ResearchResearch Area B
Research propject B04

Project B04: Coherent excitation of a nucleus

The nucleus of 229Th with its isomeric state 229mTh at about 8 eV energy is a candidate for an ultra-precise nuclear optical clock, allowing for tests of fundamental principles. Nevertheless, the exact excitation energy of the isomer is not yet known. This experiment aims at the laser excitation of the isomer via the electron shell in 229Th+, 229Th2+ ions and the direct laser excitation of the nucleus in 229Th3+ ions for an accurate determination of its transition frequency.

INTRODUCTION

Among all known isotopes 229Th possesses a uniquely low-lying nuclear excitation state at 7.8(5) eV (≈ 160 nm) with an unperturbed lifetime in the range of several 1000 s, possibly accessible by laser radiation. The comparatively high transition frequency and lifetime allows for building a nuclear optical clock with unprecedented relative uncertainty in the ≤10-19 range. In comparison to transitions in the electron shell, the transition frequencies of nuclear transitions are less sensitive to external perturbation fields due to the smaller size and moments of the nucleus, causing smaller shifts on the transition frequency and therefore a higher accuracy and long-term stability of such a clock.

The high uncertainty in the isomer energy and the lack of tunable laser sources in this energy range require for an alternative method of the nuclear excitation by laser radiation: An indirect excitation of the nucleus might be feasible, using the electron shell as an “antenna” to transfer energy into the nucleus via hyperfine interaction. These processes are known as electronic bridge or NEET (Nuclear Excitation by Electron Transition).

Experiments with Th+ and Th2+ ions at Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig aim at the excitation of the nucleus using these processes.

RESULTS

Th+, a three-electron system, possesses a dense electronic level structure. This might lead to an enhanced excitation rate as well as decay rate of the nuclear isomer. This enhancement is expected to be at least 103 and increasing several orders of magnitude if the energy of the nuclear isomer is close to an electronic transition in the electron shell. More than 160 electronic levels in the expected range of the isomer energy were found in experiments at PTB, being available for NEET excitation tests of the nucleus.

Hyperfine structures of 229Th2+ and 229mTh2+ measured in cooperation with LMU Munich

Th2+, a two-electron system, shows a less dense level structure compared to Th+. Nevertheless, in the energy range of 8 – 9 eV it possesses electrons in the 8s orbital. Since electrons in s-orbitals have the highest probability of being in the nucleus compared to other electrons being in different orbitals, a strong hyperfine interaction and therefore higher isomer excitation probability via these levels is expected. Compared to the excitation of electronic levels in Th+, the laser power used for the excitation can be significantly higher because of the absence of three-photon ionization, leading to a reduced signal-to-noise ratio. The identification of a successful excitation of the isomer in Th2+ can be done by hyperfine spectroscopy of electronic transitions: Since the isomer lifetime in Th2+ is longer than 60 s, ions in the isomeric state might be identified by their different hyperfine structure, leading to an unambiguous proof of its excitation.

FUTURE

In future experiments, sympathetically cooled Th3+ will be used and the direct laser excitation of the nucleus will be attempted. Since Th3+ ions do not possess any electronic levels in the range of the isomer energy, the nuclear transition is not perturbed by the electron shell. For the laser excitation a four-wave-mixing laser system providing tunable laser radiation in the 160 nm range is planned.

UHV-System including the rf-Paul trap for sympathetically cooled Th3+ ions

PUBLICATIONS

  • D.-M. Meier, J. Thielking, P. Głowacki, M. V. Okhapkin, R. A. Müller, A. Surzhykov, and E. Peik (2019): Electronic level structure of Th+ in the range of the 229mTh isomer energyPhys. Rev. A 99, 052514
    DOI: /10.1103/PhysRevA.99.052514
  • Mueller, R. A.; Maiorova, A. V.; Fritzsche, S.; Volotka, A. V.; Beerwerth, R.; Glowacki, P.; Thielking, J.; Meier, D.-M.; Okhapkin, M.; Peik, E.; Surzhykov, A. (2018): Hyperfine interaction with the Th-229 nucleus and its low-lying isomeric statePhys. Rev. A 98, 020503(R)
    DOI: 10.1103/PhysRevA.98.020503
  • Safronova, M. S.; Porsev, S. G.; Kozlov, M. G.; Thielking, J.; Okhapkin, M. V.; Glowacki, P.; Meier, D. M.; Peik, E. (2018): Nuclear Charge Radii of Th-229 from Isotope and Isomer ShiftsPhys. Rev. Lett. 121, 213001
    DOI: 10.1103/PhysRevLett.121.213001
  • Thielking, J.; Okhapkin, M. V.; Glowacki, P.; Meier, D. M.; von der Wense, L.; Seiferle, B.; Duellmann, C. E.; Thirolf, P. G.; Peik, E. (2018): Laser spectroscopic characterization of the nuclear-clock isomer Th-229mNature 556, pages321–325
    DOI: 10.1038/s41586-018-0011-8
All publications of the Collaborative Research Centre

PROJECT LEADER

PD Dr. Ekkehard Peik
Address
PTB, Bundesallee 100
38116 Braunschweig
Address
PTB, Bundesallee 100
38116 Braunschweig

STAFF

Dr. Maksim Okhapkin
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig
Dr. Maksim Okhapkin
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig
David-Marcel Meier, M.Sc.
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig
David-Marcel Meier, M.Sc.
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig
Johannes Thielking, M.Sc.
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig
Johannes Thielking, M.Sc.
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig
Gregor-Alexander Zitzer, M.Sc.
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig
Gregor-Alexander Zitzer, M.Sc.
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig