Problems of the synthesis of heavy nuclei

Nuclear Physics A488 (1988) 65c-82~ North-Holland, Amsterdam

PROBLEMS OP THE SYNTHESIS OF HEAVY NUCLEI Yu.Ts.Oganessian Joint Institute for Nuclear Research,

141980 Dubna, U.S.S.R.

The present review talk does not pretend to the detailed analysis of the many important aspects of the major problem of producing and investigating new elements from the end of the Periodic System. Based on the main results obtained at various laboratories, an attempt is being made to give a general review of the present state of the art, successes and hardships of recent years. The problem of superheavy elements the practicability of whose production is a debatable issue is discussed briefly. 1.

INTRODUCTION It is known that uranium is the heaviest, naturally occurr-

ing element. The lifetime of transuranium

elements decreases

sharply with growing atomic number, as a result of the considerably enhanced probability

of their radioactive

decay. In view of

the fact that they can be synthesized in complex nuclear processes, the problem of producing and studying man-made elements has grown into an extensive field of research. Where does the Periodic System end? It is not easy to give a definite answer to this question since the qualitative

calculation

of the electron structure of

a heavy atom under the conditions of superstrong electric fields is a complicated task. It is, however, clear that the existence of heavy elements is limited by nuclear stability determined by the probability

of different modes of radioactive

decay.

Prom this point of view the sharp decrease in the stability of transuranium

elements which undergo either alpha or beta

decay is not an adversity since the nuclear mass calculations lead to considerable alpha- and beta- half-lives region of very heavy elements

even in the

(Z . 120)'.

The problem of spontaneous fission is a more complicated one. The probability magnitude

of this process increases by 27 orders of in going from 23Bu to 258 Fm. An extrapolation of the

Tsf(x> dependence to the region of the heavier elements on the basis of the liquid drop model calculations 0375-9474/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

of fission barriers

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leads to the conclusion that the nuclei even with 2 ;, 104 2 should be almost unstable (Bf 0) . Perhaps one could stop at this point unless two experimental findings of the early 1960's had come as a great surprise. The spontaneous fission half-life of the doubly-even nucleus 260104 synthesized in the 242Pu + 22Ne reaction turned out to be considerably longer than it had been expected3. In this series 242mfAm of experiments the spontaneously fissioning isomer = 14 ms> was observed. Subsequently it was shown that (T1/2 similar isomeric states exist in at least 30 more isotopes of the actinides4. An explanation of this phenomenon was found in the microscopic

description taking into account the structure

of the nucleus subject to a considerable deformation on its way to separating to two fragments. According to calculations using the V.M.Strutinsky

method5 the height and shape of the fission

barrier depend not only on the relationship

between the Coulomb

and surface energies of the drop of an uniformly charged liquid, but also on the number of protons and neutrons in the fissioning nucleus. This fact plays a crucial role. The development of deformation in a charged drop can, to a considerable extent, be retarded by the inner structure of the nucleus and, as a result, lead

to the occurrence of a barrier preventing fission even

in the heaviest nuclei. So, the fission barrier and, consequently, nuclear stability are determined by the amplitude of the shell correction which reaches its maximum near the closed nucleon shells (see, for example, the doubly-magic nucleus 208Pb>. Therefore, the spontaneous fission half-lives (Z>104)maybe expectednot

tovary

of heavy nuclei

as much as in the region of the

actinides. Below it will be shown that this theoretical prediction has been perfectly confirmed in experiments. Near the following closed shells (Z=llO-114, N-.180) the stability of nuclei against spontaneous fission may be enhanced considerably, thus making the region of the possible existence of chemical elements more extensive 698 . Now the problem arises in what way these exotic nuclei can be synthesized and investigated under laboratory conditions.

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REACTIONS mADING TO ELFMENTS SYNTHESIS Eight transuranium elements were produced in USA by neutron and light charged-particle induced reactions9. Moving further along the line of long-lived nuclides one succeeded in synthesizing nuclei with Z through 100 in high-flux reactors. The isotope 257Fm (T,,2 = 100 days) is the last one produced by that technique since the 258 Fm nuclei undergo spontaneous fission with T,,2h 3 x 10 -4 s (ref. “1. This limit could not be overcome in a high-density neutron flash in nuclear explosions11 (fig. I>. 2.

