The decay properties of neutral atoms can significantly alter if most or all bound electrons are removed. While decay channels involving electrons are naturally suppressed in the absence of bound electrons, new decay pathways can also emerge.

This research program focuses on weak decays of highly charged ions, a topic that lies at the intersection of atomic physics, nuclear structure, astrophysics, and plasma physics. A comprehensive review on beta decay of highly charged ions can be found in: Yu. A. Litvinov & F. Bosch, “Beta decay of highly charged ions”, Rep. Prog. Phys. 74 (2011) 016301,                        doi: 10.1088/0034-4885/74/1/016301

Types of Beta Decays

Weak decays (β-decay) can be expressed in terms of nucleons—proton (p), neutron (n)—and leptons—electron (e−), electron neutrino (ν_e)—in a symmetric form as:

                                                           n + ν_e ↔ p + e⁻.

Taking the particle–antiparticle symmetry into account, five distinct modes of β-decay can be classified:

1. Continuum β⁻-decay (β⁻_c):          n → p + e⁻+ν⁻_e

2. Bound-state β⁻-decay (β_b):          n + ν_e → p + e⁻_b

3. Continuum β⁺-decay (β⁺_c):          p → n + e⁺ + ν_e

4. Orbital electron capture (EC):     p + e⁻_b → n + ν_e

5. Free electron capture (free EC):  p + e⁻→ n + ν_e

In β⁺_c and β⁻_c continuum decays, energy and momentum are shared between the generated leptons. In contract, EC and β_b-decay are strictly two-body processes, connected by time reversal and characterized by the annihilation (or generation) of monochromatic bound electrons e⁻_b and the generation (or annihilation) of monochromatic electron neutrinos ν_e, respectively.

Experimental Investigations

Weak decays of highly charged ions are currently studied exclusively in heavy-ion storage rings. The experiments rely on a variety of instrumentation, with non-destructive Schottky detectors being the power horse for such unique measurements. At present, two Schottky detectors are employed for measurements in the ESR: a 245 MHz Schottky detector (F. Nolden et al., NIM A 659, 69 (2011), doi: https://doi.org/10.1016/j.nima.2011.06.058) and a 410 MHz Schottky detector (M. S. Sanjari et al., Review of Scientific Instruments 91 (2020), doi: https://doi.org/10.1063/5.0009094). The present status of the research can be found in: Yu. A. Litvinov & R. J. Chen, “Radioactive decays of stored highly charged ions”, Eur. Phys. J. A59 (2023) 102, doi: 10.1140/epja/s10050-023-00978-w.

A major focus of current investigations lies in two-body beta decays. The electron capture in hydrogen-like ions has indicated that the total angular momentum of nucleus + leptons system can significantly affect the weak decay rate. Much faster EC decays (than previously anticipated) were measured in hydrogen-like ¹⁴⁰Pr and ¹⁴²Pm ions. Now, the goal is to study the lithium-like systems, where predictions of the corresponding rates remain untested: R. S. Sidhu et al., “Electroweak decays of highly charged ions”, EPJ Web of Conferences 178 (2018) 01003, doi: 10.1051/epjconf/201817801003

In a bound-state β⁻-decay the generated electron remains in a bound state of the daughter atom rather than emitted to the continuum. In neutral or moderately ionized atoms, this decay mode is largely suppressed due to Pauli blocking of inner orbitals. The results in β_b being a marginal decay branch of neutral or moderately ionized atoms. Bound state beta decay has been discovered in the ESR. Several measurements were conducted to date, for a review see: Yu. A. Litvinov & R. J. Chen, “Radioactive decays of stored highly charged ions”, Eur. Phys. J. A59 (2023) 102, doi: 10.1140/epja/s10050-023-00978-w

One of the most demanding cases---the bound state beta decay of fully ionized ²⁰⁵Tl⁸¹⁺---was measured very recently at the ESR. This measurement took almost 30 years of development at GSI to bring to fruition Fully-ionized ²⁰⁵Tl⁸¹⁺ ions were produced via nuclear reactions using a ²⁰⁶Pb primary beam accelerated to 678 MeV/u and bombarded on a Be target, with fragments separated in the Fragment Separator (FRS). The parent ²⁰⁵Tl⁸¹⁺ ions were injected into the ESR where they were accumulated, stored, and measured. Decayed by bound-state beta decay, hydrogen like ²⁰⁵Pb⁸¹⁺ ions were detected after an argon gas jet stripped electron, altering their charge state from ²⁰⁵Pb⁸¹⁺ to ²⁰⁵Pb⁸²⁺ and orbit. Using a non-destructive Schottky detector, the decay half-life was inferred from the ratio of daughter to parent ions. The measured bound-state beta decay rate has been used to address the conditions at the early Solar system and the capture probability of solar pp neutrinos by ²⁰⁵Tl. More information can be found in the GSI public press releases:

- Long-sought measurement of exotic beta decay in thallium helps extract the timescale of the Sun’s birth

- Eyes on the Sun: Naked thallium-205 ion decay reveals history over millions of years 

The corresponding publications are:

- G. Leckenby et al., “High-temperature ²⁰⁵Tl decay clarifies ²⁰⁵Pb dating in early Solar System”, Nature 635 (2024) 321-326, doi: 10.1038/s41586-024-08130-4

- R. S. Sidhu et al., “Bound-State Beta Decay of ²⁰⁵Tl⁸¹⁺ Ions and the LOREX Project”, Phys. Rev. Lett. 133 (2024) 232701, doi: 10.1103/PhysRevLett.133.232701

Next goal is to explore bound-state beta decays of fully-ionized ¹³⁴Cs⁵⁵⁺, a key isotope for understanding barium nucleosynthesis and its abundance in the solar system.

Organizationally, this subject is pursued jointly by the APPA/SPARC and NUSTAR/ILIMA collaborations and is part of the HGF/MATTER/MML program.