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Home , Superheavy element

ATTEMPTS TO SYNTHESISE SUPERHEAVY ELEMENTS - A STATUS REPORT

G. Herrmann Institut für Kernchemie, Universität Mainz, D-6500 Mainz, and Gesellschaft für Schwerionenforschung, D-6100 , 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 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 shell closure beyond 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 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 .

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 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 . 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 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 , 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 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 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 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 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 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. 5

8i+ 208pb 292118174 10-30 D + Kr 1 X 7 82 36 >_ 6 x 10" s 24

86 208pb 294118176 10-35 + Kr 6 X 3 x 3 d 82 36 10" s- 100 25

1 .5 x 10"30 2 x 10"9s - 5 h 26

238u 59 -33 + Co 297lig178 4 X lu 92 27 1 s - 30 h 27 238rj 178 + . Ni 298120 2 X 10-33 1 - 30 h 92 28 s 27

238TJ 30312il82 U -33 92 + 65 Cu 8 X 10 1 s - 30 h 27

2 X 10-33 2 h - 1 yr 28

238 65 303121182 1Q-34 6 u + Cu 4 X 2 x 10" s - 10 h 29 92U 29

232Th 76 3l+ + Ge 308122186 1 X io" _3 90 32 5 x 10 s - 1 yr 30

136 17 30612218i+ -33 Xe + 0Er 1. .5 x IG 2 x 6 54 68 10" s - 10 h 29

314124190 34 _3 + 1 X io- 5 x 10 s - 1 yr 30

311125186 9 + 5 X 10-32 10" s - 5 d 31

232Th 316126190 ia + 10-30 7 90 l> 5 X _> 6 x 10" s 24

238rj 322128194 29 7 + 8 X 10" x " s 26 92 t> >_ 6 10 11**

All the experiments with the 48Ca-on- recoiling out of the target were collected 248Cm reaction reported so far remained (foils)33). Further, the recoil were negative. The results are summarized in stopped in a gas and transported with a gas Fig. 5 giving the upper limits for production jet to a rotating-wheel counting system cross sections plotted against the assumed (VW)35). Finally, spontaneous fission decay half life of the superheavy nuclei. A in flight was searched for (dif)35). As variety of techniques has been applied in Fig. 5 demonstrates, radiochemical techniques these experiments. The Dubna (D in are most sensitive among the methods applied Fig. 5) used two different chemical sepa• so far but are limited to half lives ex• ration procedures and insepcted the ceeding several seconds. There is a strong samples by spontaneous fission (SF) and demand for techniques giving access to a-particle counting (a)23'. The Berkeley- shorter half lives with sensitivities com• Livermore collaboration applied two different parable to those of the radiochemical chemical procedures (chem)32/33) and per• approaches. formed an experiment to detect extremely volatile elements during and after bombard• Why have these experiments failed ? ment3 4). Attempts were also made to detect Either the fusion cross section must be short lived species by counting catcher extremeley low or a dramatic loss by fission foils in which the reaction products competition must occur during the neutron

- 774 - Fig. 5 Upper limits of the production cross sections for superheavy elements in the fusion reaction of 48Ca with 248Cm plotted versus the half life of the superheavy nuclei. The data are from experiments of the Dubna group (D)23) and of the Berkeley-Livermore collaboration (BL) 32-31 for details see text.

chain20) as is indicated at the top of Fig. 4: One ends in the evaporation chain with an excitation energy of 8 MeV being too low for evaporation of an additional neutron but exceeding the of 2.9 MeV calculated with the parameters leading to the steep west shore in Fig. 1b. The super• heavy nuclei will completely vanish by 173 174 175 176 177 178 179 180 fission. NEUTRON NUMBER The higher bombarding energies were chosen since theoretical treatments of heavy- Fig. 4 Neutron evaporation and ion fusion reactions predicted that an extra chain after complete fusion of lt8Ca with push of kinetic energy above the barrier is 2lf8Cm using the half lives of ref. 10. At required to fuse very heavy nuclei. For the the top excitation energies and two sets system 48Ca + 248Cm, it was estimated that of fission barriers10'1 ) for the inter• the bombarding energy was still too low by mediate nuclei formed in the neutron 10 to 30 MeV to provide for this extra evaporation process are given. push38'. Here we touch one of the conceptual problems39)associated with the question how to make superheavy elements. The problem evaporation chain. It is well known that of an extra push of kinetic energy has very fissile isotopes of the elements 105 recently been studied 4 0' with a two-dimen• to 107 can be produced by (HI.2n) reactions36) sional model allowing for dynamical defor• or even by (Hl^n) reactions37'. The (HI,2n) mations of projectile and target nuclei in reaction cross sections show an empirical the entrance channel. Recent experimental systematics if plotted versus the Coulomb data41,42) indicate that an extra push is energy between projectile and target indeed required when the entrance channel

