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Attempts to Synthesise Superheavy Elements - a Status Report ATTEMPTS TO SYNTHESISE SUPERHEAVY ELEMENTS - A STATUS REPORT G. Herrmann Institut für Kernchemie, Universität Mainz, D-6500 Mainz, and Gesellschaft für Schwerionenforschung, D-6100 Darmstadt, Fed. Rep. of Germany Abstract 2. Nuclear and chemical properties A status report is presented on Let us first consider briefly the attempts to synthesise superheavy elements nuclear and chemical properties of super• by complete fusion reactions and by damped heavy elements. For spontaneous fission collisions between heavy nuclei. Although decay the maximum stability is expected for these efforts remained negative so far, 4th-VieA doubl/l/-\nV» 1y I T magimarrî cr » nucleunnrlûiisc 12 1S 8yl 8 HOWeVer , experimenters may still feel encouraged to other decay modes have also to be con• continue with their attempts because the sidered. Figure 1 gives two examples for potential of heavy-ion fusion and transfer sets of overall half lives calculated for reactions has not fully been exhausted to superheavy nuclei. In Fig. 1a, a-decay date. dominates at Z=114 and higher atomic num• bers so that the maximum half life is expected10^ for HoX18U. Note the broad 1. Introduction shore to the west from where the island is approached in heavy-ion reactions. This Those of you who have attented the shape of the island is reproduced in more first conference of this series held in recent calculations11) although the half 1966 at Lysekil may remember the paper by lives decrease by several orders of magni• H. Meldnerl'in which he showed that the tude with a maximum value of 103 yr for next proton shell closure beyond lead îîtx18". A quite different and more unfa• (Z=82) should occur not too far from the vourable shape of the island was obtained heaviest synthetical elements, at atomic in the calculations12' summarized in Fig.1b number 114 and not at 126 as previously be• showing a steep decrease of the half lives lieved. This is one of the key papers at the west side due to the considerably which caused a tremendous research activi• lower fission barriers obtained. In any dis• ty on superheavy elements. The second one cussion of extrapolated half lives one is the paper by W. D. Myers and W. J. should keep in mind their uncertainties of Swiatecki ) published in the same year many orders of magnitudes. where it was pointed out that the stabilising effects of proton and neutron shell closures should be strong enough in some regions beyond the explored periodic H 1 1 1 1 1—I 1 1 1 1 T table to produce fission barriers compar• able to or even greater than that of uranium. The first detailed theoretical in• vestigations3 ~5' of nuclear properties re• vealed the topology of an island of rela• tively stable nuclei due to shell closures at atomic number 114 and neutron number 184. First estimates indicated half lives comparable to or even larger than the age of the earth for nuclei in the centre of the island. Thus, superheavy elements 172 178 184 190 172 178 184 190 could even exist in nature, and many groups Neutron number felt encouraged to search for such elements in terrestrial and extraterrestrial samples. First, negative results were pub• Fig. 1 Calculated half lives of superheavy nuclei lished 1969 by S. G. Thompson et al.6' who shown as contours of constant overall half also reported the first attempt to syn• life plotted versus proton and neutron thesise superheavy elements by fusion of number- after refs. 10 (a) and 12 (b). 2l*8Cm with t*°Ar to form a compound nucleus of element 114. A review written in 1974 contains already 329 references7'. When superheavy elements are formed in nuclear reactions they carry excitation In the following status report, I energy and angular momentum. Both these properties decrease the effective fission sua 11 focus on attempts to synthesise super• 13 1 heavy elements in the laboratory but shall barrier. As theoretical calculations ' **' skip the search in nature since there will indicate, the barrier should disappear at be a contribution to this conference from about 50 MeV excitation energy and 30 units the Dubna group8' which made the strongest of angular momentum. efforts in this direction over many years. For a more detailed discussion of relevant In spontaneous fission of superheavy questions the reader is referred to review nuclei, the fission fragments should carry articles, e.g. ref.9'. an unusually high kinetic energy, 200-230 MeV, and evaporate an unusually large - 772 - number of neutrons, about ten15). In the second process illustrated by More recent experimental data on fission the lower branch in Fig. 2, the two inter• properties of heavy nuclei indicate16) that acting nuclei stick toqether for a very the kinetic energy may be even higher, short time, about 10~2Ís, forming a composite about 270 MeV, but the neutron multiplici• system and separate again. During the ty may be lower, about five. Hence, the