P5 229Th nuclear spectroscopy in solid-state

The 8 eV nuclear transition of the Thorium-229 nucleus presents an exciting and unique system for precision laser spectroscopy. A multitude of fundamental investigations and practical applications have been suggested, ranging from searches for physics beyond the standard model to the construction of an optical nuclear clock.

Prerequisite for these prospects is the ability to (coherently) manipulate the nuclear 2-level system with laser light, which so-far has remained elusive due to two main challenges:

  1. The transition wavelength is 148,7(4) nm in the vacuum ultraviolet regime, where no conventional laser spectroscopy sources are available.
  2. The nuclear transition is dipole-forbidden (M1 and weak E2), reducing the interaction cross section with light and hence the nuclear excitation probability by many orders of magnitudes compared to typical electron shell transitions. A narrow linewidth (order millihertz for a bare nucleus) and corresponding lifetime of hundreds of seconds makes absorption or fluorescence laser spectroscopy a formidable challenge.

In this project our team aims to overcome these challenges and realize laser spectroscopy of the 229Th nuclear transition by combining a solid-state crystal sample approach with a tailormade pulse train VUV laser system.

By doping 229Th into VUV transparent crystal matrices, the number of laser-interrogated nuclei can be pushed into the >1015 range. Even more important for this project, the thorium-crystal interaction leads to the emergence of additional quantum levels, both, in the nuclear (quadrupole structure) and the electronic (defect/colour centres) degrees of freedom. The key innovative aspect of this subproject is to exploit these additional quantum states to enhance the nuclear excitation probability or detection sensitivity by several orders of magnitudes.

In particular, we will exploit multi-photon excitation schemes and optical polarization of nuclear levels using light carrying OAM (OAM beams) or circular polarization as developed by the group from subproject P6. These approaches are intrinsically linked to the specific properties of the dedicated pulse train VUV laser system developed by the team of subproject P4.

There has been a rapid progress in determining the 229Th nuclear excitation wavelength and (radiative) lifetime in the last years. The latest value of 148,7(4) nm and 600(30) s (both in crystal environment), respectively, were published in April 2023. The population of the excited state however remains indirect. So-far, the direct, resonant excitation of the 229Th isomeric state using (laser) light has not been successful.

Even if one or several of the approaches that are made by other research teams we know of, are successful, they all probe the interaction of the nuclear 2-level system with linear polarized, resonant laser light.

Within COMB.AT, we will go beyond by introducing OAM beams and multi-photon excitation scheme. The key main goal of COMB.AT in the “nuclear pilar” is the excitation of the 229Th isomer using laser light and a determination of the transition wavelength to GHz resolution. The overarching challenge in this objective is to realize a high nuclear excitation rate despite the weak interaction cross section of the M1 (E2) transition with (conventional) laser light.

We will employ the VUV pulse train laser system developed by the team of subproject P4, which will deliver nuclear excitation rates of order 10-8-10-7 Hz, 1-2 orders of magnitudes higher than currently existing or planned approaches. With close theory support by the team of subproject P6, we will investigate whether the excitation rate can be further enhanced using OAM beams.

Team of P5

The sub-project P5 “229Th nuclear spectroscopy in solid-state” is led by Thorsten Schumm at the Institute of Atomic and Subatomic Physics of TU Wien. He can be considered as one of the leading figures in the applications of the 229Th nuclear transition, having initiated the most comprehensive research networks on the topic. Correspondingly, the team believes to be in direct contact with every group pursuing this topic around the globe.

The experiments concerning the excitation of the 229Th isomer will take place at TU Wien. Sample production, fabrication, and characterization of detection chambers will take place at the Institute for Atomic and Subatomic Physics (site of P5), laser development and excitation of the sample will take place at the Photonics Institute (site of P4). To our knowledge, we are the only group producing radioactively doped single crystal samples in Europe.

The following personnel were already part of the Schumms Quantum Metrology group before the start of the SFB COMB.AT and will continue to help the subproject team in their endeavors:

  • Thorsten Schumm (Professor): PI of the subproject P5
  • Enikoe Seres (Senior staff scientist): Laser specialist, working on solid-state VUV frequency comb
  • Josef Seres (Senior staff scientist): Laser specialist, working on solid-state VUV frequency comb
  • Adrian Leitner (Lab technician): Permanent staff for crystal growing and characterization
  • Stephan Schneider (Senior staff scientist): Project administration (finances, reporting)
  • Nadine Hilmar (Technical staff): Project presentation and outreach (website, meetings)
  • Heinz Matusch (Technical staff): Mechanical workshop
  • Andrew Pelczar (Technical staff): Electronics workshop
  • Martin Pimon (Postdoctoral Researcher)

They are currently operating 6 quantum optics labs and 1 crystal growing lab at the premises of the Institute for Atomic and Subatomic Physics, as well as 1 quantum optics lab in the 4th underground of the central Physics building at Karlsplatz (metrology hub lab). Note that 3 of these labs have special permits and installations for handling radioactive materials.

Thanks to the start of the SFB COMB.AT, 2 PhD students could join the team. One of them will focus on in house crystal sample development for the spectroscopy samples. Together with the lab technician (A. Leitner) and the DFT theory team (M. Pimon, A. Grüneis), the PhD student will identify the most promising host materials, develop fabrication processes, and characterize the obtained samples for optical and radiological properties. In particular, the presence and exact energy of defect centers is of utmost relevance and already represents an important scientific result.

The other PhD student will focus on the 229Th Isomer detection using Fluorescence detection and NQRS. Once the optimal host material(s) is/are selected, the detection schemes need to be adapted. In the case of optical detection, background rejection schemes need to be tailored to the material properties. In the case of NQR detection, the NQR splittings are exclusively caused by the local electric field gradient produced by the host material (at the nucleus position). These need to be calculated, and then the NQR electronics need to be adapted for the highest sensitivity in the expected wavelength regime.