Shell closures

THESE SECTIONS ARE MANUFACTURERS STANDARD AND FEATURE METAL TO METAL JOINTS AT UNION AND SHELL CLOSURES. ... 

Figure 10. Electron shell closure fails to coincide with the dosing of periods in the periodic table because the shells do not fill in strictly sequential order As shown here, die fourth shell begins to fill before the third shell has been completed. The resumption of third-shHl filling accounts for the appearance of the first transition-metal series, beginning with scandium and ending with zinc.

These shell closures have a profound influence on nuclear properties, in particular the binding energy (adding terms not accounted for in Eq. 2.2), particle separation energies and neutron capture cross-sections. The shell model also forms a basis for predicting the properties of nuclear energy levels, especially the ground... 

Figure 7.2 Variation of a-decay energies indicating the effect of the IV = 126 and Z = 82 shell closures along with the IV = 152 subshell.

We should also note that the double-shell closures at Z = 82 and N = 126 lead to especially large positive Q values, as already shown in Figure 7.2. Thus, the emission of other heavy nuclei, particularly 12C, has been predicted or at least anticipated for a long time. Notice also that 12C is an even-even nucleus and. v-wave emission without a centrifugal barrier is possible. However, the Coulomb barrier will be significantly larger for higher Z nuclei than that for a particles. 

The first question to be answered was what guest compound would be trapped inside during the shell closure This question is akin to asking whether two soup bowls closed rim-to-rim under the surface of a kettle of stew would net any stew. The answer was that [6.97] contained essentially every kind of component of the medium present during ring closure.10... 

The 132 n region has recently shown itself to be a very good region in which to develop an effective interaction. This is a region with a strong double-shell closure, stronger than all shell closures beyond 0. There is also a large set of experi-... 

By studying the a decay of mass-separated nuclei in the region around Z=82 we have extended the knowledge of shell-model intruder states. The allowed a-decay branches of the odd-odd Bi nuclei connect across the Z=82 shell closure initial and final states of the same single-particle character. 

Shown in Fig 2 is a schematic representation of the shell model states for the region near 1 8Gd. There has been a great deal of interest in this mass region since it was proposed [0GA78] that a shell closure occurs for Z-64. 

Nuclear chart including the major shell closures and approximative isodeformation curves for e = 0.2. Hfs and IS measurements have been performed at ISOLDE in long isotopic chains of the elements indicated by arrows. Shorter sequences of the elements 35Br, 53I, 0gTm and glTl, not indicated here, have also been studied at ISOLDE. 

Shell model calculations predict a quasi-shell closure at 96Zr. Therefore, it is of interest to measure g-factors of states in 97Zr and test whether they can be described by simple shell model configurations. The 1264.4 keV level has a half-life of 102 nsec, and its g-factor was measured by the time-differential PAC method at TRISTAN [BER85a]. The result, g-0.39(4), is consistent with the Schmidt value of 0.43, which assumes no core polarization and the free value for the neutron g factor, g g free. This indicates that the 1264.4 keV level is a very pure single-particle state, thus confirming the shell model prediction of a quasi-shell closure at 96Zr. 

We have shown that g-factors of excited states in nuclei far from stability can provide important nuclear structure information, such as purity of wave functions, configuration-mixing, number of active protons, dissipation of shell closures, values of g, gv. 

Nuclear-structure studies around the expected double shell-closure at... 

Bryant JA et al (1991) Host guest complexation 55. Guest capture during shell closure. J Am Chem Soc 113 2167-2172... 

Cram DJ, Karbach S, Kim YH, Baczynskyj L, Kallemeyn GW (1985) Shell closure of two cavitands forms carcerand complexes with components of the medium as permanent guests. J Am Chem Soc 107 2575... 

The magic numbers were successfully explained by the nuclear shell model [5,6], and an extrapolation into unknown regions was reasonable. The numbers 126 for the protons and 184 for the neutrons were predicted to be the next shell closures. Instead of 126 for the protons also 114 or 120 were calculated as closed shells. The term superheavy elements, SHEs, was coined for these elements, see also Chapter 8. 

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.

The lower energy of the outer-shell arrangement of the fourth water was confirmed spectroscopically by Robertson etal. (2003). Those experiments showed shell closure by the ligating water molecules when three water molecules are hydrogen bonded to the HO ion. 

Because it was assumed that the next protonic magic number was 126 (by analogy with the neutrons), early studies of possible superheavy elements did not receive much attention [12—15), since the predicted region was too far away to be reached with the nuclear reactions available at that time. Moreover, the existence of such nuclei in nature was not then considered possible. The situation changed in 1966 when Meldner and Roper [16, 17) predicted that the next proton shell closure would occur at atomic number 114, and when Myers and Swiatecki [18) estimated that the stability fission of a superheavy nucleus with closed proton and neutron shells might be comparable to or even higher than that of many heavy nuclei. 

The dissociation energies of alkali-metal clusters show variations due to even-odd effects and shell closures. This is not the case for mercury. The dissociation energies and the mass spectra show no trace of similar rapid variations. The dot-dashed line is from the tight-binding calculation of Pastor et a/. - for neutral Hg clusters. It deviates from the experimental result already for n = 3 and shows a much smoother transition. The experiment points to a more abrupt transition between the different regions of binding. 

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