Hot fusion reactions

A combined team of scientists from the Lawrence-Livermore National Laboratory (LLNL) in California, and from the laboratory in Dubna, Russia, reported the following hot fusion reaction ... 

In Figure 15.4, we show current measurements (filled squares) of the cross sections for cold fusion reactions as a function of the atomic number Z of the completely fused system. (The cold fusion point at Z = 118 is an upper limit.) Also shown (as open circles) are the cross sections for hot fusion reactions. Clearly, future efforts will have to focus on experiments at the 0.1- to 1.0-pb cross-section level or lower. Current technology for cold fusion reaction studies would require 12 days to observe one event at a cross-section level of 1 pb. Similarly, a cross section of 1 pb in a hot fusion reaction would require 6—19 days to observe one event. From examining the data in Figure 15.4, it would also appear that hot fusion reactions might be the reactions of choice in pursuing future research in this area. 

Figure 15.4 Plot of the observed cross sections for the production of heavy elements by cold and hot fusion reactions.

In the following sections a detailed description is given of the set-ups of the physical experiments used for the investigation of SHEs. (The instrumentation based on chemical methods for the study of heavy elements is presented in subsequent chapters of this book.) Experiments are presented, in which cold and hot fusion reactions were used for the synthesis of SHEs. 

Table 1. Nuclides from hot fusion reactions [50] used in chemical investigations.

Hot fusion reactions with the deformed nuclei of the actinides (e.g. or Pu) as targets lead to high excitation energies of the compound nuclei (Ex 50 MeV), emission of about 4 to 5 neutrons, high probability of fission and relatively low values of i7fus. On the other hand, the neutron numbers of the reaction products are relatively higli and the half-lives relatively long. 

The characteristic circular-shape of the tokamak reactor is clearly seen here. The reactor uses strong magnetic fields to contain the intensely hot fusion reaction and keep it from direct contact with the interior reactor walls. 

For element 114, gas-phase chromatography experiments similar to those for element 112, are also planned for the near future by the PSI/GSI collaboration. A long-lived isotope 114 (3 s) will be produced in the hot -fusion reaction from Pu with a Ca beam. Element 112, a decay product of element 114, will be studied on its volatility in relation to that of Hg using the same technique as described above. In this way, the Z number of element 114 will be proven in an indirect way. 

In order to make isotopes of new elements having high N/Z ratios, there has been more recent interest in hot fusion reactions involving nuclei such as and Es as targets and light ions... 

Abstract The Island of Stability of spherical superheavy nuclides exists at the extreme limit of the Chart of the Nuclides, beyond regions of nuclear stability associated with deformed nuclear shapes. In this chapter, the reactions that are used to synthesize these transactinide nuclides are discussed. Particular emphasis is placed on the production of nuclides with decay properties that are conducive to a radiochemical measurement. The cold- and hot-fusion reactions that lead to the formation of evaporation residues are discussed, as are the physical techniques that have been used in production experiments. Recent results from " Ca-induced fusion reactions are included. Speculative methods of producing the more neutron-rich nuclides that populate the approaches to the center of the Island of Stability are also presented. 

Hot-fusion reactions were employed in the discoveries of the elements beyond mendelevium as far as element 106, producing the first three members of the domain of superheavy elements. Higher transactinides have also been synthesized in these reactions. As before, the general trends with increasing atomic number were shorter half-lives and smaller production cross sections, a consequence of decreased survival probability in the evaporation process [132, 133]. The probability of decay from the nuclear ground state by spontaneous fission became significant in these elements. The techniques used in the experiments still included radiochemistry and off-line radiation counting [134]. As half-lives dropped below minutes into seconds it became more common to use direct techniques like transportation in gas jets to mechanisms like wheels and tapes (see Sect. 3.3 and Experimental Techniques ). Detection of new nuclides resulted from the detailed... 

Rutherfordium, element 104, was first synthesized in hot-fusion reactions by research groups in Dubna (Russia) and Berkeley (USA). Early work in Dubna in which a spontaneous fission activity was assigned to °Rf [137-140] could not be reproduced by others. Later chemical experiments involving the formation of Rf in the Pu( Ne,jcn) reaction and the detection of a non-isotope-specific spontaneous fission activity in the chemical form of a volatile chloride demonstrated the fundamental change in chemical properties occurring beyond the end of the actinide series [141, 142]. The decay of Rf was eventually determined to be spontaneous fission with a half-life of 21 ms, produced in the reactions... 

Ongoing work suggests that the 2.1-s activity may be an observation of and that the ground state may be significandy shorter lived. The isotope Th2 = 8 s) can only be produced indirectly. At first it was reported as a 15-min SF activity in the " Bk( 0,4n) reaction, as the decay daughter of a small EC branch in 27-sec Db. However, 3% of the 6 nb evaporation-residue cross section results in an effective cross section of 180 pb for the production of the 15-min activity [161]. A single event consistent with an 8-s half-hfe was observed in an a decay chain of Hs (see below), which is considered to be the more reliable observation of Rf. Work is needed to sort out the decay properties of this isotope. Several of the Rf isotopes are also produced indirectly in hot-fusion reactions as daughters of the a decays of Sg isotopes (see below). 

Bohrium, element 107, has been produced in hot-fusion reactions. The isotopes... 

In hot-fusion reactions, the cross section for producing heavy-element nuclides is determined by the probability that the highly excited compound nucleus will avoid fission in the deexcitation process. Cold fusion near the reaction barrier is qualitatively different the formation of the compound nucleus comes about in two separate steps [105, 107]. The reacting nuclei come into contact, captured into a dinuclear configuration, which is separated from an equilibrated compound nucleus by a potential-energy barrier which is not reproduced by the one-dimensional Coulomb-barrier model [94, 95, 210, 219, 220]. This extra barrier diverts the trajectory of the reaction through multidimensional deformation space toward quasifission, making reseparation much more likely than complete fusion. 

