Discovering transuranic elements at Dubna - heavy element research in Soviet Union

Science News, Jan 30, 1988 by Dietrick E. Thomsen

Discovering Transuranic Elements at Dubna

The search for transuranic elements, chemical species heavier than uranium, is an attempt to extend the frontiers of both chemistry and nuclear physics. An extremely difficult endeavor for the experimenters involved, it is nevertheless fairly easy for outside observers to follow. Each achievement in turn can be marked on a scorecard -- the periodic table of the elements. One by one the blanks are being filled in on the way to the so-called "island of stability," whose existence theorists predicted 20 years ago. This is a range of relatively stable nuclei with atomic numbers just above 110. Possibly they might be stable enough to be of some use for something.

The transuranics are all inherently unstable, subject to spontaneous fission or radioactive decay -- as is uranium, atomic number 92, the heaviest element known to exist in detectable amounts on earth. In the search for transuranics, experimenters take two general approaches. Either they try to manufacture them by setting up fusion reactions involving lighter nuclei, or they look for traces of them in somewhat exotic minerals, such as meteoric debris or certain hot brines.

Up to now all the successes -- all the detections of new elements that have gained the acceptance of the scientific community -- have come from induced fusion reactions or bombardment of nuclei with energetic neutrons. The other approach has not been as fruitful. According to one of the leaders in the field, Georgy N. Flerov of the Joint Institute for Nuclear Research at Dubna in the Soviet Union, searches of minerals have shown evidence for fission events that cannot be assigned to known species, but no kind of identification has been possible.

So far the heaviest element claimed by any researchers in the field is element 110, which has been the object of experiments by a Dubna group led by Yuri Oganessian. Flerov recently sent SCIENCE NEWS two papers -- one on Dubna's latest work on this element, authored by Oganessian and 15 other scientists from the Soviet Union, romania, France and the German Democratic Republic, the other a review of the program in general, in which Flerov expresses his belief in the feasibility of going for atomic numbers higher than 110.

A number of laboratories in several countries have participated in the general search for elements heavier than uranium, but up to now the main laboratories involved in claims of discovery have been Dubna, where Flerov has guided the world for decades; the Lawrence Berkeley (Calif.) Laboratory, where Glenn T. Seaborg has been the overall leader; and the Gesellschaft fur Schwerionenforschung (GSI) at Darmstadt, West Germany. GSI, which has an unconfirmed claim to element 109, is now trying to confirm the apparent discovery of element 110.

The earliest discoveries of transuranic elements came out of weapons research during World War II. Later, scientists using the technique of bombarding selected targets with accelerated ions in the hope of making two nuclei fuse into a bigger one experienced several successes, including the generally accepted discoveries of elements 104 and 105 as well as claims to the discovery of some of the higher numbers. Then followed a long period of difficulty to which Flerov refers in one of the papers he recently sent to SCIENCE NEWS. The difficult period led many experts to conclude that the island of stability would never be reached, but Flerov remains optimistic. In his review paper he writes, "In our view, however, attempts to produce new elements with [atomic numbers equal to or higher than 110] can and must be pursued." Dubna's first evidence for element 110 was reported a little less than two years ago (SN: 5/17/86, p.319).

The experiments are usually done by selecting a fusion reaction calculated to yield a particular transuranic element. Experimenters determine the appropriate reaction by adding up neutrons and protons in the reacting elements, making allowance, where nuclear dynamics demands it, for the release of one or more free neutrons in the reaction. Then they bombard targets made of one element with accelerated ions of the other.

The key to finding a new transuranic element is the observation of a source of nuclear fission or of alpha-particle decay that does not correspond to any nuclear species already known. Such activity can be attributed to the particular transuranic element being sought, and on this basis several claims have been made, including Dubna's claim to element 110, as related in the previously mentioned paper by Oganessian et al.

For certainty, however, the scientific community at large requires specific kinds of confirmation, particularly the identification of the products of the fission or the alpha decay to see that they are the nuclei that nuclear dynamics would expect to come from the transuranic element in question. The identification is extremely difficult, as the quantities involved are minute--often no more than a few atoms. Discoveries of elements 104 and 105 are generally accepted on this basis. Claims to higher atomic numbers, particularly 110, still await that kind of confirmation.