36 48 2 nucleus ^. The Coulomb energy between Ca model parameter exceeds (Z /A)eff - 33. and 248Cm is lower than that for similar For the 48Ca + 248Cm system one obtains a systems successfully applied for the syn• value of 33.9 for this parameter corres• thesis of isotopes of the elements 105 to ponding to an extra push of a few MeV. On 107; from the empirical systematics one the other hand, it has been observed43) reads a cross section of 10~33 cm2 for the that fusion can occur with sizeable cross (48Ca,2n) reaction on 248Cm. However, the sections at projectile energies below the experiments were carried out at higher classical interaction barrier. This means energies corresponding to a (48Ca,4n) that fusion through barrier penetration can channel. In this case, a fission catastrophe occur with compound nucleus excitation could occur during the neutron evaporation energies considerably lower than those

- 775 - obtained at or above the barrier. Of course, fission as is evident from the broad fission- this can only occur at the cost of up to product distribution between atomic numbers several orders of magnitude in the total 30 and 70 and the steep decrease of the fusion cross section. The problem of optimi• cross sections for elements beyond uranium sation then, is to find a bombarding which could be followed45' over eight orders energy where the loss in the fusion cross of magnitude up to (Z=100). Before section is overcompensated by the largely fission the element distribution in the increased survivability of the compound damped collision should be symmetric around nucleus resulting from an evaporation chain uranium. This primary distribution can easily considerably shorter than the one indicated be reconstructed45). The resulting in Fig. 3. distribution is reproduced by calculations in which the nucleón transfer in damped collisions is treated as a diffusion 3.2 Damped collisions process46^, as can be seen in Fig. 6. Extra• polation to the light complementary fragment An alternative pathway to the super• with Z=70 to a cross section of about heavy elements was opened by the first 10~28 cm2 for a split into elements 70 plus studies44'45) of the interaction of two 114 in the decay of the composite system. colliding uranium nuclei at the Unilac accelerator. These studies demonstrated that However, only a very small fraction of the superheavy island can be reached by the superheavy fragments would survive nucleón transfer during the short contact fission. Calculations with the diffusion time of the composite system. If the doubly model predict at 8.3 MeV/u bombarding ener• magic nucleus ^ ®X18 4 would be formed in a gy production cross sections of about binary reaction the complementary fragment 10 cm2 for element 114 nuclei with less would be an ytterbium isotope: than 30 MeV excitation energy46', an energy range for which the survival probability 2 3 8TT1 4 6 2 3 8TT1 4 6 against fission should be larger than 50% for the first neutron-evaporation step pro• Fig. 6 shows the element distribution in vided the neutron number is close to 184. the reaction between two U nuclei as Neutron numbers close to this magic con• obtained in radiochemical studies45'. The figurations are estimated for element 114 element yields peak at uranium in a narrow fragments originating from uranlum-on-urariium distribution due to quasi-elastic transfer collisions if one applies the rules ob• of a few nucléons. The underlying, broader served47' for the distribution of neutrons distribution results from nucleón transfer and protons between fragment pairs in very in damped collisions. Reaction products from heavy systems. This leads to a much closer quasi-elastic and damped collisions undergo access to the centre of the island than in fusion reactions, as is demonstrated by Fig. 3. Taking all estimates together, one may expect a cross section of about 10~35 cm2 for element 114. This corresponds, with the accessible beam intensities, to production rates of a few atoms per week,

Sequential fission a rate just exceeding the detection limit 1040 mb of the most sensitive methods available at present.