I

P--delayed

flsslon

explosions

90 I

150

Neutron 160

/

-I

number I

170

FIGURE 1 The Chart of Transuranium Nuclides produced by different types of reactions (indicated in the figure). The curves show the calculated amplitudes of the shell correction (in MeV) to the liquid-drop deformation energy12

J

180

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Light ion induced fusion A move into the region of transfermiwn elements became possible in complete-fusion reactions induced by heavy ions although it was associated with considerable difficulties. The production in an accelerator of multiply charged ion beams with intensities comparable to the neutron density in a high-flux reactor presents a serious problem. Moreover, the high ionization losses of heavy projectiles decrease sharply the amount of the target material used.(down to 10m3 g/cm*>. However, more significant limitations are imposed by the high excitation energy of the compound systems (Ex 40-50 MeV), to be removed by the emission of 4 or 5 neutrons. In the process of neutron emission, as a result of the strong competition between fission and.neutron evaporation, only a small fraction (IO-8 2.1.

IO"') of the nascent nuclei de-excite. At the same time, five transfermium elements with Z through 106 were successfully produced by bombarding targets made of the isotopes of actinides ranging from Pu to Cf by ions from carbon to neon. Note that the production cross section of element 106 nuclei in the 24gCf(180,4n)263106 reaction is equal to about 2~10~~~ cm2 (ref.13>. It is difficult to explain the reasons why that method. has not been employed in the synthesis of the heavier elements. Possibly, that could.be due to the technical difficulties in using highly radioactive targets, to a high level of the background from the decay of the products of competing reaction channels, to a relatively low yield.of compound nucleus evaporation residues under the conditions of a limited ion beam intensity, and so on. However, at that time a possibility arose of increasing the yield of evaporation residues with Z>,IO~ by lowering the excitation energy of the initial nucleus. fusion 2.2. v0laff It is possible to reduce the excitation energy of the compound nucleus by substantially increasing the Q value of the completefusion reaction. This, in principle, can be achieved only for more symmetric reaction partners. These processes resemble inverse fission reactions. The most advantageous case is when the interacting nuclei lie near the closed nucleon shells.

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instead of actinide isotopes, one can use the magic Pb or 2ogBi as target material and ions heavier than nuclei argon as projectiles'4. After it has been demonstrated that reactions of Pb and Bi nuclei with Ar, Ti and Cr ions can result in the production of isotopes with 2=100-106 (ref.15), this kind of reactions found a wide application in synthesizing heavier elements up to 2 110 (refs. 16,17). In "coldt'fusion reactions the composite system has a low excitation energy so that its de-excitation may be accompanied by the emission of as few as one or two neutrons. This apparent advantage exists until the limitations to the process of fusion of heavy nuclei begin to manifest themselves. Unfortunately these limitations appear already in systems with 2~104 (fig. 2). Therefore,

208

z,xz2 1.6

II

I

I

18 I

2.0

1

!

2.2

I

,

/

2,4,x103 ,,

+ - %Cri

.

‘*Fe

. 5s

P -

I !

100

I,,

,

105 Atomic

,

,

,

,

,

,

110 number

FIGURE 2 (a) The cross section for the formation of 256Fm in 248Cm induced reactions, as a function of the projectile. (b) The dependence of production cross sections for the isotopes of transuranium elements in the reactions 238U + 238U (open circles) and 254Es + 22Ne (closed circles). The curves are drawn through experimental points to guide the eye

