We describe a method for determining density of helium via measurements of optical refractivity. In combination with the nist-equation of state, this allows realization of the pascal. Our apparatus is based on the integration of a gas triple-cell into a quasi-monolithic heterodyne interferometer: the stability of the interferometer is +/- 50 pm over 10 h. We claim the contribution of cell window thinning to pathlength uncertainty can be canceled within an uncertainty of 0.37 fm/Pa per window pass, which for our 25 cm cell length corresponds to a fractional error of 9.3 x 10^-6 in the measure of helium refractivity. We report the ratio (n-1)_N2 / (n-1)_He = 8.570354(13) at p = 367.420(4) kPa, T = 293.1529(13) K and λ = 632.9908(6) nm, which can be used to calibrate less accurate refractometers. By measuring helium refractivity at a pressure realized by a piston gage, we determined the Boltzmann constant with standard uncertainty k_B = 1.380652(17) x 10^-23 JK^-1.
atomic and molecular physics, interferometry, pressure measurement