Signatures of theZ=82Shell Closure inα-Decay Process

Abstract

In recent experiments at the velocity filter Separator for Heavy Ion reaction Products (SHIP) (GSI, Darmstadt), an extended and improved set of α-decay data for more than 20 of the most neutron-deficient isotopes in the region from lead to thorium was obtained. The combined analysis of this newly available α-decay data, of which the (186)Po decay is reported here, allowed us for the first time to clearly show that crossing the Z = 82 shell to higher proton numbers strongly accelerates the α decay. From the experimental data, the α-particle formation probabilities are deduced following the Universal Decay Law approach. The formation probabilities are discussed in the framework of the pairing force acting among the protons and the neutrons forming the α particle. A striking resemblance between the phenomenological pairing gap deduced from experimental binding energies and the formation probabilities is noted. These findings support the conjecture that both the N = 126 and Z = 82 shell closures strongly influence the α-formation probability.

Decay of201–203Raand200–202Fr

“…2(a)] and their interpretation is discussed in Ref. [22]. We deduced the reduced α-decay width using the Rasmussen prescription [21] assuming L = 0 and obtained δ [23] and radon [24,25] -decay width (keV) α reduced …”

N=Znuclei: a laboratory for neutron–proton collective mode

“…[115] compared the α-decay reduced widths for Xe and Te nuclei with that of 212 Po and neighboring Po isotopes and an enhancement by a factor of 2-3 is seen. We also noticed that the |RF (R)| 2 value of 194 Rn is larger by a similar factor compared to the |RF (R)| 2 of the textbook α-decay isotope 212 Po [117,118]. This faster α decay would change the borderline of accessible neutron deficient α-decaying nuclei and might be a important question and motivation for further experimental work.…”

“…[117,118] where a generic pattern for the systematics of the formation amplitude F (R) was also proposed. It was found that, when going from one isotope to another, the α-particle formation probability usually varies much less than the penetrability.…”

Confirmation of the new isotope Pb178

“…The general finding for Pb nuclei is that the α-decay reduced widths are smaller than those of Rn, Po, Hg, and Pt neighbors and that deviations from this trend are brought about by the presence of intruder states and configuration mixing, due to shape coexistence. Andreyev et al [6] discussed the α-particle preformation probability in relation to the neutron pairing energy for the nuclei in this region, which demonstrated the effect of the shell closures of N = 126 and Z = 82 on these values. As reported in Ref.…”

“…Systematic studies of α-decay reduced widths of even-even lead nuclei and also of radon, polonium, mercury, and platinum nuclei, which are two and four protons away from the closed shell, have been discussed previously in Refs. [3][4][5][6]. The general finding for Pb nuclei is that the α-decay reduced widths are smaller than those of Rn, Po, Hg, and Pt neighbors and that deviations from this trend are brought about by the presence of intruder states and configuration mixing, due to shape coexistence.…”