Weak Decays in Highly Charged Ions
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 (νe)—in a symmetric form as:
n + νe ↔ 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 (βb): n + νe → p + e−b
3. Continuum β+-decay (β+c): p → n + e+ + νe
4. Orbital electron capture (EC): p + e−b → n + νe
5. Free electron capture (free EC): p + e−→ n + νe
In β+c and β−c continuum decays, energy and momentum are shared between the generated leptons. In contract, EC and βb-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 νe, 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 βb 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
Recent highlights
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:
- 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.