Superheavy nucleus

One isotope of element 114, with 184 neutrons, is predicted to be another doubly magic nucleus, and is therefore expected to sit right in the middle of an island of stability in the space of superheavy nuclei (Fig. 13). Nuclear scientists suspect that it may have a half-life of as much as several years. Element 114 has thus become a kind of Holy Grail for element-makers. If it turns out to be stable, this would show that these researchers are not necessarily doomed to search for increasingly fleeting glimpses of ever heavier and less stable new elements. There might be undiscovered elements out there that you can (in principle, at least) hold in your hand. 

We now know these predictions were wrong, in part. While we believe there are a group of superheavy nuclei whose half-lives are relatively long compared to lower Z elements, we do not believe they form an island of stability. Rather, we picture them as a continuation of the peninsula of known nuclei (Fig. 15.1 lb). We also believe that their half-lives are short compared to geologic time scales. Therefore, they do not exist in nature. The most stable of the superheavy nuclei, those with Z = 112, N 184, are predicted to decay by a-particle emission with half-lives of 20 days. 

Some have taken the viewpoint that, without the special stability associated with nuclear shell structure, elements as light as Z = 106-108 would have negligibly short half-lives. The mere existence of these nuclei with millisecond half-lives is said to be a demonstration that we have already made superheavy nuclei, according to this view. The shell stabilization of these nuclei, which are deformed, is due to the special stability of the N = 162 configuration in deformed nuclei. (The traditional superheavy nuclei with Z 114, N = 184 were calculated to have spherical shapes.)... 

The "island of stability" near 114 protons and 184 neutrons corresponds to a group of superheavy nuclei that are predicted to be stable. The first member of this group was reported in 1999. 

The model, with major extensions to nuclear fission, leads to surprising predictions for the lifetimes of superheavy nuclei. Suggestions for heavy-ion synthesis are presented. 

In view of experimentalists disappointing experiences with the predictions for superheavy nuclei, we hesitate to suggest new experiments. 

Hopefully, our results (some of which have been presented previously [KUM83]) for the superheavy nuclei will also stimulate other theorists to recalculate fission lifetimes, and, especially, fusion cross-sections for the suggested target-projectile combinations. These cross-sections are expected to be small and the corresponding experiments are expected to be quite difficult. 

Smolanczuk, R., Sobiczewski, A. Shell effects in the properties of heavy and superheavy nuclei . In Proceedings of The XV. Nuclear Physics Divisional Conference on Low Energy Nuclear Dynamics , St.Petersburg, Russia, 18-22 April 1995,... 

In 1955, J.A. Wheeler [1] concluded from a courageous extrapolation of nuclear masses and decay half-lives the existence of nuclei twice as heavy as the heaviest known nuclei he called them superheavy nuclei. Two years later, G. Scharff-Goldhaber [2] mentioned in a discussion of the nuclear shell model, that beyond the well established proton shell at Z=82, lead, the next proton shell should be completed at Z=126 in analogy to the known TV = 126 neutron shell. Together with a new A=184 shell, this shell closure should lead to local region of relative stability. These early speculations remained without impact on contemporary research, however. 

Fig. 1. Topology of the island of superheavy nuclei around the shell closures at proton number Z=114 and neutron number N=184 as predicted in 1969. Thick solid lines are contours of spontaneous fission half-lives, broken lines refer to a-decay half-lives. Shaded nuclei are stable against p-decay. Reproduced from S.G. Nilsson, S.G. Thompson and C.F. Tsang [10], Copyright (2002), with permission from Elsevier Science.

Due to the topology of the island, superheavy nuclei should decay by spontaneous fission, either immediately or after a sequence of other decay steps. In a detailed theoretical exploration [12] of the Z-N plane around the island, the longest-lived nuclide again turned out to be Z=110, A=184, decaying with 3xl09 y half-life by a-particle emission to 290108. From there, two subsequent (T-transitions should lead via 290109 to 290110, where the chain should terminate by spontaneous fission with 140 d half-life. The doubly magic 298114, half-life 790 y, should also decay into 290110 by two a-particle emissions via 294l 12 as the intermediate. 

Since spontaneous fission is extremely rare in Nature, detection of fission events in natural samples would give a strong hint. Alpha-particle spectra would be less specific, because the energies predicted for superheavy nuclei fell into the range covered by the natural decay series deriving from uranium... 

Within a few years, many aspects of superheavy nuclei and elements were touched. A review [21] covering the literature until the end of 1973 was based on already 329 references, and status reports [22-27] published from time to time illustrate how the field developed. During this exploratory period continuing until the early eighties the optimistic perspectives of occurrence in Nature and easy synthesis at accelerators were proven in a... 