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The experimental results obtained at Darmstadt and at Dubna suggest that with the increasing atomic number of the projectile (and consequently, of the composite system) the cross section for forming "cold" fusion products decreases substantially. The heaviest nuclide was produced in the 2ogBi(58Fe,n)266109 reaction18. The attempts to synthesize element 110 in 59Co and 64Ni bombardments have set only the upper limits of cross sections at a level of I-4 pb 'gP20. 2.3. Incomplete fusion Incomplete fusion of the interacting nuclei offers another possibility of decreasing the energy of the initial nucleus. It is known that a neutron (or two neutrons) populate effectively the low-lying states of the final nucleus in the pick-up reactions of the type (d,p) or (t,p). In these reactions heavy fermium isotopes with masses 258 and 259 (ref. *') have been synthesized. However, for producing transfermium elements a considerable increase in the A and Z of the projectile is required. This in turn leads to considerable changes in the character of the interaction. It has been shown experimentally that the interacting nuclei can transfer many nucleons even near the Coulomb barrier22. The probability of nucleon transfer is of a stochastic nature and depends on the energy available for the inelastic channel of the reaction. Therefore, as the number of the nucleons transferred from the projectile to the target nucleus increases, the excitation energy of the heavy product also grows. As a result, the survival probability for heavy nuclei turns out to be extremely low. This seems to account for the fact that, irrespective of the projectile mass, the final heavy nuclei have atomic numbers not far from the target one23*25 (fig. 3). In this situation it is necessary to use the heaviest available nuclei as target material. In2ih;; direction experiments were carried out by K.Hulet et al who succeeded in synthesizing new heavy isotopes with Z=lOl, 102 and 103 by bombarding 254Es nuclei by I80 and 22Ne ions. The properties of these nuclides turned out to be rather interesting (bimodal spontaneous fission, the high stability of doubly-odd nuclei with Z=159, and so forth).

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11-t 0

0

loo

Projectile

FIGURE

200 mass

Cross sections evaporation 208pb, 209

3

for the formation of

residues

in the reactions

Bi (HI, n), as functions

of the atomic number of the composite system.

loop+, lo-=

: u.

11

250 Mass

+

+T ’ ;so’ ’ ( number

However, the difficulties in accumulating the target material 254Es (T,,2 = 276 days) and the drastic cross section decrease in the Z--103 region are the main obstacles to the synthesis of new elements. In the Table Itofollow the conclusions drawn from the above analysis of the possibilities of synthesizing elements by different types of nuclear reactions are summarized. 3.

RADIOACTIVE DECAY PROPERTIES OF TRANSACTINIDE NUCLEI As is seen from Table I, a move from the neutron capture to heavy ion-induced reactions has made it possible to synthesize 9 transfermium elements by increasing 2 and almost without changes in the neutron number (N 157). In the liquid drop model this fact leads to a substantial increase in the fissility parameter and, consequently, to a sharp decrease in nuclear lifetime with respect to spontaneous fission. At the same time, fig. 4 shows that the experimental values of the spontaneous fission half-lives of the doubly-even isotopes of transfermium elements considerably differ from those

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TABLE I Summary Reaction

Projectile mass

Sequential neutron capture

Limitation

Heaviest nuclide z N

n0

break of chain

100

157

High-flux neutron pulse

nf

spontaneous fission of Fm isotopes NY>158

100

157

Light ion induced fusion Z,Z2.-800

,%22

low yield of evaporation res.Ex 40 MeV

106

157

Incomplete fusion

c22

lack of target heavier than

103

159

109

157

254Es Cold fusion 1500:.z,z2~2200

50

$,

dynamical hindrance of fusion Ex 20 MeV

o,~o ,

,”, , ,1 0.85

Fissility

0%

Ix)

FIGURE 4 Spontaneous fission half-lives of doubly-even nuclei, as functions of the fissility parameter. The liquid-drop model calculation is shown by a dash-dotted curve. The full curves connect experimental points

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predicted by macroscopic theory. The spontaneous fission probability for the isotopes of the heavy actinides depends on neutron number; around N=152 the spontaneous fission half-lives vary by a factor of IO". No effect of that kind has been observed for transactinide elements (Z X-.104)the spontaneous-fission stability of which practically remains unchanged as the atomic number increases. This fact is direct evidence for the presence of the fission barrier due to the shell structure even in the heaviest nuclei synthesized. As a resul:80known isotopes with Z 104, including the doubly-even nuclei 106 and 264 108, undergo mainly decay 29,3O)* (refs. The -;-spectraof a number of isotopes of transactinide elements were measured on a few atoms by P.Armbruster, G.Mcnzenberg et al. 17331. Because of very poor statistics it is difficult to draw any conclusion about the structure of these nuclei. However, these measurements carry important information about the ground-state masses of heavy nuclei. This makes it possible to compare empirical data with the results of calculating nuclear masses and thus to do the llbestl' extrapolation to the region of still heavier elements (fig. 5).

