Isomers in superheavy nuclei Ackermann, D.; Antalic, S.; Heßberger, F. P.
The European physical journal. ST, Special topics,
2024/6, Letnik:
233, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Isomeric states in atomic nuclei are a well-known phenomenon all over the complete chart of nuclei. Their properties deliver valuable information on the structure of the nuclei. A region of specific ...interest are the very heavy and superheavy nuclei, where the occurrence and properties of isomeric states will have an impact on the prediction of localization and strength of the spherical superheavy proton and neutron shells. In this review, an overview of the present situation is given. Some specific features are discussed for selected examples.
A very exotic process of β-delayed fission of 180Tl is studied in detail by using resonant laser ionization with subsequent mass separation at ISOLDE (CERN). In contrast to common expectations, the ...fission-fragment mass distribution of the post-β-decay daughter nucleus 180Hg (N/Z=1.25) is asymmetric. This asymmetry is more surprising since a mass-symmetric split of this extremely neutron-deficient nucleus would lead to two 90Zr fragments, with magic N=50 and semimagic Z=40. This is a new type of asymmetric fission, not caused by large shell effects related to fragment magic proton and neutron numbers, as observed in the actinide region. The newly measured branching ratio for β-delayed fission of 180Tl is 3.6(7) × 10(-3)%, approximately 2 orders of magnitude larger than in an earlier study.
.
The reaction
54
Cr$ + $
248
Cm
was investigated at the velocity filter SHIP at GSI, Darmstadt, with the intention to study production and decay properties of isotopes of element 120. Three ...correlated signals were measured, which occurred within a period of 279ms. The heights of the signals correspond with the expectations for a decay sequence starting with an isotope of element 120. However, a complete decay chain cannot be established, since a signal from the implantation of the evaporation residue cannot be identified unambiguously. Measured properties of the event chain are discussed in detail. The result is compared with theoretical predictions. Previously measured decay properties of even element super-heavy nuclei were compiled in order to find arguments for an assignment from the systematics of experimental data. In the course of this review, a few tentatively assigned data could be corrected. New interpretations are given for results which could not be assigned definitely in previous studies. The discussion revealed that the cross-section for production of element 120 could be high enough so that a successful experiment seems possible with presently available techniques. However, a continuation of the experiment at SHIP for a necessary confirmation of the results obtained in a relatively short irradiation of five weeks is not possible at GSI presently. Therefore, we decided to publish the results of the measurement and of the review as they exist now. In the summary and outlook section we also present concepts for the continuation of research in the field of super-heavy nuclei.
At CERN-ISOLDE, high-purity radioactive ion beams of 219Fr and 221RaF were investigated with α-decay spectroscopy at the CRIS and ASET experiments in the course of three different experimental ...campaigns. The half-life of 215At, α-decay daughter of 219Fr, is measured to be 36.3(3)9μs, and that of 221Ra was determined to be 26.2(1)6s, both of which are well in line with the trends in this region of the nuclear landscape but at odds with some of the reported literature.
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.
At CERN-ISOLDE, high-purity radioactive ion beams of
Fr and
RaF were investigated with α-decay spectroscopy at the CRIS and ASET experiments in the course of three different experimental campaigns. ...The half-life of
At, α-decay daughter of
Fr, is measured to be 36.3(3)9μs, and that of
Ra was determined to be 26.2(1)6s, both of which are well in line with the trends in this region of the nuclear landscape but at odds with some of the reported literature.
Alpha-gamma decay studies of 247Md Heßberger, F. P.; Antalic, S.; Giacoppo, F. ...
The European physical journal. A, Hadrons and nuclei,
2022/1, Letnik:
58, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Alpha decay and spontaneous fission of the ground-state and the isomeric state of
247
Md were investigated with specific emphasis to identify the ground-state of the daughter nucleus
243
Es. The ...decay studies were accompanied by measuring
α
-
γ
coincidences. On the basis of the measured data an improved decay scheme of
247
Md is proposed. Spontaneous fission half-lives of
247
g
Md and
247
m
Md were determined and the fission hindrance of the high spin ground-state (7/2
-
) relative to the low spin isomeric state (1/2
-
) was estimated.
The decay of the 13/2+ isomeric state in 183Hg was observed for the first time following the a decay of the 13/2+ isomer in 187Pb produced in the 142Nd(50Cr, 2p3n) reaction. Using $\alpha - \gamma$ ...delayed coincidence measurements, the half-life of this isomer was measured to be 290(30) μs. This isomer is proposed to deexcite by an unobserved low-energy M2 transition to the known 9/2- member of a strongly prolate-deformed 7/2-514 band, followed by a 105-keV M1 transition to the bandhead. A lower limit of B(M2) >= 0.018 W.u. was deduced for the unobserved transition. The presumed retardation is proposed to be due to the notable shape change between the initial, nearly spherical, and the final, strongly deformed, states. A similar scenario is also considered for the 13/2+ isomer in 181Hg, suggesting both are cases of shape isomers. The B(M2) systematics of neutron transitions across the nuclear chart is discussed.
The radioactive element astatine exists only in trace amounts in nature. Its properties can therefore only be explored by study of the minute quantities of artificially produced isotopes or by ...performing theoretical calculations. One of the most important properties influencing the chemical behaviour is the energy required to remove one electron from the valence shell, referred to as the ionization potential. Here we use laser spectroscopy to probe the optical spectrum of astatine near the ionization threshold. The observed series of Rydberg states enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV. New ab initio calculations are performed to support the experimental result. The measured value serves as a benchmark for quantum chemistry calculations of the properties of astatine as well as for the theoretical prediction of the ionization potential of superheavy element 117, the heaviest homologue of astatine.
Prompt and delayed γ-ray spectroscopy of the neutron-deficient, semi-magic isotope 187Pb has been performed using the recoil-decay and isomer-decay tagging techniques at the Argonne Gas-Filled ...Analyzer. A new 5.15(15)-μs isomeric state at only 308 keV above the spherical 3/2− ground state is identified and classified as a shape isomer. A strongly-coupled band is observed on top of the isomer, which is nearly identical to the one built on the prolate 7/2−514 Nilsson state in the isotone 185Hg. Based on this similarity and on the result of the potential-energy surface calculations, the new isomer in 187Pb is proposed to originate from the same configuration. The retarded character of the 308-keV (7/2−)→3/2gs− transition with a deduced B(E2)=5.6(2)×10−4 W.u. can be well explained by the significant difference between the prolate parent and spherical daughter configurations, leading to the shape isomerism. The excitation energy of the isomer is surprisingly low, being roughly half of the excitation energies of the known 0+ intruder bandheads in the neighboring 186,188Pb isotopes. The combined results of the present work and the previous α-decay and laser spectroscopy studies present evidence for triple shape coexistence at low energy in the negative-parity configurations of 187Pb, which is well reproduced by the potential-energy surface calculations.
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