predicts truly stupendous changes for alpha decay. Take the alpha emitter radium-226, which has a half-life of 1600 years. The model hints that by encasing it in a metal and chilling it to about 4 kelvin, it might be possible to slash its half-life by a factor of 1000, to less than two years. If that turns out to be what happens, it could have huge practical implications.

Radium-226 is a component of spent fuel from nuclear power stations and the sludge left over from uranium mining. It will pose a health hazard for more than 50 generations to come. 鈥淥ur great-great-grandchildren may have to pay the price, and this is a justified criticism of nuclear power,鈥� says Rolfs. If his theory is correct, it might be possible to make the radium safe in our own lifetimes simply by mixing it into a pure metal and chilling it (see 鈥淲aste disposal鈥�).

This is a radical prediction, to say the least, and other physicists are finding it hard to swallow. 鈥淚鈥檓 deeply sceptical about it,鈥� says Phil Walker, a nuclear physicist at the University of Surrey in Guildford, UK. Despite this he welcomes what Rolfs is doing. 鈥淚 always like people who are prepared to stick their neck out,鈥� he says. 鈥淗e challenges the way we鈥檙e thinking, and even if he鈥檚 not right it will make us reconsider things and understand why he鈥檚 not right.鈥�

Nikolaj Zinner, a theorist at the University of Aarhus in Denmark, says that physicists generally accept Rolfs鈥檚 findings regarding nuclear fusion in metals. Zinner is also prepared to accept his idea that it might be possible to tweak beta decay rates in the way Rolfs suggests, but says alpha decay is an entirely different kettle of fish. That鈥檚 because while beta decay is governed by the weak nuclear force, it鈥檚 the strong force that governs alpha decay. To escape a nucleus, a positively charged alpha particle has to overcome the attractive strong force of other nuclear particles around it by penetrating an energy barrier, a process called quantum tunnelling. Zinner accepts that conduction electrons from a metal would, as Rolfs suggests, lower that barrier by increasing the density of negative charge surrounding the nucleus. The catch, Zinner says, is that they would also effectively decrease the energy of an alpha particle inside the nucleus, making it harder for the particle to escape.

Zinner says these two effects pretty well cancel each other out. If anything, he calculates, implanting a radioisotope in metal should slow down its alpha decay by a minuscule amount, which is not what we want. 鈥淭he lifetime should actually increase, if anything, but by a very small amount of about 1 per cent or less in most realistic cases,鈥� he says.

This isn鈥檛 the only reason put forward to suggest that Rolfs鈥檚 model may be on shaky ground. Other physicists question its prediction that the rates of alpha decays will increase as the temperature falls. The model assumes that the average energies of conduction electrons in a metal are proportional to their temperature, and so they become more sluggish and crowd closer to a positively charged nucleus when they are very cold. This, however, flies in the face of theories which say that even at normal temperatures the electrons will already be crammed into their lowest possible quantum energy states. Only by heating them to tens of thousands of kelvin will they move to higher energy states.

鈥淲arming them up from 1 to 300 kelvin makes almost no difference at all,鈥� says Nick Stone of Oak Ridge National Laboratory in Tennessee. If alpha decay rates could really be changed as radically as Rolfs suggests, we鈥檇 already know about it, he adds. For more than four decades his group has studied the decay of many radioisotopes, including the alpha emitter radium-224, in iron chilled to a whisker above absolute zero. They did this to test fundamental theories about how radioisotopes decay when aligned by strong magnetic fields experienced by trace nuclei surrounded by cold iron atoms. If there had been dramatic changes in alpha decay rates at low temperatures, they should have stuck out like a sore thumb during the checks and balances in his experiments, he says. 鈥淓ither we鈥檝e been totally asleep, or this huge change in alpha decay rates just isn鈥檛 there.鈥�

Nevertheless, Rolfs is standing by his results. His preliminary experiment on alpha decay has shown some promise. For this, he implanted the alpha emitter polonium-210 into copper and chilled the samples to 14 kelvin (see Diagram). His measurements, as yet unpublished, suggest that the polonium half-life shortened by about 10 per cent. That鈥檚 still a far cry from the 95 per cent his model predicted, but he is confident that refinements to the experiment should cut the half-life further. Improving the purity of the metal samples, for instance, and implanting the radioisotopes deeper inside should help, he says. Most of all, he hopes to run tests with a long-lived element like radium-226. Experiments, not theory, are the key to understanding nature, he insists. 鈥淚f the alpha decay of radium-226 can be made very short, I don鈥檛 need any theory. It will just be a fact.鈥�

Rolfs is now working on more tests with physicists at CERN. A facility there called ISOLDE creates and accelerates beams of tailor-made nuclei. ISOLDE could easily implant radium-226 nuclei into metals, and equipment at CERN or elsewhere could then test the alpha-decay rates at temperatures as low as 0.01 degrees above absolute zero. 鈥淭he experiment would actually be very easy 鈥� it鈥檚 beautifully simple,鈥� says Karsten Riisager from the ISOLDE team, which plans to seek approval for the experiment from a CERN committee later this year. 鈥淲ithin one year, we鈥檇 hopefully have the first scientific results, and within a couple of years we should have settled the matter,鈥� he says.

Rolfs says that if his theory stands up it might not only help rid us of radioactive waste but could also shed light on a long-standing puzzle about the Earth. Some of the heat produced in our planet鈥檚 interior comes from the radioactive decay of uranium and thorium. Yet there is more heat than anyone can account for (New 杏吧原创, 7 August 2004, p 26). Rolfs suggests that in the molten metallic environment of the Earth鈥檚 core, uranium and thorium could be contributing to the excess heat by decaying faster than we imagined. Tweaking radioactive decay rates might also change estimates of the ages of the oldest stars, and hence the universe itself.

No one has yet thought these ideas through, because the research is still only months old. 杏吧原创s need far more proof that half-lives really can change radically. 鈥淓ven if the effect is only 10 per cent, it would still be scientifically interesting,鈥� says Walker. 鈥淎nything that opens up a new avenue with nuclear physics could go down surprising routes.鈥�

For Rolfs himself, it is the possibility of dealing with the nuclear waste problem that is a bigger pull. 鈥淎ll my life, I used taxpayers鈥� money to study something completely impractical,鈥� he says. 鈥淵ou cannot buy or sell anything from stars. If at the end of my career I can do something useful, this would be a wonderful payback.鈥