Exotic nuclear decay discovered The discovery, nearly a century after Becquerel, ofa novel mode of radioactive decay is a surprise, but one that confirms a decay as the chief means by which heavy nuclei shed mass.
THOSE who decorate their offices with wall observations now reported, there are particle exists within the nucleus it will then charts showing the isotopes, stable and roughly 1,000 million times as many escape, or the rate of the corresponding otherwise, of the elements are in for a-particles and 14C nuclei in the decay of disintegration process, is thus a function of trouble. As things are, these elaborate mRa, perhaps as vivid a proof as there the height of the potential barrier, its width diagrams usually show by means of a could be of the dominance of the familiar and of the total decrease of the potential colour scheme of some kind which radio mechanisms of decay. energy of the system once the disinte actively unstable nuclei decay by which The rarity of these events is also the gration has taken place. means. Decays in which the a- or explanation why this novel form of radio This calculation, first made in simple {J-particles (electrons) are emitted activity has not previously been found. So form by Gamow, has more recently been predominate, but the diagrams must also much can be told from the account by Rose much refined and accounts for the make room for other less common and Jones of their observations, which "Gamow factors" used by Rose and Jones processes - positron emission, internal entailed more than half a year of running as part of their reason for believing that conversion (of an electron in an inner shell) time with detectors arranged so as to their disintegration products are 14C and and even fission. Now the same wall charts distinguish between single 14C fragments not some other isotope of carbon. But even will have to accommodate radioactive and more frequent occurrence of groups of on this simple picture, one huge uncer decays in which unstable nuclides emit sub three a-particles from the nearly tainty persists - the chance that a particle stantially heavier nuclear fragments than simultaneous decay of separate 223Ra of the kind that eventually tunnels success the familiar a-particle. nuclei. (The fact that one of the two fully through the barrier can be held, if That is the burden of the discovery (see detectors seems at one stage to have been only momentarily, to exist. What emerges p.245) reported this week from the Uni put out of action by accumulated radiation from what Rose and Jones now say is that versity of Oxford by H.J. Rose and G.A. damage is a telling measure of the numbers the "preformation probability" of I2C Jones of a handful of unmistakable carbon of a-particles involved, even if the fault would have to be comparable with that of nuclei among the decay products of appears to have been an ambition to avoid the much simpler a-particle if it were to be radium-223 (223 Ra), hitherto marked in the buying new material for this purpose.) In produced in a distintegration. That cannot charts as a simple a-emitter with a half-life the circumstances, there is no reason to be the case, at least on this simple picture. It of 11.2 days. What they have shown is that think that the discovery that 223 Ra can is relevant, but sobering, also to recall that 223 Ra nuclei occasionally, but significantly, decay with the emission of a 14C particle the preformation probability of 14C decay with the loss not of an a-particle but will quickly be followed by similar obser inferred from the results now reported is of a carbon-14 (14C) fragment. Anthro vations in respect of other nuclei. Rose and substantially greater than might have been pomorphically, from the point of view of Jones say they are looking at another expected from simple considerations, the nucleus so to speak, this novel arrange candidate nucleus, but there will be few which is a reminder that the calculation of ment has obvious advantages. The decay groups with the stamina that compels the absolute rates of even a decay, for scheme of Z23Ra involves the successive competition. example, is still not feasible. emission of three a-particles (to give Some, no doubt, will even ask whether What, in these circumstances, may be radon-219, polonium-215 and lead-211) this search for exotic and necessarily rare made of the discovery now reported? If, in followed by an electron, to give mechanisms of decay can be worth the the years ahead. there should accumulate a bismuth-2II. (This decay scheme, which trouble, now at least that it has been handful of examples of exotic kinds of begins with uranium-235, predominantly demonstrated that such mechanisms are radioactive decay in unstable heavy nuclei, leads to lead-207 by the further emission of not entirely excluded. The simple answer, it should at least be possible to glean some two a-particles and an electron.) So why as always, is that the neglect of phenomena empirical information on the chance that should not an unstable 223Ra nucleus on the simple ground8 that they are only various groups of nucleons will assemble shorten this tortuous process by elimin rare, or that their effects are small, is never into different potential disintegration ating a substantial part of the charge and justified. On this occasion, several fragments in neutron-rich heavy isotopes mass that must ultimately be lost in a single important questions arise, not the least of such as 221 Ra. package? What now emerges is that that which is why the first exotic fragment to Hopes that exotic disintegration routes option is not as rigorously forsworn as the have been found in the decay of a radio may have some practical value in other conventional wall charts suggest. active nucleus should have been the fields, perhaps in some novel technique of So how can it be that the best part of 90 unstable 14C nucleus and not the stable 12C, geochronology, seem, however, to be slim. years has passed since Becquerel's itself simply a combination of three Exotic disintegrations of the kind now discovery of radioactivity before it has a-particles. identified are simply too few compared been recognized that a- and {3-emission are Superficially, the calculation of the rate with the more familiar adisintegrations for not the whole of radioactivity? That is the of some specified radioactive decay is a their daughter nuclei and their products to obvious question that will now arise. The simple problem in elementary quantum be recognizable among the end products of proper response, as a glance at the article mechanics, often found in introductory the three principal radioactive series. (And by Rose and Jones will show, is that the textbooks. An a-particle, for example, can 14C is in any case unstable.) But at this stage discovery of 14C emission from 223Ra, escape from an unstable nucleus only if it it would be rash to rule out such possi important though it is, serves chiefly to can tunnel through the potential barrier bilities. Too many confident predictions emphasize that a-particle emission is the against disintegration caused by the that new phenomena have no practical chief means by which radioactively nuclear forces that hold even unstable significance have too often been falsified unstable nuclei lose mass. Indeed, in the nuclei together. The chance that if such a for that course to be wise. John Maddox
0028-0836/84/0J0207·0 I so I .00 Cl 1984 Macmillan Journals Ltd