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Atomic clocks are currently the gold standard when it comes to timing. These devices measure time based on transitions between two states of an atom. In two papers in Nature, Masuda et al.1 and Seiferle et al.2 signal progress toward a clock that uses transitions between two states of an atomic nucleus instead. Such a nuclear clock could surpass existing atomic timepieces and have applications in basic and applied physics.
Humans are trying to measure the time that has passed for thousands of years. From the sundial to the hourglass to the pocket watch, we have constantly tried to improve our ability to quantify and normalize time. In the early 1900s, scientists were struggling to define time consistently and to put in place various standards to help synchronize humanity. What was missing was a natural reference point that could be used, regardless of its location on Earth. We had to define what really meant a second: something fundamental that remains precise in every space, every millennium.
Scientists have realized that the properties of atomic transitions are independent of their location in space or time. This recognition has led to the idea of using a known transition between two atomic states as a means of defining time. If a normalized second could be defined as a specific and agreed number of atomic transitions, the time could be quantified. The researchers decided to do it in the 1930s and by the late 1940s the world had its first atomic clock.3.
Over the past 70 years, atomic clocks have been continually improved and currently have a precision4 about 1 part out of 1018. But what if we could do better than these devices? And if we could make a clock 100,000 times smaller, less sensitive to its environment and possibly a precision of 1 part out of 10?19? An atomic nucleus, about 100,000 times smaller than an atom, could provide such a device5.
Since 2003, researchers around the world have been trying to make a nuclear clock using the nucleus of a thorium atom 229.6. This nucleus, unlike all other known ones, has an excited state (called isomeric state) which is only a few electronvolts (eV) in energy above its ground state7. As a result, the transition between these two states is accessible using specialized lasers. The problem is that the exact energy of the isomeric state is currently unknown. Masuda et al. and Seiferle et al. have made progress in understanding the accuracy of the thorium-229 isomeric transition by conducting experiments that extend previous work7.
In Masuda and colleagues' experiments, a high-intensity x-ray beam passed through a pair of silicon crystals that narrowed the X-ray energy range to 0.1 eV. These X-rays were then used to irradiate a thorium nucleus 229 which was in the ground state (Figure 1). The nucleus has gone into a second excited state which has a much higher energy than that of the isomeric state. The narrow X-ray energy range allowed the authors to determine the exact energy of this second excited state: 29.19 keV. Finally, the nucleus is disintegrated directly in the isomeric state. Masuda's approach et al. to produce this state more efficiently than was previously possible.
Figure 1 | Low energy states and transitions of the thorium-229 nucleus. Masuda et al.1 report a technique for producing nuclei of thorium-229 atoms in an excited state called isomeric state. The authors irradiated a thorium nucleus 229 in the ground state with X-rays, which caused the nucleus transition to a second excited state of an energy of 29.19 keV (electron eV). volts). The core then is disintegrated in the isomeric state. Seiferle et al.2 observed a process known as internal conversion, in which a core of thorium-229 in the isomer state disintegrated in ground state and the neutral atom emitting an electron. By studying the energy of emitted electrons, the authors estimated the energy of the isomeric state at about 8.28 eV. These two studies could lead to ultra-precise clocks based on thorium-229 cores.
In the Seiferle experiment and his colleagues, a thorium-229 ion beam was generated from the natural decay of uranium-233 ions. About 2% of the thorium ions were in the isomeric state. These ions were then neutralized to allow them to disintegrate in the ground state through a process called internal conversion. In this process, a nuclear disintegration that typically produces a gamma ray will cause the neutral atom to emit an electron (Fig. 1). However, the internal conversion is complicated because the electron can come from many different energy levels in the neutral atom.
To observe the electrons ejected from the internal conversion, Seiferle and his collaborators used a magnetic field to bend the trajectory of these particles towards an electron detector. They applied an electric field to the electrons until the voltage associated with this field was large enough to stop the electrons. The final voltage was equal to the initial energy of the electrons. Seiferle et al. then used a theoretical model to interpret the spectrum of electronic energy, which is the first spectrum of energy observed from the decay products of the isomeric state. Their analysis indicated that the energy of the isomeric state was 8.28 ± 0.17 eV.
Although the ultimate and revolutionary goal of directly observing the isomeric transition of thorium-229 remains difficult to achieve, substantial progress continues to be made. The results of Masuda et al. and Seiferle et al. are key breakthroughs. Let's hope the observation is not too far away, as teams of scientists work to make the world's first nuclear clock, which would offer unprecedented accuracy. This discovery would allow a multitude of experiences and discoveries over the coming decades. For example, a nuclear clock could have applications in dark matter research8 and in observing possible variations of the fundamental constants of physics9.
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