Helium: When You Must be Sure it’s Ultra-Pure

A new method detects residual contaminants in ultra-pure helium gas, critical to nuclear physics experiments.

Image courtesy of R.C. Pardo, Argonne National Laboratory.

The Science

The gas that makes balloons float is also vital to scientific experiments. In these experiments, natural helium (He) is purified, but it contains a tiny bit of a slightly different form of helium, known as the isotope ³He. A sample can contain just one ³He in every million helium atoms. That’s too much for many experiments. Many experiments require ultra-pure helium, with a ³He component at least another million times smaller, or one in a trillion of the He atoms. Although techniques are believed to produce ultra-pure helium, until recently no experimental methods have confirmed that the amount of ³He present in a sample is indeed that small. Now, scientists at the ATLAS facility at Argonne National Laboratory have used accelerator mass spectrometry (AMS) to precisely measure the very small concentrations of ³He present.

The Impact

Scientists need ultra-pure helium for a wide range of experiments. For example, they use ultra-pure helium to study the longevity and other properties of a free neutron. Free neutrons can provide insights into the formation of the universe and physics beyond the Standard Model, if measured accurately. To determine the purity of the helium for this study, the team demonstrated an approach that reaches a level of precision several orders of magnitude beyond that of any other technique. The team also found that measuring the amount of troublemaking ³He in purified helium samples intended for neutron studies suggest the need for significant experimental corrections, due to neutron absorption by the residual ³He present.

Summary

Answering difficult scientific questions about the nature of the universe requires isotopically puriﬁed helium (⁴He). The isotope ³He can contaminate the helium. Accurately measuring the amount of ³He requires determining the ³He/⁴He ratio at values well below those that can be achieved with standard mass spectroscopy techniques. Accelerator mass spectrometry provides the only way to directly measure the ³He content in purified helium samples at the level of sensitivity required for the neutron lifetime experiment, which seeks to determine how long a free neutron survives. Scientists used the ATLAS facility to demonstrate measurements of ³He/⁴He ratios as small as 10⁻¹⁴, or 1 in 100,000,000,000,000. In this work, scientists tuned the ATLAS accelerator, which serves as an ultra-precise mass filter, with specialized carbon ions. They scaled the accelerator components to ³He⁺. To reduce atmospheric ³He contamination, the team produced the ³He⁺ ions in a new radio frequency helium discharge source that reduced naturally occurring background sources of ³He. They monitored the final accelerator tune by regularly switching to H₃⁺ ions from high-purity hydrogen. They eliminated H₃⁺ ions and ions consisting of paired deuterium and hydrogen atoms by dissociation in a gold foil, after acceleration to 8 MeV. After stripping the second electron from the ³He⁺ ion, they dispersed the ions in a magnetic spectrograph and counted the ³He²⁺ ions. The team anticipates that these observations will also guide the design of future neutron experiments. Based on known improvements, an ultimate sensitivity to ³He/⁴He ratios as small as 10⁻¹⁵ appears to be feasible.

Contact

Richard C. Pardo
Argonne National Laboratory
[email protected]

Funding

We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing support for facilities used in this work. This work was also supported in part by the National Science Foundation under grant PHY-0855593 and by the U.S. Department of Energy (DOE), Office of Science, Oﬃce of Nuclear Physics, under contract DE-AC02-06CH11357. This research used the resources of the ATLAS facility, which is a DOE Oﬃce of Science user facility at Argonne National Laboratory.

Publications

H.P. Mumm, M.G. Huber, W. Bauder, N. Abrams, C.M Deibel, C.R. Huﬀer, P.R. Huﬀman, K.W. Schelhammer, R. Janssens, C.L. Jiang, R.H. Scott, R.C. Pardo, K.E. Rehm, R. Vondrasek, C.M. Swank, C.M. O’Shaughnessy, M. Paul, and L. Yang, “High-sensitivity measurement of ³He-⁴He isotopic ratios for ultracold neutron experiments.” Physical Review C 93, 065502 (2016). [DOI: 10.1103/PhysRevC.93.065502]