12t

6’

’ ’ MO

’

’

’

’ ’ ’ ’ ’ I 105 110 Atomic number

FIGUFLE5 The experimental values of Q'c.for the isotopes of transfermium elements with N=157. The curves are the values calculated by Liran and Zeldes (LZ)32 and by Kolesnikov and Demin (KD>33 and by Mbller, Leander and Nix (MLN)34

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All the applies to region of neutron-deficient produced mainly tlcold"fusion This method an advance the N>157 for elements Z//110 alone, the heavy 64Ni, 7oZn and 76Ge are used as projectiles. Unfortunately, this way is blocked because of a strong decrease in the reaction cross section for Z,Z2:r1800. 4.

LIMITATIONS AND POSSIBILITIES OF FUSION REACTIONS 4.1. Theory In recent years several theoretical approaches of macroscopic type have been elaborated to investigate the dynamics of the fusion process. .35 and subsequentThe calculations carried out by W.Swiatecki ly by other authors36-38 show that as the charges (and, to a lesser extent, masses) of the partner nuclei reach certain values, their further increase leads to a considerable erihancement of the fusion barrier and, consequently, the excitation energy of the composite system. This can serve as a qualitative explanation of the strong decrease in cross sections for forming "cold" fusion reaction products. In this situation it is natural to try more asymmetric targetprojectile combinations in which the3;oulomb interaction energy decreases noticeably (see G.N.Plerov >. In terms of the synthesis of, say, element 110 this would necessitate the use, instead of the 208Bb + 64Ni _r272110 --2300) reaction, of more asymmetric systems such as :@+ 4oAr 2, 275 110 (ZlZ2---1700) and 249Cf + 26Mg --)2751,O (z1z2-1100). It is however difficult to foretell how effective the proposed systems will prove to be. A slight variation in the model parameters changes the situation to a great extent (fig. 6). For the U + Ar.\ 110 reaction, the calculations of W.Swiatecki et al 34, and of R.Davies et al 36 suggest that the fusion reaction threshold should not increase whereas the calculated results of J.P.Blocki et al 38 completely eliminate the formation of evaporation residues with Z=llO (Ex > 100 MeV). At the same time, the reaction threshold decreases significantly in the approach used by P.Fr?ibrich JO.

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FIGURE 6 The increase of the dynamical threshold of the fusion reaction (extra push energy) as a function of the parameters determined by the atomic numbers and masses of the interacting nuclei and projectile. The dots are the calculated values for the reaction 238U + 40Ar. The open circle is the result the calculation by Frzbrich using the surface-friction mode13' Clearly, neither of the calculations is qualitatively perfect so far. On the other hand, the use of targets of uranium isotopes instead of 208Pb leads to a decrease in the & value and hence to an about 20 MeV increase in the excitation energy of the composite system. In this case fission begins to play a more essential role in the process of composite system de-excitation. It should also be noted that the shell structure effects diminish with growing excitation energy and, as a result, heavy composite systems become "barrierless". The mechanism of the formation and decay of such systems has not been clarified as yet. It can be assumed that in this case the probability of the formation of final products should depend on dynamical factors to a considerable extent.

