Accurate determination of the absolute 3He/4He ratio of a synthesized helium standard gas (Helium Standard of Japan, HESJ): Towards revision of the atmospheric 3He/4He ratio

TL;DR

This study precisely measures the absolute 3He/4He ratio of the Japanese helium standard (HESJ) and the atmospheric ratio, providing crucial data for isotope applications and revising previous estimates.

Contribution

It directly determines the absolute 3He/4He ratio of HESJ and atmospheric helium, improving accuracy for isotope-based applications.

Findings

Absolute 3He/4He ratio of HESJ: 27.36 +/- 0.11 ppm

Atmospheric 3He/4He ratio: 1.340 +/- 0.006 ppm

Fabrication of control samples with 0.25-0.38% accuracy

Abstract

The helium standard of Japan, referred to as HESJ, is an inter-laboratory standard for the 3He/4He ratio. While the ratio of 3He and 4He of the HESJ was previously determined by a relative comparison to atmospheric helium, the absolute value of the 3He/4He ratio of the HESJ has not been directly determined yet. Therefore, it relies on the early measurements of that of atmospheric helium. The accuracy of the absolute 3He/4He ratios of the atmosphere and other working standards including HESJ is crucial in some applications of helium isotopes, such as tritium-3He dating, surface-exposure age determination based on cosmogenic 3He, and the accurate measurement of the neutron lifetime. In this work, new control samples of helium gases with 3He/4He ratios of 14, 28, and 42 ppm were fabricated with accuracy of 0.25-0.38% using a gas-handling system for a neutron lifetime experiment at Japan Proton Accelerator Research Complex (J-PARC). The relative 3He/4He ratios of these samples and the HESJ were measured using a magnetic-sector-type, single-focusing, noble gas mass spectrometer with a double collector system. As a result, the absolute 3He/4He ratio of the HESJ was determined as 27.36 +/- 0.11 ppm. The atmospheric 3He/4He ratio was determined as 1.340 +/- 0.006 ppm, based on this work.