Exotic Indium Nuclei

The isotope tin-100 is of interest for nuclear structure due to its closed-shell proton and neutron configurations. It is also the heaviest nucleus comprising protons and neutrons in equal numbers. In new research, physicists from the ISOLTRAP experiment at CERN’s Isotope mass Separator On-Line (ISOLDE) facility performed direct mass measurements of indium-99 and indium-100, neighboring nuclei of tin-100. The results appear in the journal Nature Physics.
High-precision mass measurements of neutron-deficient indium isotopes with ISOLTRAP: radioactive atoms were produced by nuclear reactions of 1.4 GeV protons impinging on a thick lanthanum carbide target; short-lived indium atoms diffusing from the target were selectively ionized using a two-step laser excitation scheme, provided by the ISOLDE RILIS, which excited one electron above the indium ionization potential (IP); the extracted ion beam was mass separated and injected into a radiofrequency quadrupole (RFQ) ion trap sitting on a high-voltage (HV) platform, where it was bunched and cooled; the beam was then processed by an MR-ToF MS to separate the indium ions from the isobaric contaminants; when the precision Penning trap was used for the mass measurement, further cooling and purification of the beam was achieved using a helium buffer-gas-filled preparation Penning trap; a position-sensitive microchannel plate (MCP) detector was used to record the time of flight and/or the position of the ion after ejection from the precision Penning trap; in the case of indium-99, for which the production yield was too low, the MR-ToF MS was used to perform the mass measurement. Image credit: Mougeot et al., doi: 10.1038/s41567-021-01326-9.
Atomic nuclei have only two ingredients, protons and neutrons, but the relative number of these ingredients makes a radical difference in their properties.
Certain configurations of protons and neutrons, with ‘magic numbers’ of protons or neutrons arranged into filled shells within the nucleus, are more strongly bound than others.
The rare nuclei with complete proton and neutron shells, which are termed doubly magic, exhibit particularly enhanced binding energy and are excellent test cases for studies of nuclear properties.
The new theoretical calculations and experimental results from the ISOLTRAP team shed light on one of the most iconic doubly magic nuclei: tin-100.
With 50 protons and 50 neutrons, tin-100 is of particular interest for studies of nuclear properties because, in addition to being doubly magic, it is the heaviest nucleus comprising protons and neutrons in equal number — a feature that gives it one of the strongest beta decays, in which a positron is emitted to produce a daughter nucleus.
Studies of the beta decay of tin-100 suffer from difficulties in producing it.
Moreover, the two most recent such studies — a 2019 study by RIKEN and a 2012 study by GSI — yield different values for the energy released in the decay, resulting in discrepant values for the mass of tin-100.
In the new study, Dr. Maxime Mougeot of the Max-Planck-Institut für Kernphysik and colleagues measured the mass the exotic odd-proton nucleus indium-100, the beta-decay daughter of tin-100, and of indium-99, with one proton less than tin-100.
“The mass of tin-100 can be derived from that of indium-100 and the energy released in the beta decay of tin-100 into indium-100,” Dr. Mougeot said.
“So our indium-100 mass measurement grabbed this iconic doubly magic nucleus by the tail.”
The new mass measurement of indium-100 is 90 times more precise than the previous one, magnifying the discrepancy in the values of the tin-100 mass deduced from the most recent beta-decay studies.
The researchers then made comparisons between the measured masses of the indium nuclei and new sophisticated ab initio theoretical calculations that attempt to describe nuclei from first principles.
These comparisons favor the beta-decay energy result from GSI over that of the RIKEN team.
Moreover, they show excellent agreement between the measurements and the calculations, giving the researchers great confidence that the calculations capture the intricate nuclear physics of tin-100 and its indium neighbors.
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M. Mougeot et al. Mass measurements of 99-101In challenge ab initio nuclear theory of the nuclide 100Sn. Nat. Phys, published online September 23, 2021; doi: 10.1038/s41567-021-01326-9

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