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In the process of collective motion the composite system can de-excite by emitting neutrons and _c-rays4'.This fact should result in an enhanced fusion probability and, consequently, in an increase in the cross section for producing the evaporation residues. It is, however, difficult to judge the roles of these effects; only straightforward experiments can give a quantitative answer we made an attempt to to the question. Therefore, in 1986-87 synthesize the element 110 nuclei in reactions induced by Ar and Ca ions. 4.2. Experiment The experiment was designed to record new nuclei with the maximum possible detection efficiency which was achieved by the kinematic selection of evaporation residues and by recording their decay (or the daughter products of their cc-decay) by spontaneous fission detection. That is why the reactions 232Th(44Ca,xn)276'xl?0and 236U(40Ar,xn)276-X110were chosen. In a long bombardment (with an integrated ion flux of about IO") rare spontaneous fission events with half-lives of several milliseconds have been detectedb2. Despite a very small production cross section - about 5-10 picobarns - and poor statistics, it was possible to determine the kinematical conditions required for the maximum yield of a given activity to be obtained. These results, as well as the data of control experiments, were close to those expected for the evaporation residues of a composite system with Z=llO. Of course, that experiment was the first step beyond the limitations of 7tcold11 fusion reactions. This was made at the limits of the experimental facilities. Therefore, it is important to reproduce and complement these results in other independent experiments, which has not been done as yet. Therefore we shall try to measure the energy of paired spontaneous fission fragments and genetically linked .d.-particles from the decay of complete-fusion products in the reaction 236U + 40Ar and later in the 24gCf + 26Nlgreaction, by using recoil selectors and different types of detectors. In proceeding along this line the study of the formation of known isotopes with 2 Cl10 in "hotl'fusion reactions is of great interest. For this purpose the cross sections for evaporation residues formed in the reactions 23zTh(27A~,5n)254~03and

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2369238LJ(26Mg,4n)25892601042;;re deterynyd. Preliminary results 107 production cross have been obtained for the 105 and sections in the 31P bombardments of 232Th and 235TJ.The data obtained are presented in fig. 7 together with the known results.

“I

dl lO”t_

i? ;

+ 22Ne.

+27AI.

Atomic

number

FIGURE 7 Production cross sections for transfermium nuclei in tlcold'l fusion reactions (open symbols) and in trhotrr fusion reactions (closed symbols). The curves are drawn through experimental points From comparing the cross sections of the reactions Pb, Bi (HI, n) and Th, U (HI,+5n) which lead to the formation of the same nuclides, it is possible to conclude that the "cold" fusion reactions have an advantage in the synthesis of transactinide elements up to Z,---108. However, as a result of the above-mentioned limitations to fusion; this advantage disappears in the region of the heavier nuclei. Of course, the limitations due to excessive Coulomb forces in the entrance channel of rrhot'V fusion reactions can also manifest themselves as the Z and A of the target-projectile system increase. However, the atomic numbers and masses of the actinide target nucleus and projectile can vary over a wide range without a substantial increase in the Coulomb energy. This fact, in principle, allows one to approach the island of stability of superheavy elements.

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4.3. The SHX problem At the same time, as is seen from the data presented in fig.7, the cross sections of the 244-248Cm + 48Ca reactions are not expected to be higher than of order of picobarns and that may be the reason why all the previous attempts have only set the upper limit of the production cross section for superheavy nuclei43 (fig. 8).

FIGURE 8 The curve showing the upper limit of production cross sections for SHE isotopes in the reaction 248Cm + 48Ca, as a function of the half-life of the final product Evidence for that has been provided once again by the results of the recent experiments aimed at synthesizing element 113 in the 48Ca bombardments of 237Np. In the 237Np(48Ca,3n)2s2113 reaction, a series of sequential -.-decaysleads to the formation of the isotope 262103 which, according to K.Hulet et al27 , undergoes spontaneous fission with !I!,,,‘*4 hours. The upper limit for spontaneous fission cross sections'-wasset at a level of about 40 picobarns for all nuclei in this chain under the assumption that they are either X.emitters or spontaneous fission activities with T l,2> 50 msa Therefore, the problem of synthesizing superheavy elements is associated with a considerable increase in experimental sensiti-

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vity, in the first place, by increasing the intensity of the 4*Ca ion beams. This is a difficult but feasible task. We hope that the joint efforts of several groups engaged in the synthesis of new elements at various laboxatories will allow to start these experiments in a not too distant future. REFERENCES 1) E.O. Diset and J.R. Nix, Nucl. Phys. A193 (1972) 647. 2) W.D. Myers and W.J. Swiatecki, Nucl. Phys. 81 (1966) I Ark, Pys. 36 (1967) 343. 3) S.A.E.Johansson, Report UCRL-~0474, Berkeley, 1962. 4) S. Bjornholm and J.E. Lynn, Rev. Mod. Phys. 52 (1980) 725. 5) M. Bra&

et al. Rev, Mod. Phys. 44 (1972) 320,

6) W.D. Myers and W.J. Swiatecki, Nucl. Phys. 81 (1966) 1. 7) V.M.Strutinsky, Yad. Fiz. 3 (1966) 614. 8) H. Meldner, Ark. Pysik 36 (1967) 593. 9) G.T. Seaborg and W.D. Loveland, in: Treatise on Heavy Ion Science, vol. 4, ed. D.A.Bromley (Plenum Press, New York, 1985) pm 255. IO>