The results of direct searches for superheavy elements in the uranium-on- uranium reaction are depicted in Fig. 7. As in Fig. 5, upper limits for the production cross sections are plotted versus the assumed half life. In the experiments of a Darmstadt-Mainz collaboration, two different chemical separation procedures were applied (chem), and recoil atoms were transported with a wheel system during bombardment to plastic foil fission track detectors (wheel)48). In the latter experiments a background due to known spontaneously Fig 6 Element formation cross sections (dots) in fissioning actinide nuclei was observed the reaction of 238U with 238U as obtained which sets the limits for the detection 49 by a radiochemical analysis of thick targets sensitivity. A Marbura-Gießen collaboration ) 27 bombarded with 7 .5 MeV/u uranium ions'*8 ). and a München group ' applied a gas jet The reconstructed pre-fission distribution for rapid transport of the recoil atoms for elements around uranium is shown by to the detector system (jet). In the experi• triangles. The dashed curve shows the ments of^ a Darmstadt-Heidelberg collabo• 44 distribution calculated with the diffusion ration ) the recoil atoms were implanted in• model for damped collisions'* 6 ) and its to a surface barrier detector (Ree). All these extrapolation to a split into elements experiments remained negative. As in the 48 248 70 + 114 in the decay of the composite case of the Ca + Cm reaction, one system. notes a lack of sensitive measurements for short lived nuclei which would require, as the wheel experiments show, a separation from actinide nuclei also produced in the damped collision process.

- 776 - 238,, 248r U+ Cm

10"" -

10""-

] HT351 i i il i i i 11 i i i 11 i i 111 i i i il i i i il 110~ 10T1 10° 101 102 HALF-LIFE [dl

Fig. 8 Upper limits for the production cross section of superheavy elements in the

238y + 2«f8c reaction plotted versus 3 1 1 3 5 7 9 10~ 10" 10 10 10 10 10 half life52}. HALF-UFE [s]

Fig. 7 Upper limits for the production cross of 208Pb with 238U carried out by a Marburg- section of superheavy elements in colli• Gießen-Darmstadt collaboration55' using a sions of two uranium nuclei plotted rotating wheel system (wheel), the gas jet versus half life. Data from refs. 27, 44, method (gas jet), and chemical procedures 48, 49; see text. (chemistry) with results shown in Fig. 9.

These searches have recently been ex• tended into two directions: According to theoretical estimates with the diffusion model, the cross section should increase by 23 208 two orders of magnitude if 248Cm instead of V Pb- 238U is bombarded with 238U ions46). Experi• ments with the 238U + 248Cm reaction were begun by a Berkeley-Darmstadt-Livermore- Mainz-Oak Ridge collaboration. Preliminary data of these experiments50-52) which again showed no evidence for superheavy elements are displayed in Fig. 8. Three different chemical procedures were used: collection of gases volatile at room tempe• rature (noble gases), evaporation of vol- - atile species at higher temperatures" (gas phase chemistry),. and aqueous solution chemistry. The experiments were hampered by the limited stability of the targets in the uranium beam51). There• fore, the sensitivity that could be reached —I 1 1 1 1 1 1 L_ with present time beam intensities has 10 10* 106 10B 1010 not yet been achieved. Further experiments Half-life [si will be carried out soon.

In the other series of experiments a theoretical suggestion was followed that Fig. 9 Upper limits for the production cross when fissile nuclei are chosen as target or section, of superheavy elements in the projectile they may undergo fission during 2 0 8pb + 2 3 8u reaction plotted versus close contact with a reaction partner, and half life55). one of the fission fragments may fuse with the projectile to form a superheavy nucleus ). This suggestion has been shown by a München group54' to produce negative results in the reaction of 208Pb with 136Xe where an upper cross-section limit of 1.2x1O~30cm^ was obtained for half lives between 10~12 and 104 s, and in the reaction

