Thorium-229: How the first nuclear transition can be excited with lasers in the visible wavelength range

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Production of 229 mThese isomers through (a) resonant excitation or (b) excitation through the excited second nuclear state. The second scenario requires a large Lorentz factor of the ion group, which could be achieved at the Large Hadron Collider (LHC). Credit: Physical review research (2023). DOI: 10.1103/PhysRevResearch.5.023134

The isotope of thorium with the mass number 229 (229Th) is very exciting in many respects for fundamental physics and for future applications, for example in the sense of a nuclear clock.

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An international German-Chinese-American research group with the participation of Prof. Dr. Dmitry Budker’s group at the Johannes Gutenberg University in Mainz has now proposed a completely new approach to the study 229th in detail. The researchers want to use thorium ions which have only three electrons left in their shell out of the 90 present in a neutral atom.

Such a system offers many advantages, the researchers report in the current issue of the journal Physical review research, in particular that the first nuclear transition can be excited with conventional lasers in the visible wavelength range. This, however, requires the ions to circulate in a relativistic storage ring.

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A test laboratory for the new physics

The special feature of thorium-229 is that its atomic nucleus, with the metastable isomeric state of thorium-229m, has by far the lowest excited energy level of all the approximately 3,800 currently known atomic nuclei. It is therefore the only nuclear transition that can potentially be interrogated with lasers, even without the use of storage rings. The extremely precise measurement of this transition and of the two nuclear states opens up promising and different perspectives.

To this end, researchers led by Dmitry Budker are now proposing a new approach both in terms of “study object” and experimental setup: they are using highly charged ions, or HCI for short, specifically those in which only three electrons are present remained in the electronic shell. In such highly charged thorium ions, the interaction between the electron and the nucleus opens up some new transitions which can be used to “populate” the nuclear isomeric state efficiently.

The idea is to accelerate these thorium ions to nearly the speed of light in a particle accelerator. In this way they develop, so to speak, a leverage effect to excite them as effectively as possible with a conventional laser and thus be able to study them very precisely. More importantly, multiple excited states can be addressed and used to “populate” the isomeric state that is actually of interest.

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Most of the previous studies on Thorium-229m have dealt with non-relativistic atoms or ions in low charged states, which places high demands on the light source required for excitation because an extremely short wavelength laser is needed in the deep ultraviolet range. “The fact that we can use a laser in the conventional visible wavelength range makes spectroscopic studies easier,” explains Dmitry Budker.

“That this is possible is related to the fact that thorium ions are accelerated to nearly the speed of light. Due to relativistic effects, they perceive a laser beam directed at them from the front as a beam with a much longer wavelength short: to them, conventional laser light looks like a UV laser,” adds first author Junlan Jin, currently a Ph.D. student at Princeton University, who previously worked closely and successfully with Dmitry Budker’s group as part of a remote internship.

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In the present publication, the authors describe the various steps required to realize their method: starting with the generation of an accelerated beam of highly charged thorium ions, with possible accelerator rings at the FAIR facility under construction at GSI in Darmstadt, Germany, or the planned Gamma Factory at CERN, the authors of the current Thorium publication are also involved in the conceptual proposals for realizing such a “super light source”.

They then discuss in detail various scenarios for achieving the fullest possible excitation of thorium nuclei, before focusing on the detection of the excited states produced and transferability to similar systems.

The conclusion of the research team: according to their estimation, the energy of the isomeric state can be measured with an accuracy better than 10-4 or even down to less than 10-6, which is an order of magnitude improvement in the present value. This would pave the way for further improvements in the determination of isomeric state energy and help answer fundamental physics questions using the thorium system.

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‘The development of a nuclear clock is not so much the focus of our proposal, because our new method involves several technological challenges for its realization,’ adds Dmitry Budker.

“For us, however, thorium is a very large ‘playground’ for tackling questions of fundamental physics, a testing laboratory for new physics, so to speak. For example, we want to answer the question whether some fundamental constants of nature do not are perhaps so constant, but drift or oscillate with time or with location. Furthermore, one can imagine tests of fundamental symmetries and searches for particles or fields that go beyond the Standard Model.”

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More information:
Junlan Jin et al, Excitation and Probing of Low-Energy Nuclear States in High-Energy Storage Rings, Physical review research (2023). DOI: 10.1103/PhysRevResearch.5.023134

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