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12) P. Mailer et al..Z. Phys. A323 (1986) 41. 13) A.Ghiorso et al. Phys. Rev. Lett. 33 (1974) 1490. 14) Yu.Ts. Oganessian et al. JINR Preprint D7-8194, Dubna (1974); H.Gaeggeler et al. Z. Phys. A289 (1979) 415. 15) Yu.Ts. Oganessian, in: Classical and Quantum Mechanical Aspects in Heavy Ion Collision, Lecture Notes in Physics, vol. 33 (Springer-Verlag, Heidelberg, 1975) p. 221. 16) Yu.Ts. Oganessian et al. Radiochimica Acta 37 (1984) 113. 17) P. Armbruster, Ann. Rev. Nucl, Part. Sci. 35 (1985) 735. 18) G. Mkzenberg et al. 2. Phys. A309 (1982) 89. 19) Yu.T.s.Oganessian, in: Proc. Int. School-Seminar on Heavy Ion Physics, Dubna, 23-30 September 1986, JINR (Dubna) 1987, pe 103.

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G. Mhzenbesg, in: Proc. Int. School-Seminar on Heavy Ion Physics, Dubna, 23-30 September 1986, JINR (Dubna) 1987, p. 127.

21) D.C. Hoffman et al. Phys. Rev. C24 (1981) 495. 22) A.G. Artukh et al. Yad. Fiz. 28 (1978) 1154. 23) D. Lee at al. Phys. Rev. C25 (1982) 286. W. Schroder and J.R. Huizenga, in: Treatise on Heavy Ion Science, vol. 2, ed. D.A.Bromley (Plenum Press, New York, 1985). 24) D. Hoffman et al. Phys. Rev. C31 (1985) p. 1763. 25) R.B. Welch et al. Phys. Rev. C35 (1987) 204. 26) M. Sch&del et al. Phys. Rev. C33 (1986) 1547. 27) R.W. Lougheed et al. LLNL Nuclear Chemistry Division, FY.87 Annual Report UCAR 10062/87 Evermore (1987) 28) D.C. Hoffman, in: Proc. Symp. IO Years of Uranium Beam at the UNILAC, Darmstadt, April 2-4, 1986, eds. N.Angert and P.Kienle, GSI 86-19, p. 265. 29) A.G...Demin et al. Z. Phys., A315 (1984) 197; G. Munzenberg et al. Z. Phys. A322 (1985) 227. 30) Yu.T$. Oganessian et al. Z. Phys. A319 (1984) 215; G. Munzenberg et al. Z. Phys. A324 (1986) 489. 31) G. Mcnzenberg, Rep. Prog. Phys. 51 (1988). 32) S. Liran and N. Zeldes, At. Data Nucl. Data Tables 17 (1976) 431. 33) N.N. Kolesnikov and A.G. Demin, VINITI, No. 7309-87 Dep. (1987). 34) ii Mb'ller, G.A. Leander, and J.R. Nix, 2. Phys. A323 (1986) . 35) W.J. Swiatecki, Phys. Sci. 24 (1981) 113. 36) K.T.R.Davies et al. Phys. Rev. C28 (1983) 629; J.R. Nix and A.J. Sierk, Preprint LA-UR-87-133 Los-Alamos (1985). 37) S. Bjornholm and W.J. Swiatecki. Nucl. Phys. A391 (1982) 471. 38) J.P. Blocki et al. Nucl. Phys. A459 (1986) 145. 39) G.N. Flerov, in:Proc. Int. Conf. on Nuclear Physics, Florence (Italy), August 29 - September 3, 1983, vol. 11, eds. P. Blazi and R. Ricci (Tipogr. Corn. Bologna, 1983) p* 365. 40) P. Frb;brich, C. Phys. Rep. 116 (1984) 337.

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