short contact, the kinetic energy of the observation of high total kinetic energy projectile can be partially or completely and neutron multiplicity should constitute transformed into internal excitation and a characteristic fingerprint for super• rotation. Although the atomic and mass heavy nuclei. For short lived superheavy number of the reaction products are in nuclei, detection of energetic a-particles general close to those of the interacting may be a characteristic and sensitive nuclei, a substantial number of protons and 10 12 method * ) . neutrons can be transferred between the interacting nuclei. Hence, a characteristic Concerning the chemistry of super• feature of these damped collisions is the heavy elements I should restrict myself to broad distribution of excitation energy, the remark that their position in the angular momentum, atomic and mass number of periodic table has been predicted by quan• the outgoing products. The reaction tum-mechanical calculations of their ground products deexcite by particle evaporation state electronic structure17). Accordingly, and, in heavy systems, by fission. element 110 should be a homologue of pla• tinum, element 112 a homologue of mercury, and element 114 a homologue of lead. De• 3.1 Fusion reactions tails of their chemical behaviour have been 18 discussed ) which form the basis of chemi• Since complete fusion was to success• cal separation procedures that will be ful for the synthesis of the heaviest ele• mentioned below. ments, it was quite natural to use this type of reaction in the first attempts to produce superheavy elements in the labora• 3• Searches at accelerators tory. The problem lies in the extreme neutron excess of the nuclei located in the Heavy-ion reactions seem to be the centre of the island. This is illustrated only practical way of producing superheavy by Table 1 which contains the fusion reac• elements in the laboratory: one tries to tions tried together with the upper limit jump from the peninsula of known nuclei in for the production cross section and with one step over the sea of instability to the the range of half lives covered. As one can superheavy island. Two different ap• see from the compound nuclei listed in the proaches can be used as is outlined in Table, the vicinity of element 114 can only Fig. 2. The upper branch illustrates com• be reached in combinations with neutron plete fusion of the interacting nuclei. numbers far below 184 whereas this magic The compound nucleus is excited since, in neutron number can only be produced with general, more kinetic energy is required to atomic numbers as high as 122. Neutron overcome the Coulomb barrier than energy evaporation from the compound nuclei will is consumed in the fusion process. Part of further increase this dilemma. A more de• the excitation* energy is carried away by tailed discussion of these reactions can be particle evaporation, mostly of neutrons. found in a recent review article20). For very heavy compound nuclei, fission into two fragments of comparable size do• Among the systems listed in Table 1 minates, however. the reaction 2 k 8 nrn 8 0 152 . <*8na v 2 9 6 v* + 2 0 Ua 9&Cm has been studied most extensively since it Evaporation provides the closest approach to the island Otesidue if both the proton and neutron numbers are considered. Let me illustrate this point by Fig. 3a which is identical with Fig. la but shows in addition the landing place after the k 8Ca-on-2 **8 Cm reaction. We overshoot the centre of the island and lose, with the If8Ca projectile energies applied in the ex• periments, four neutrons. Then, a short lived ot-emitter is produced followed by two Strongly Dumped electron capture transitions which lead to• 9 Collision wards the region of relatively long half 176 lives. The final nucleus, Hlx should decay by fission with a half life of about 1 h. This sequence is shown in more detail Fig. 2 Schematic picure of the interactions in Fig. 4. Experiments with this reaction are difficult, however, since neither the between heavy nuclei leading to fusion to 2U a compound nucleus (upper branch) and to very neutron rich actinide isotope Cm with 3.6x10^ yr half life nor the very a damped collision via a composite system 1 8 (lower branch) with characteristic times neutron rich projectile * Ca with 0.19% in units of 10~21 s (after ref. 19). natural abundance are generally available. - 773 - TABLE 1 Attempts to synthesise superheavy elements by complete fusion reactions Upper limit for the production Half life Compound cross -section range System nucleus (cm2) covered Ref 232Th 280llo170 + 3 X lo"" 3 90 20 >_ 3 x 10" s 21 231p-, 279111168 35 3 + 20ua 4 X 1er >_ 3 x 10" s 21 281112169 10-35 + 7 X > 2 h 21 288 17l+ ,oAr 32 2 + 114 3 9> 2 X io- lQ~ s -Id 6,:2 2 2 -Pu + 290lli+176 1 X 10-35 20 2 h - 1 yr 23 + 291115176 2 X IO-35 20 2 h - 1 yr 23 48 294116178 35 2 + Ca 2 X IO- 2 h - 1 yr 9> 20 23 248 Cm + 296116180 See 96 20 Fig.
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