Direct Detection of the 229Th Nuclear Clock Transition

From left to right: Peter G. Thirolf, Lars v.d. Wense, Benedict Seiferle.

Authors: Lars von der Wense¹, Benedict Seiferle¹, Mustapha Laatiaoui^2,3, Jürgen B. Neumayr¹, Hans-Jörg Maier¹, Hans-Friedrich Wirth¹, Christoph Mokry^3,4, Jörg Runke^2,4, Klaus Eberhardt^3,4, Christoph E. Düllmann^2,3,4, Norbert G. Trautmann⁴, Peter G. Thirolf¹

Affiliations:
¹Ludwig-Maximilians-Universität München, 85748 Garching, Germany.
²GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany.
³Helmholtz Institut Mainz, 55099 Mainz, Germany.
⁴Johannes Gutenberg Universität, 55099 Mainz, Germany.

The measurement of time has always been an important tool in science and society [1]. Today’s most precise time measurements are performed with optical atomic clocks, which achieve a precision of about 10^-18, corresponding to 1 second uncertainty in more than 15 billion years, a time span which is longer than the age of the universe [2]. By comparing two of such clocks, which are shifted in height by just a few centimetres, also the time dilation due to general relativistic effects becomes measurable [3].

Despite such stunning precision, these clocks could be outperformed by a different type of clock, the so called “nuclear clock” [4]. The nuclear clock makes use of a nuclear transition instead of an atomic shell transition as so far applied. The expected factor of improvement in precision of such a new type of clock has been estimated to be up to 100, in this way pushing the ability of time measurement to the next level [5]. The reason for the expected improvement is the size of the nucleus, which is orders of magnitude smaller than the size of the atom, thus leading to significantly improved resilience against external influences.

Many potential applications for a nuclear clock are currently under discussion. These include practical applications such as improved satellite-based navigational systems, data transfer, gravity detectors [6] as well as fundamental physical applications like gravitational wave detection [7] and testing for potential changes in fundamental constants [8].

Using existing technology, there is only one nuclear state known, which could serve for a nuclear clock. This is the first excited nuclear isomeric state of ²²⁹Th. Among all known (more than 175,000) nuclear excitations, this isomeric state exhibits a unique standing due to its extremely low excitation energy of only a few electronvolts [9]. The energy is that low, that it would allow for a direct laser excitation of the nuclear transition, which is the prerequisite for the development of a nuclear clock.

The existence of this isomeric state was shown in 1976, based on indirect measurements [10]. However, despite significant efforts, the direct detection of the isomeric decay could not be achieved within the past 40 years [11]. In the recently presented work [12], our group was able to solve this long-standing problem, leading to the first direct detection of the ²²⁹Th nuclear clock transition. This direct detection is important, as it paves the way for the determination of all decay parameters relevant for optical excitation of the isomeric state. It is thus a breakthrough step towards the development of a nuclear clock.

Figure 1: (click on the image to view with higher resolution) Experimental setup used for the production of a purified ²²⁹Th ion beam and the direct detection of the isomeric state. For details we refer the reader to the text and to Ref. [12].

The detection was achieved by producing a low energy, pure ²²⁹Th ion beam, with a fractional content of ²²⁹Th in the isomeric state. The isomer was produced by making use of a 2% decay branch of the alpha-decay of ²³³U into the isomeric state. The setup used for ion beam production is shown in Fig. 1 and will be described in the following section. The ions were collected with low kinetic energy onto the surface of a micro-channel-plate (MCP) detector, triggering the isomer’s decay and leading to its detection at the same time. The obtained signal is shown in Fig. 2. A high signal-to-background ratio could be achieved owing to the concept of spatial separation of the ²³³U source and the point of isomer detection. Many comparative investigations were performed in order to unambiguously show that the detected signal originates from the ²²⁹Th isomeric decay [12].

Figure 2: ²²⁹Th isomeric decay signal as observed during 2000 second integration time on the MCP detector allowing for spatially resolved signal read out.

For the production of a low-energy ²²⁹Th ion beam, a ²³³U source was used, which was placed inside of a buffer-gas stopping cell, filled with 40 mbar of ultra-pure helium. ²²⁹Th isotopes, as produced in the alpha-decay of ²³³U, are leaving this source due to their kinetic recoil energy of 84 kiloelectronvolts. These recoil isotopes were stopped in the helium buffer-gas, thereby staying charged due to the large ionization potential of helium. The low-energy ²²⁹Th ions, produced in this way, were guided through the helium background towards the exit of the stopping cell by electric fields, provided by a radio-frequency funnel system. The exit of the stopping cell consists of a Laval-nozzle system, leading to the formation of a supersonic gas jet. This gas jet injects the ions into a radio-frequency quadrupole (RFQ) ion-guide, leading to the formation of an ion beam. This ion beam is further purified with the help of a quadrupole mass-separator (QMS). In this way, a low-energy, pure ²²⁹Th ion beam was produced, possessing a fractional isomeric content of about 2%.

The next envisaged steps towards the development of a nuclear clock will be performed within the framework of the EU-funded Horizon 2020 collaboration named “NuClock” (www.nuclock.eu). Experiments will be carried out that aim for a precise determination of the isomer’s energy and half-life as being the basis for the first direct laser excitation of a nuclear transition.

References:
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