188Hg, cas limite des noyaux critiques

Abstract

Le 18880Hg est étudié par conversion interne et émission γ (spectre direct, en coïncidences, et corrélations angulaires) à partir de la désintégration du 188 81Tl (T½ = 71 s) isotopiquement séparé en ligne à ISOCELE (Orsay). En plus d'une bande fondamentale quasi sphérique et d'une bande déformée (O+) déjà vues par ions lourds, une bande γ et une bande de parité négative 5- 7- 6- 8- 9- sont mises en évidence; des états de vibration octupolaire sont suggérés. Cet ensemble montre comment s'effectue la transition entre noyaux de forme aplatie instable et noyaux critiques (β2 γβ 3 Δ instables)

Systematic decay studies of even–even 132–138Nd, 144–158Gd, 176–196Hg and 192–198Pb isotopes

“…Also from Figs. 6 and 7 it can be seen that for the 136 Nd parent the 16 O, 30 Si decay half lives are comparable with the alpha decay half life while for the 138 Nd parent the 12 C, 16 O, 22 Ne, 26 Mg, 28 Mg, 30 Si, 32 Si, 34 Si and 36 S decays are more probable than the alpha decay. However the decay half lives in all these cases is very large, T 1/2 > 10 50 s. Thus, although a number of shell effects are evident from the cluster decay results, the Nd parents are, on the whole, very stable against cluster decays.…”

“…The graphs for Nd parents show that there are almost equal number of N = Z and N = Z cluster emissions (six N = Z clusters and eight N = Z clusters) from them while in the case of the Gd parents there are more N = Z clusters being emitted than the N = Z clusters (five N = Z clusters and nine N = Z clusters). When we analyze the corresponding half life values we can see that for the Nd parents except for few emissions ( 12 C, 16 O, 28 Si, 30 Si, 34 S from 132 Nd) almost all the cluster decays (both N = Z and N = Z) have their T 1/2 > 10 50 s and for the Gd parents except for two or three N = Z clusters ( 12 C from 150,152 Gd and 16 O from 154 Gd) all the other cluster decays have their T 1/2 > 10 50 s. On comparing the cluster emissions from the Hg and Pb parents we find that there are equal numbers of N = Z and N = Z clusters (seven N = Z and seven N = Z clusters) getting emitted from Hg parents while there are more number of N = Z cluster emissions than N = Z emissions (three N = Z clusters and eleven N = Z clusters) from Pb parents. On making a study of the half lives of the cluster decays from Hg and Pb parents we can see that more than half the decays (both N = Z and N = Z) from the Hg and Pb parents have their T 1/2 > 10 50 s. Thus we may infer that as the N : Z ratio of the parent nuclei increases the number of N = Z clusters emitted are becoming equal to or greater than the number of N = Z clusters.…”

“…Amongst these 16 O cluster has the lowest half life of 1.68 × 10 46 s. The reason for the 16 O decay from 132 Nd being the most probable among the Nd parents is due to the proximity to the Z = 50 magic shell coupled with the midshell effect of the N = 64 neutron shell in the 116 Te daughter. Interestingly the 30 Si and 34 S heavy cluster decays from 132 Nd have their half lives (1.84 × 10 47 s and 2.96 × 10 49 s respectively) lower than the 12 C light cluster decay (1.15 × 10 50 s). It must be the stabilising effects of the N = 56 semi-magic neutron number [70] which is the reason for the low half lives of the 30 Si and 34 S decays.…”

“…Then there are dips for the 8 Be and 16 O emission from the 150 Gd and 154 Gd parents respectively. 8 Be emission from 150 Gd isotope is due to the spherical N = 82 shell closure of the daughter 142 Nd while the 16 Coming to the N = Z clusters, all the decays are either stable or very stable with the lowest half lives for the 30 Si decay from 144 Gd and the 14 C decay from 154 Gd. The low value for the 30 Si decay is due to the Z = 50 spherical shell closure in the 114 Sn daughter while the 14 C decay is bringing forth the N = 82 shell closure in the 140 Ce daughter which is also evident as a dip in the log 10 (T 1/2 ) versus mass number of parent graph.…”

The interacting Boson Model applied to the mercury isotopes

“…The first value is for positive interference, the second for negative interference. f Normalized to present or past experimental value [17,18,[32][33][34] for each isotope. (See (2) and Table 1.)…”