Triple-Point Temperatures of ²⁰Ne and ²²Ne

Abstract

Interest in the triple points of neon isotopes has recently arisen as a result of the multi-institute project to understand how variations in the isotopic composition of natural neon influence the triple-point temperature of a particular sample. Given the challenges in determining with sufficiently low uncertainty both the relative isotopic concentrations, particularly of ²²Ne relative to ²⁰Ne, and the temperature differences among cells filled from different gas sources in order to determine the sensitivity coefficient with adequate confidence, modeling of the isotopic influence becomes an attractive alternative for correcting the triple points of “natural” neon samples to a common composition. The modeling requires, among other things, knowledge of the triple-point temperatures of ²⁰Ne and ²²Ne. The triple points of these pure neon isotopes have utility in their own right as secondary reference temperatures on the ITS-90, and one or both of these could replace “natural” neon in a revised International Temperature Scale or approximation to the ITS. The neon isotope triple-point temperatures (on the IPTS-68) reported by Furukawa, Kemp, and Sakurai in various publications from 1972 to 1986 predate the ITS-90, and so there is utility in contemporary measurements (on the ITS-90) with gas samples of recent production. At NRC, we find the triple point of ²⁰Ne to be at 24.54230 K (13.80 mK below the triple point of “natural” neon) and that of ²²Ne to be at 24.68889 K (132.79 mK above the triple point of “natural” neon), assuming the reference sample of “natural” neon realizes the ideal ITS-90 temperature of 24.5561 K.