ERC HIThor: Highly Ionized Trapped 229-Thorium
A New Paradigm Towards a Nuclear Clock
The HiThor Project
Owing to its exceptionally low-lying first excited level, the isotope Thorium-229 is the only viable candidate for a frequency standard based on a nuclear transition. Such a “nuclear clock” can be used for ultra-precise metrology, or as a means to find answers to fundamental questions in science such as: “Have the fundamental constants of nature changed since the birth of the universe?” Several groups and metrology institutes world-wide work on such a nuclear clock that eventually may even become a new standard of time and frequency.
Our concept contrasts the hitherto existing approaches: Within the ERC funded HiThor (Highly Ionized Trapped 229-Thorium) project we focus on very highly ionized thorium-229, i.e., the 229Th nucleus with only a few or no remaining electrons. Thus, we are dealing with very fundamental quantum systems composed of only very few “ingredients”.
The 229Th Nucleus
The Thorium-229 isotope has a first excited nuclear level (“isomer”) just 8.355 733 554 1034(83) eV above the ground state. This very precise measurement was accomplished in 2024 by means of laser spectroscopy of a CaF2-crystal doped with Thorium-229 by a team lead by the JILA/NIST in Boulder, USA. However, this energy is not the “true” nuclear transition energy: In a Thorium-229-nucleus without any electrons, i.e., in bare 229Th90+ the transition energy is only about 8.272(22) eV, due to the density effect of the electrons in the doped 229Th:CaF2crystal. Ground and isomeric state are connected by M1 transition with a lifetime of the isomeric state in vacuum of roughly 2500 s compared to 630(15) s in the crystal environment.
How to Produce Highly Ionized 229Thq+
Thorium-229 is a radioisotope with a half-life of about 7920 years. The present world-wide abundance of Thorium-229 is only a few hundred grams essentially from breeding in nuclear reactors. Alternatively, some of the present experiments use thorium-229 that is formed during the a-decay of an 233U-source.
In HiThor we produce the highly charged 229Thq+ ions in-flight in the accelerator chain of GSI, using a nuclear fragmentation reaction of either an 238U or,-- in the near future---, a 232Th primary. The primary beam impinges with typically 70% to 90% speed-of-light onto a thick production target. At this high energy not only nuclear fragmentation occurs with high probability but also most or all electrons are stripped of. In the reaction a cocktail of radioisotopes is produced, that subsequently is injected into the storage ring ESR where the separation of the isotopes is then performed.
The Essential Building Bricks of HiThor
The HiThor project rests on three major pillars:
1) Initial laser experiments with fast 229Th ions at the storage ring ESR to verify and characterize Nuclear Hyperfine Mixing.
Up to this date, nuclear hyperfine mixing has been predicted theoretically but has not been prooven experimentally. In the ESR, highly charged Thorium ions will be stored and overlapped with a laser for spectroscopy measurements. Taking advantage of the Doppler shift of the high velocity ions, the transition wavelength is tuned to the laser wavelength.
Two experimental runs will be performed at the ESR to commission the experimental setup and for data acquisition.
(contact: Dr. Carsten Brandau, GSI)
2) Development of a state-of the art VUV laser and frequency comb
The frequency comb method is a common tool to produce narrowband lasers with a well-known spectrum. The laser which will be used in the experiment consists of a home-build ytterbium-based infrared (IR) laser frequency comb including a low-noise oscillator and a chirped-pulse amplifier chain. The wavelength-tuning of the IR comb is accomplished by means of ultrafast serrodyne optical frequency shifting which enables a unique coverage of a wavelength range from 1000 nm to 1060 nm, i.e., a span of ± 30nm around a central wavelength of 1030 nm. The IR amplifier is followed by a passive enhancement cavity in which the IR laser can be converted to its 7th harmonic. Employing this unique wavelength tuning scheme, the VUV-wavelength band spans 151.429 nm down to 142.857 nm corresponding to 8.1876 eV to 8.6789 eV.
(contact: Dr. Christoph Heyl, HI Jena/DESY)
3) Experiments with ions at rest at the spectroscopy Penning trap SPECTRAP on the HITRAP platform
Ions at rest are an ideal setup for precision measurements of laser transitions as the linewidth reduces to the natural linewidth only. The transition is excited by the laser and the fluorescence is measured perpendicularly to the excitation laser over the entiry azimuthal angle. This is achieved by omitting the ring electrode. The harmonicity of the trap’s electric field is ensured by six electrodes, the geometry of which has been adjusted accordingly. Furthermore, the trap has been designed to capture fast ions from a beamline and cool them within a few milliseconds. The fluoreszence photons from theexcited stored ions.
(contact: Stefan Ringleb HI Jena / GSI)
