Refractive index measurement of nonpolar rarefied gas in quantum vacuum metrology standard

In the face of the historical change of international measurement system, the classical physics based physical standard corresponding to many measurement parameters develops toward "natural standard", namely quantum standard. In order to further improve the reproducibility and accuracy of vacuum value, the latest research uses quantum technology to realize the measurement and characterization of vacuum value. In this method, Fabry-Perot cavity is used to accurately measure the refractive index of the gas. The density can be calculated by the refractive index and inversed to obtain the corresponding vacuum value. The measurement of the gas refractive index is the key to the accuracy of the vacuum value. The macroscopic permittivity of nonpolar gases is related to the microscopic polarization parameters of atoms through quantum dynamics. In recent years, with the rapid development of ab initio theory and methods on the electromagnetic and thermodynamic properties of monatomic molecules, the calculation accuracy of relevant parameters was constantly improved, which can further reduce the measurement uncertainty of the above methods. In this paper, the theoretical value of helium refractive index is calculated accurately based on the first principle with known pressure and temperature. The relationship between gas pressure and refractive index is obtained, and the relative uncertainty of the theoretical value of refractive index is 6.27 x 10(-12). Then, the refractive index of helium in a range of 10(2)-10(5) Pa is measured by the vacuum measuring device which is based on Fabry-Perot cavity, and the uncertainty of measurement is 9.59 x 10(-8). Finally, the discrepancy between the theoretical and measured values of helium refractive index is compared and analyzed. It can be concluded that the the uncertainty of helium refractive index measurement originates from the deformation of the cavity caused by helium permeation. Therefore, solving the problem of helium permeation is the key to establishing a new vacuum standard. In this paper, the change of cavity length caused by helium penetration in the cavity is corrected. The refractive index coefficient is corrected at various pressure points in a vacuum range of 10(3)-10(5) Pa, and its pressure-dependent expression is obtained The variation of cavity length caused by gas pressure is further quantified. The relationship between the change of cavity caused by gas pressure and that caused by the refractive index is obtained. The correction parameter of cavity length is calculated to be 3.12 x 10(-2). In the future experiment of helium refractive index measurement by means of Fabry-Perot cavity, the refractive index correction coefficient at each pressure point given in this paper can be used to correct the refractive index measurement results, thereby eliminating the influence of helium penetration on the refractive index measurement, and obtaining the gas pressure with high accuracy.