- 777 - One may argue that these experiments 7) Herrmann, G., Int. Rev. of Science, have failed because in damped collisions of Inorg. Chemistry, Ser. 2, Vol. 8, 238U with 248Cm at dissipated energies of Radiochemistry (ed. Maddock, A.G.) 150 MeV it was suggested that the heaviest p. 221 (Butterworth, London, 1975). fragments might be formed with elongations 8) Flerov, G. N., these Proceedings. outside their fission saddle point5 ). How• ever, as is discussed in more detail in a 9) Herrmann, G., Nature 280, 543 (1979). separate contribution to this conference57' 10) Fiset, E. 0.,and Nix, J. R., Nucl. the production cross sections for Phys. A193, 647 (1972) . up to z=101 in the 238U + 238U and 8U + 248Cm reactions are consistent with 11) Sobiczewski, A., in Proc. Int. Symp. the decay by multiple chance fission of on Superheavy Elements (ed. Lodhi,~M. fully equilibrated fragments with minimum A. K.), p. 274 (Pergamon, New York excitation energies of 30 to 40 MeV 1978). corresponding to the evaporation of 3 to 4 12) Randrup, J., Larsson, S. E., Möller, neutrons from the primary fragments. This P., Sobiczewski, A.,and #ukasiak. A., excitation energy is in agreement with an Phys. Ser. 10A, 60 (1974), and estimate from a one-body dissipation plus Sberg, S., private communication (1979). particle-hole model58). The same model predicts minimum excitation energies of 13) Moretto, L. G., Nucl. Phys. A180, 337 25 MeV for Z=114 in the 238U + 248Cm re• (1972) . action if fragment deformations are absent39'. 14) Mustafa, M. G., in Proc. Int. Symp. on This should lead to somewhat shorter neutron Superheavy Elements (ed. Lodhi, M. evaporation chains than indicated in Fig. 3 A. K.), p. 284 (Pergamon, New York with final nuclei having half lives long 1978). enough for detection even if the pessimistic picture of Fig. 3b holds. 15) Schmitt, H. W.,and Mosel, U., Nucl. Phys. A186, 1 (1972). 16) Hoffman, D. C., in Proc. Int. Symp. on 4. Outlook Superheavy Elements (ed. Lodhi, M. A. K.), p. 89 (Pergamon, New York Faced with so many unsuccessful attempts 1978). to make superheavy elements in the laboratory one may feel pessimistic about the possibi• 17) Fricke, B., Structure and Bonding 21, lities to reach the island or may doubt 89 (1975). whether it exists at all. This seems to be 18) Keller, 0. L.,and Seaborg, G. T., Ann. premature, however: We have not yet fully Rev. Nucl. Sei. 27, 139 (1977). explored the potential of heavy-ion re• actions; complete fusion at or below the 19) Brix, P., Max-Planck-Inst. für Kern• barrier or damped collisions between heavy physik Heidelberg Report MPI H - 1977 target nuclei and projectiles distinctly - V41 . lighter than the very heavy ones applied so 20) Seaborg, G. T., Loveland, W.,and far may be considered promising alternative Morrissey, D. J., Science 203, 711 approaches. There is a need for sensitive (1979). techniques covering the region of short half lives, and there may be alternative 21) Ter-Akopyan, G. M., Bruchertseifer, H., chemical procedures in experiments aimed Buklanov, G. V., Orlova, O. A., Pleve, at reaching the outmost detection sensi• A. A., Chepigin, V. I.,and Choi Val tivity. Whatever the final outcome, we will Sek, Yad. Fiz. 29, 608 (1979) learn more about the forces that terminate [Sov. J. Nucl. Phys. 2j9, 312 (1979)]. the periodic table at its upper end. 22) Thompson, S. G., Swiatecki, W. J., Gatti, R. C., Bowman, H. R., Moretto, L. G., Jared, R. C.,and Latimer, R. M., Univ. of California Radiât. Lab. References Berkeley Report UCRL 18667 (1969) ,p. 283. 1) Meldner, H. , Arkiv Fysik 36.' 593 23) Oganessian, Yu. Ts., Bruchertseifer, (1967) . H., Buklanov, G. V., Chepigin, V. I., 2) Myers, W.D.,and Swiatecki, W. J., Choi Val Sek, Eichler, B., Gavrilov, Nucl. Phys. 82, 1 (1966). K. A., Gäggeler, H., Korotkin, Yu. S., Orlova, O. A., Reetz, T., Seidel, W., 3) Nilsson, S. G., Sobiczewski, A., Ter-Akopyan, G. M., Tretyakova, S. P., Szymanski, Z., Wycech, S., and Zvara, I., Nucl. Phys. A294, 213 Gustafson, C., and Möller, P., (1978). Nucl. Phys. A115, 545 (1968). 24) Colombani, P., Gatty, B., Jacmart, J. 4) Nilsson, S. G., Tsang, C. F., C., Lefort, M., Peter, J., Riou, M., Sobiczewski, A., Szymanski, Z., Stephan, C.,and Tarrago, X., Phys. Wycech, S., Gustafson, C., Lamm, I.L., Lett. 42B,208 (1972) . Möller, P.,and Nilsson, B., Nucl. Phys. A131, 1 (1969). 25) Wirth, G., Ahrens, H., Bögl, W., Franz, G., Kratz, J. V., Marx, D., 5) Mosel, U.,and Greiner, W., Nickel, F., Warnecke, I.,and Weber, W., Z. Physik 222, 261 (1969). Ges. für Schwerionforsch. Darmstadt 6) Nilsson, S. G., Thompson, S. G.,and Ann. Rep. 1976, p. 72, and unpublished Tsang, C. F., Phys. Lett. 28B, 458 results. (1969) .

