A Russian scientist from Skobelitsyn Research Institute of Nuclear Physics, MSU theoretically substantiated that the speed of transition of thorium-229 from ground to excited state may be managed depending on external conditions. The frequency of transitions may be increased or decreased by dozens of times. This effect will help create extremely precise clocks exceeding even the best atomic ones. The article was published in Physical Review Letters journal.

The most precise modern clocks are atomic ones in which time is registered on the basis of electron transition between energy levels. Recently scientists suggested switching from electron to nuclear transitions that may considerably increase the precision of clocks due to higher frequency.

However, in the majority of cases this frequency and corresponding energy are too high for the method to be applied. The main candidate to be used in such clocks is the nucleus of thorium-229.

Its low-energy transitions are unique and lead to the emanation of an UV-spectrum photon. The work with nuclei is complicated due to internal conversion that causes the energy released in the course of nuclear transition to be transferred to one of the electrones and not released as a photon.

The probability of an electron gaining energy instead of its transition to a photon in a thorium-229 atom is a billion times higher. However, if the atom is placed in a crystal with a wide band gap, the situation changes.

"My idea is that in a crystal electronic sheath may be completely rearranged, allowing us to observe nuclear radiation without conversion," - explained the author of the work Evgeny Tkalya from RINP, MSU.

In his new work he theoretically reviewed the transitions of a thorium-229 nucleus in a crystal with the whole system covered with an isolator, a thin dielectric film, or metal.

The author concluded that spontaneous emission can be controlled if the nucleus is placed within such bodies. This phenomenon is well-known for optic electron transitions and is called Purcell effect. Analysis has shown that the cover, depending on its size and properties, may change the transition speed up to 50 times. This process is specifically interesting in clocks, as the emission line becomes narrower as well allowing the mechanisms to keep time more accurately.

"This may increase the precision by an order of magnitude compared to thorium-based clocks that do not take this effect into account," - said the scientist. "Using these additional physical phenomena, we may reach relative precision over 10-20."

The main issue that hinders the development of a nuclear clock prototype is the lack of knowledge about transition energy. Currently the inaccuracy of measurements for this value is tenths of electron-volt (eV), and to efficiently excite the nuclei with external radiation, the inaccuracy should be reduced to the level of the exciting laser line width (about 10-5 eV).

The scientist also shared the results of experiments carried out by a group of researchers at MEPhI showing that the radiation can be controlled and proving theoretical provisions of his work.

Research paper

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Lomonosov Moscow State University
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En route to the optical nuclear clock
Munich, Germany (SPX) Apr 19, 2018
The nucleus of thorium-229 possesses a property that is unique among all known nuclides: It should be possible to excite it with ultraviolet light. To date, little has been known about the low-energy state of the Th-229 nucleus that is responsible for this property. Together with their colleagues from Munich and Mainz, researchers at the Physikalisch-Technische Bundesanstalt (PTB) have now performed the first-ever measurements - using optical methods - of some important properties of this nuclear ... read more