Abstract
Today’s most precise time and frequency measurements are performed with optical atomic clocks. However, it has been proposed that they could potentially be outperformed by a nuclear clock, which employs a nuclear transition instead of an atomic shell transition. There is only one known nuclear state that could serve as a nuclear clock using currently available technology, namely, the isomeric first excited state of 229Th (denoted 229mTh). Here we report the direct detection of this nuclear state, which is further confirmation of the existence of the isomer and lays the foundation for precise studies of its decay parameters. On the basis of this direct detection, the isomeric energy is constrained to between 6.3 and 18.3 electronvolts, and the half-life is found to be longer than 60 seconds for 229mTh2+. More precise determinations appear to be within reach, and would pave the way to the development of a nuclear frequency standard.
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Acknowledgements
We acknowledge discussions with D. Habs, T. W. Hänsch, T. Udem, T. Lamour, J. Weitenberg, A. Ozawa, E. Peters, J. Schreiber, P. Hilz, T. Schumm, S. Stellmer, F. Allegretti, P. Feulner, J. Crespo, M. Schwarz, L. Schmöger, P. Micke, C. Weber, P. Bolton and K. Parodi. We thank T. Lauer for the Ti sputtering of the Si wafers and the MPQ for the temporary loan of the MCP detector. We thank I. Cortrie, L. Black and J. Soll for graphic design support. This work was supported by DFG (Th956/3-1), by the European Union’s Horizon 2020 research and innovation programme under grant agreement 664732 “nuClock” and by the LMU department of Medical Physics via the Maier-Leibnitz Laboratory.
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L.v.d.W., B.S. and P.G.T. performed the experiments. M.L. and J.B.N. did preparatory experimental work. H.-J.M. and H.-F.W. produced the radioactive source 1. C.M., J.R., K.E., C.E.D., N.G.T. and L.v.d.W. produced the radioactive sources 2 and 3. L.v.d.W., P.G.T. and B.S. wrote the manuscript with input from all authors.
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Extended data figures and tables
Extended Data Figure 1 Schematic of the experimental process.
Daughter nuclides of the 233U decay chain leave the 233U source owing to the kinetic recoil energy transferred to the nucleus during the α decay. Only those nuclides produced by α decay have enough kinetic recoil energy to leave the 233U source material efficiently. The maximum layer thickness through which recoiling nuclei can pass is a few tens of nanometres. The α-recoil nuclei are thermalized with helium and extracted from the stopping cell. The process of electron capture during thermalization leads to the formation of ions in the 1+, 2+ or 3+ charge states. Subsequently, an ion beam is formed and purified with a quadrupole mass-separator such that only 229Th remains. The thorium ions are collected by soft landing on the surface of a microchannel-plate (MCP) detector and the isomeric decay is detected. Major components shown here are described in detail in Extended Data Fig. 2 legend.
Extended Data Figure 2 Overview of the experimental setup.
The buffer-gas stopping cell houses the 233U source, which is mounted onto the front end of a DC cage electrode system56. The 229Th α-recoil ions emitted from the source are stopped in the buffer-gas stopping cell filled with 40 mbar helium. These ions are then guided by an electric RF+ DC funnel system towards the exit of the stopping cell formed by a supersonic Laval nozzle, which injects them into a radio-frequency quadrupole (RFQ) ion guide, where an ion beam is formed by phase-space cooling due to the remaining helium pressure of 10−2 mbar. Following the RFQ, the ion beam is purified after a mass-to-charge separation with a quadrupole mass-separator (QMS). Behind the QMS a microchannel plate (MCP) allows for the detection of the low-energy internal conversion (IC) electrons emitted in the 229Th isomeric decay. Boxed area at right is shown magnified in inset.
Extended Data Figure 3 Intensity profile measurements.
Upper panel, mass spectrum in the range of the 2+ ion species as performed with the chemically unpurified 233U source 1 and an MCP detector (Methods) operated in single-ion counting mode. Lower panel, ion impact profile measurement (−900 V MCP surface voltage, 1 s exposure time) performed with 233U source 1 and an MCP detector allowing for spatially resolved read-out (Methods). The 229Th and 233U mass peaks can clearly be separated.
Extended Data Figure 4 Different classes of decay events as observed during ion accumulation on the MCP surface.
In order to suppress any ion-impact signal, soft landing of the ions is guaranteed at −25 V MCP surface voltage. Single frames of 4 s exposure time are shown. The MCP detector used (Methods) allows for spatially resolved image read-out. The extracted ion species is chosen by mass-to-charge separation with the help of the QMS. a, Alpha decays originating from 221Fr. b, Beta decays originating from 209Pb. c, Isomeric decay of 229Th. d, Isomeric decay of 235U. In the frames shown all ions were extracted in the 2+ charge state from the chemically unpurified 233U source 1.
Extended Data Figure 5 Chart of nuclides potentially contained in the source material.
The chart includes all elements from curium (Cm, Z = 96) to mercury (Hg, Z = 80). All nuclides drawn are taken into consideration for the exclusion of a potential nuclear background. For completeness, all potentially populated nuclides are shown, even if their activity can be assumed to play a negligible role owing to a small branching ratio or a long half-life of the mother nuclide. These nuclides are shown without colour. Nuclides that can potentially recoil from the source as populated via α decay are assigned a white circle. Nuclides that possess one or more isomeric states carry a white star. A complete list of potentially contributing excited isomers is given in Extended Data Table 3. The short forms a, d and s are used for years, days and seconds. SF is short for spontaneous fission.
Extended Data Figure 6 α-energy spectra of different Si-detector-based measurements, each accumulated for 7,200 s.
A silicon charged particle detector (Methods) is used for detection. The extracted ion species is chosen by mass-to-charge separation by the QMS. a–d, The accumulated counts are shown for extraction from the chemically unpurified 233U source 1 for 213Bi2+ (a), no extraction (that is, dark counts, b), 229Th2+ (c) and 229Th3+ (d). No signal above the background is detected for 229Th in the 2+ and 3+ charge states. This clearly excludes any α decay as signal origin.
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von der Wense, L., Seiferle, B., Laatiaoui, M. et al. Direct detection of the 229Th nuclear clock transition. Nature 533, 47–51 (2016). https://doi.org/10.1038/nature17669
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DOI: https://doi.org/10.1038/nature17669
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