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DISCUSSION where you have a small excitation energy. So these 4n de-excitation channels what we always thought we What are the chances that any spec• G. Goldhaber: would have in the last five or six years, then I troscopic information on any of the new heavy iso• think the chances are much smaller to survive. But topes may be obtained in the foreseeable future? now having these chances that there may be the In G. Herrmann: What one can measure are the alpha and 2n channels gives us our optimism we have gained energies and fission half life. Even the question in the last year. of fine structure in the alpha decays is very diffi• You would like to observe a 4n re• cult to investigate at the level of a few atoms per J. Jastrzebski: action from a compound nucleus. Do you have some week production rate. I think that one should be experimental evidence that the compound nucleus it• quite pessimistic. The limit at the moment is not self was formed in the reaction you were studying? the beam intensity but is the target problem. We can produce more beam than we can apply to the tar• G. Herrmann: The heaviest systems in which this has get. The UNILAC is a very efficient way to bore been demonstrated were presented by holes into everything. Gottfried Münzenberg and by the Ar on Pb in the Dubna data. The evaporation residue would be a If we could measure the properties of D. Hoffmann: superheavy element. If we would find it, it would the spontaneous fission that is taking place, that be OK. would certainly be most interesting and most useful. Some years ago we talked more about When you discussed calcium 48 on D. Hoffmann: 0. Schult: making odd Z superheavy elements and now most of curium 248 right at the reaction, you are with the the discussions centre on even Z. Do you have a energy way above the fission barrier. Now I wonder comment on that? how well one knows how fast fission occurs on com• petition with neutron emission. The question is do G. Herrmann: I think this has to do with the target you get the 4n channel at all? and projectiles that are available. lf8Ca is an ex• tremely neutron-rich nucleus, 248Cm is the heaviest G. Herrmann: That is the main problem. It has not not too radioactive nucleus you can get in milligram been observed and according to the cross-section amounts. For the deep inelastic reactions, this systematics, it should have been found. One answer does not play a role because you produce a broad is that fission barriers are much lower than ex• distribution in atomic number. So this argument may pected. Unfortunately, the barriers go twice into still hold for the deep inelastic that chances are the whole business, in the reaction and in the half better for an odd Z element. life. The results and the chances to make the super- heavies are very sensitive to the fission barrier D. Hoffmann: And the possibility of using an odd Z heights. target such as has also been proposed. Again there are grave target problems. P. Armbruster: May I comment on this once more. These results on the heavy elements 100 to 107 gave G. Herrmann: The problem is then of course that what us some optimism that you can produce these heavy one earns in cross-sections one may lose in the elements in the super heavy region by reactions target thickness and the number of atoms in the target.

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