Fig. 4. Nucleosynthesis of superheavy nuclei in the r-process rapid neutron capture alternating with IT-transilions during a supernova explosion. Shown in the Z-N plane is the r-process path of very neutron-rich nuclei extending to ZA100, from where p -decay chains directed towards the belt of p-stable nuclei would lead to Z 114, N=184 nuclei (dots). From D.N. Schramm and W.A. Fowler [28], reprinted with permission from Nature, Copyright (2002) Macmillan Magazines Limited.

But even if the half-lives of superheavy nuclides would not exceed the 108y level, there was hope to discover them in Nature. Although now extinct, they may have left detectable traces such as fission tracks or fission products in certain samples. Another possible source could be the cosmic radiation impinging on Earth whose heavy component may be formed by r-process nucleosynthesis in our galaxy not longer than 107 y ago [33] and may, hence, contain superheavy nuclei with half-lives down to some 105 years. 

Fig. 5. Neutron counting as detection method for spontanous fission events of superheavy nuclei. The recorded neutron rates (points) were found to follow the relative cross sections of cosmic-ray induced spallation reactions (curve) and were, thus, due to background events. The numbers are rates for natural uranium and thorium. From W. Grimm, G. Herrmann and H.-D. Schiissler [40].

More advanced applications of neutron counting were based on the expectation that spontaneous fission events of superheavy nuclei should be accompanied by the emission of about ten neutrons [41,42], distinctly more than two to four observed for any other spontaneous fission decay. Such neutron bursts can be recognized by recording neutron multiplicities - events with several neutrons in coincidence - with 3He-filled counting tubes [43,44] or large tanks filled with a liquid scintillator sensitive to neutrons [45],... 

For the synthesis of superheavy nuclei by complete fusion, larger projectiles with at least twice as many protons and neutrons are required. It soon became obvious that in such reactions the deficit in the yields is even larger than can be explained by post-fusion losses. This problem stimulated systematic studies of heavy-ion reactions all the way down from the first touch of two interacting nuclei until final fusion. New types of reactions -... 

Would incomplete transfer processes open an alternative approach to the synthesis of superheavy nuclei Would, e.g., in the collision of two 238Ui46 nuclei one partner take up enough protons and neutrons to grow to the doubly magic 298114lg4, whereas the complementary partner would shrink to 17870io8> a known neutron-rich isotope of ytterbium ... 

R. Vandenbosch, J. R. Huizenga, Nuclear Fission, Academic Press, New York 1973 J. R. Nix, Calculation of Fission Barriers for Heavy and Superheavy Nuclei, Annu. Rev. Nucl. Sci. 22, 65 (1972)... 

The results show that, as one moves away from the double closed-shell nucleus 298114, the calculated spontaneous fission half-hves in Fig. 4a decrease from lO s y for nuclei on the inner contour to lO" y (about 5 min) for nuclei on the outer contour. With respect to spontaneous fission, the island of superheavy nuclei is a mountain ridge running north and south, with the descent being most gentle in the northwest direction. The calculated a-decay half-hves in Fig. 4 b, however. 

The distribution of doublets at E>1019 3 eV (Fig.3c) is more isotropic. It is possible that at these energies the formation of doublets starts at the expense of superheavy nuclei fragmentation. To appreciate the origin of doublets we considered the distribution of showers parallel with the distribution of doublets (Fig3b) in the same energy interval. In the distribution of showers the some maxima are observed at b <3° (exceese of the number of showers relative to expected is 2.7g=(29-17.5)/, at 21°>b>15° etc. However, the maximum in the distribution of doublets is seen at b <3° only. This means that the maximum number of doublets appears where there is the exceed particle flux. Other maxima in the particle distribution are most likely formed by accident and therefore maxima from these directions in the distribution of doublets do not observed. So, the arrival directions of doublets (clusters) can be an indicator of cosmic rays anisotropy. It is important in the case when the cosmic ray anisotropy cannot be detected because of the small statistic. 

K. Rutz, M. Bender, T. Biirvenich, T. Schilling, P.-G. Reinhard, J. Maruhn, W. Greiner, Superheavy nuclei in selfconsistent nuclear calculations , Preprint (1996), Institut fiir Thcoretische Physik, JWG Uni-vcrsitat, Frankfurt a M, to be published. 

D. A., Strellis, D. A., and Wilk, P. A. 1999. Observation of superheavy nuclei produced in the reaction of 86Kr with 208Pb. Phys Rev Lett 83, 1104—1107. 

Smolanczuk, R. 1997. Properties of the hypothetical spherical superheavy nuclei. Phys Rev C 56, 812-824. 

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