OVERVIEW: HISTORY AND PERSPECTIVE OF THE SEARCH FOR SUPERHEAVY ELEMENTS

ABSTRACT Although superheavy elements had been postulated earlier, the current wave of international interest in these elements began in the latter part of the 1960's as a result of predictions of unusual nuclear stability around element 114. Another island of stability was later postulated around element 164. Theoretical studies of half-lives then led to investigations of the dynamics of heavy ion reactions that might produce these superheavy elements at accelerators. Also theoretical studies concerning the astrophysical r process have evaluated the probability that superheavy elements may have been formed in a supernova event or in a man-made nuclear explosion. These latter calculations were needed because predicted half-lives of some superheavy elements would allow them to exist in nature. An important immediate result of this new field of endeavor, motivated by the search for superheavy elements, has been a much better understanding of the previously poorly understood fission process. The theoretical studies have been paralleled by experimental programs concerning searches for superheavy elements in nature and at heavy ion accelerators. Searches using heavy ion reactions have culminated recently in investigations at Berkeley of the reaction 248 Cm + 48 Ca. This reaction is thought to be the most favorable accessible one. So far reported results have been negative and a move toward shorter half-lives is in progress. The unique uranium beam at the UNILAC in Darmstadt is now being used to bombard uranium targets. Perhaps higher beam intensities and the use of heavier targets (such as 248 Cm) may bear fruit. In the meantime, the work at the new heavy ion accelerators is opening new insights into the interplay and characteristics of competing mechanisms in the reactions of very heavy nuclei. Extensive searches have also been made in nature. These searches sometimes have used predictions of chemical and physical properties of the superheavy elements to ascertain the most probable ores or natural formations to look in; sometimes they have used unusual and unexplained natural phenomena as possible indications of superheavy elements; and sometimes they have used hopeful, blind luck. So far none of these approaches has produced a generally accepted superheavy element discovery although workers at various sites (including Dubna and Stanford) have recently obtained results which are encouraging. Spontaneous fission has been used most usually for the searches in nature and at accelerators because of the extreme sensitivity it allows for detection. Predictions that the average number of neutrons per fission might be about 10 caused several programs in neutron multiplicity counting of natural samples to commence. Many kinds of samples have been counted. Among the more interesting are meteorites. At Dubna some possible signs of a superheavy element have been seen in the Allende meteorite which was earlier shown to exhibit an unusual xenon isotope ratio in work at the University of Chicago. Alpha activities not corresponding to known nuclides have also been reported from time to time. Perhaps the most notable of these, from the historical point of view, are ones around 4.5 and 3.0 MeV. The search for superheavy elements has served as the motivation for the development of several techniques for identifying in a definitive fashion a very small number of the atoms of an element. Perhaps the most notable of these is the development of apparatus to allow the use of synchrotron radiation to excite x-ray fluorescence for an identification on the basis of what must be regarded as an extension of Moseley's method. Thus, although to date no superheavy elements have been discovered, the possibilities still appear strong; and the motivation generated by the search is, in any case, resulting in definite advances in the techniques of theory and experiment.