Physicists at the University of California at Los Angeles, Yale University and Los Alamos national laboratories (all – USA) presented a fresh version of the standard rate, working on a nuclear transition of thorium-229.
The accuracy of atomic clocks is determined, we recall the natural width of the transition, on which a count, frequency, corresponding to the transition, and how to get low impact of disturbing external factors. The most accurate are the so-called optical frequency standards and time working on the transitions with frequencies in petagertsevom (1015) range. In the case of conventional cesium clock frequency is “only” 9.2 GHz.
Thorium isotope 229Th has isomeric level, which lies very close to the ground state, only (7,6 ± 0,5) eV from him. Such a low energy corresponds to a small wavelength that falls in the vacuum ultraviolet, and very high frequency.
The clock on the nuclear transition will differ markedly from the usual atomic, in the working area which is a rarefied gas: thorium to be placed in the volume of solid material. “Since the nuclear transitions are often less dependent on outside influences than the atomic, we expect a frequency standard using impurity 229Th in high-quality crystal, – says one of the sponsors, Eric Hudson (Eric Hudson). – Difficult and expensive installation, occupying an entire room will be replaced by a single crystal, operating, at room temperature. The limit of accuracy in this case, adds Mr. Hudson, would ask the interaction with electrons, determined by temperature changes of the crystal. “Unfortunately, even the most detailed calculations do not allow to accurately assess the significance of this effect, – said the scientist. – You can only build a working model and to experiment.
Researchers, however, confident that the clock in any case would not budge on the accuracy of modern atomic.
It is worth noting that the frequencies of nuclear transitions are also highly sensitive to changes in some physical constants – for example, the fine structure constant. “They are about six orders of magnitude more sensitive than electronic transitions in atomic and optical clocks, – says Mr. Hudson. – Consequently, and at not too high measurement accuracy, we can establish new, narrower limits of variation of constants.
Among the obvious requirements to crystals, which will contain thorium, are transparent in the vacuum ultraviolet region of the spectrum and the chemical “compatibility” with ions Th4 +. These properties demonstrate Na2ThF6, LiCaAlF6, LiSrAlF6, LiYF4, CaF2, and a number of other crystals. The authors have created a model of hours using LiCaAlF6 and more affordable than 229Th, isotope 232Th.