A new primary method of gas thermometry based upon Doppler-broadened mercury spectroscopy in the UV region


Funded by: Ministero dell’Istruzione, Università e Ricerca (MIUR)  
Calls: PRIN 2015
Start date: 2017-02-05  End date: 2020-02-04
Total Budget: EUR 768.201,00  INO share of the total budget: EUR 253.868,00
Scientific manager: Livio Gianfrani   and for INO is: Cancio Pastor Pablo

Organization/Institution/Company main assignee: Seconda Università di Napoli

other Organization/Institution/Company involved:

other INO’s people involved:

Abstract: In November 2014, at its 25th meeting, the General Conference for Weights and Measurements (CGPM) adopted a Resolution on the future revision of the International System of Units (SI). In the new SI, four base units (namely, kilogram, ampere, kelvin and mole) will be redefined fixing the numerical value of a set of fundamental Physical constants (Planck constant, elementary charge, Boltzmann constant and Avogadro constant), letting researcher free to chose the best practical realization of the units as long as they can be referred to the constants.
This is done to eliminate any artifact or material dependency and ensure the long term stability of the units.
In the same Resolution, the CGPM encouraged scientists to keep working on these constants to obtain refined data.
Among them, the Boltzmann constant (kB) shows the largest uncertainty.
Presently, the most accurate way to access the value of kB is from measurements of the speed of sound in a noble gas inside an acoustic resonator.
After many decades of research and technical developments, acoustic gas thermometry has recently provided kB determinations with a relative uncertainty smaller than one part per million (ppm).
Nevertheless, a refined value of kB should ideally be determined by at least three different methods, with comparable uncertainties.
The recent literature of the field has demonstrated that Doppler broadening thermometry (DBT) has the potential for being one of them.
DBT consists in retrieving the Doppler width from the highly accurate observation of the shape of a given atomic or molecular line in a gas sample at thermodynamic equilibrium.
If implemented at the temperature of the triple point of water, DBT can give an optical determination of kB, provided that a refined theoretical model is fitted to the measured data.
This project is aimed to develop a new primary thermometer based upon Doppler-broadened precision spectroscopy of mercury atoms in the UV region.
We intend to probe the 6 1 S 0 ->6 P transition at 253.7 nm of the 200 Hg bosonic isotope, with a twofold motivation. On the one hand, we plan to perform a spectroscopic determination of kB with an overall uncertainty of about 10-6, so as to contribute to the redefinition of kelvin.
On the other hand, we intend to provide a new powerful tool for the realization of the future international temperature scale, linking some fixed points to an absolute frequency in the UV region.
This experiment, once the kelvin will be redefined, will provide a primary realization of the kelvin.
The main requirement for attaining the target uncertainty is the availability of an absolute and highly reproducible frequency scale underneath the Hg spectrum.
This requirement will be satisfied by using the most advanced methods of frequency stabilization, measurement and control at deep-UV wavelengths.