High-sensitivity measurement ofHe3−He4isotopic ratios for ultracold neutron experiments

Abstract

Research efforts ranging from studies of solid helium to searches for a neutron electric dipole moment require isotopically purified helium with a ratio of 3 He to 4 He at levels below that which can be measured using traditional mass spectroscopy techniques. We demonstrate an approach to such a measurement using accelerator mass spectroscopy, reaching the 10 −14 level of sensitivity, several orders of magnitude more sensitive than other techniques. Measurements of 3 He/ 4 He in samples relevant to the measurement of the neutron lifetime indicate the need for substantial corrections. We also argue that there is a clear path forward to sensitivity increases of at least another order of magnitude.

Accurate Determination of the Absolute 3He/4He Ratio of a Synthesized Helium Standard Gas (Helium Standard of Japan, HESJ): Toward Revision of the Atmospheric 3He/4He Ratio

“…The 3 He/ 4 He ratio of the HESJ was determined as 27.36 ± 0.11 ppm. This result can contribute to the improvement of the accuracy of neutron lifetime experiments (Arimoto et al, ; Nagakura et al, ; Mumm et al, ). With the present result and the averaged R HESJ / R a of 20.415 ± 0.029, the 3 He/ 4 He in the atmospheric helium was determined as 1.340 ± 0.006 ppm, which is consistent with the recent IUPAC recommendation value (Meija et al, 2016), but not with some of other previous determinations (Clarke et al, ; Mamyrin et al, ) with discrepancy of ~4%.…”

Optimizing neutron moderators for a spallation-driven ultracold-neutron source at TRIUMF

“…Reaching the required isotopic purity for dozens of liters of helium is difficult with conventional purification methods [41]. Instead we rely on commercially available helium with a 3 He abundance of 10 −12 [42,43], resulting in an absorption lifetime τ abs of several thousand seconds, so we assumed its contribution to be negligible. See table 2 for the range of assumptions made in the optimizations.…”