In the example above in which isotopes of nobelium are produced by hot and cold fusion, the difference between the observed cross sections and the geometrical cross sections derive from two different effects. In the hot-fusion reaction, the compound nucleus is unlikely to survive the competition between fission and each of the four neutron-evaporation steps, leading to a small cross section. In the coldfusion reaction, the probability that the compound nucleus avoids the fission process is orders of magnitude higher than in the hot-fiision reaction, but the dynamical hindrance to complete fusion results in a lower probability for formation of that compound nucleus [227, 235-237]. It is a matter of some serendipity that the nobelium evaporation-residue cross sections for the two reaction types are approximately the same. 

In spite of the lower excitation energies obtained in cold-fusion reactions, hot-fusion reactions produce evaporation residues that are more neutron rich, a consequence of the bend of the line of fi stability toward neutron excess. For the purposes of studying nuclei whose stability is more strongly influenced by the spherical 184-neutron shell clostrre, hot fusion is the more viable path. If nuclei were constrained to be spherical, or deformed into simple quadrupole shapes like those that influence the properties of the actinide isotopes with N — 152, one would expect cold-fusion reactions to quickly veer into ZJ space where nuclides would be characterized by very short partial half-lives for decay by spontaneous fission. In fact, there is a region of nuclear stability centered at Z = 108 and N — 162 [12, 19-21], removed from the line of fi stability toward proton excess, where the nuclei derive a resistance to spontaneous fission from a minor shell closure associated with complicated nuclear shapes, making a emission their most probable decay mode [133, 240]. 

Decay by emission of a particles (with ANIAZ — 1) is a proton-rich process, and in the known transactinides results in daughter nuclei that lie closer to the line of /i stability than do their parents. The decays of the superheavy cold-fusion nuclei lead to long chains of sequential a emissions and a progressive increase in neutron richness in the lower members of the chain. In this way, cold fusion can be used to produce isotopes that rival the neutron richness of those produced in hot-fusion reactions. This has been referred to as overshooting [22, 47]. For example, 10-s Hs can be produced directly in the Cm( Mg,5n) reaction with a cross section of 7 pb [179, 180]. The most neutron-rich isotope of hassium that can be produced directly by cold fusion is Hs, in the Pb( Fe,n) reaction [241-243]. However, Hs is also the third member of the Cn decay chain. 

Simple addition of protons and neutrons in the reactants indicates that the transactinide products of " Ca-induced fusion reactions derive the same advantage in neutron number over cold-fusion products that was observed in more asymmetrical hot-fusion reactions (see Sect. 2.2). In reactions that produce copemicium (Z = 112), the switch from the cold-fusion Pb( n,n) Cn reaction to the hot-fusion U(" Ca,3n) Cn reaction effectively adds 6 neutrons to the evaporation residues. In terms of exploring Z,N space toward the center of the Island of... 

Besides the effect of shell stabilization in the entrance channel on the of the compound nucleus, the mechanism of " Ca-induced hot-fusion reactions shares another aspect of the character of cold-fusion reactions. While deexcitation of the hot compound nuclei is dominated by the competition between fission and neutron emission, attempts to reproduce the evaporation-residue cross sections by a simple r /ry treatment results in values that are much higher than those that are observed experimentally [300-302]. It is necessary to invoke a significant dynamical hindrance to fusion and a two-step mechanism [303, 304] to reproduce the cross sections for " Ca-induced reactions that result in transactinide nuclides [305, 306], which increases as the atomic number of the target nuclide increases. Like the cold-fusion reaction intermediate, the reaction trajectory from nuclei in contact to a compound nucleus can be diverted into a more probable path leading to quasifission, even though the potential energy of the compound nucleus is lower than or approximately equal to that of the reacting nuclei in contact [8, 105,123,174,220, 301, 307-312]. Only a small number of dinuclear intermediates reach the compact shape associated with the compound nucleus. 

Probing superheavy element space by " Ca-induced hot-fusion reactions is characterized by advancing beyond the = 162 deformed subshell closure toward nuclei that are spherical and tightly bound. The macroscopic-microscopic model characterizes the ground-states of nuclei with > 175 as having a prolate deformation parameter 62 < 0.1, making them nearly spherical [8, 58, 60]. At neutron numbers below N — 175, any cross section benefit of the " Ca-induced hot-fusion approach to the Island of Stability is expected to decrease, as the shell stabilization of the ground state of the compound nucleus decreases. If a coldfusion path to a particular superheavy nuclide is available, it is expected to be the better one however, very little experimental evidence of this is available. As an example, the attempt to produce Cn (Z — 112) in the UC Ca,4n) reaction was unsuccessful (cross section limit <0.6 pb) [8, 316], in contrast to its production in the ° Pb( °Zn,n) reaction (cross section 0.5 pb) [263, 271]. 

Though the " Ca-produced transactinide elements can be relatively resistant to spontaneous fission decay, the local increase in a-decay Q value with an increase in atomic number results in an overall decrease in half-life [329]. The decays of the " Ca-produced superheavy nuclei lead to chains of sequential a decays to longer lived daughter nuclei lying closer to the line of stability, some of which have surprisingly long half-lives. As an example, the decay of 115, produced in the AmC Ca,3n) reaction, results in a chain of a-emitting superheavy nuclei that culminates in Db (Z = 105), a nuclide with a half-life of one day [285, 286, 330]. This nuclide contains 7 neutrons more than the heaviest dubnium isotope that can be produced by either cold fusion or more asymmetric hot-fusion reactions. 

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