Helium Detection in Technical Materials

Materials used to study nuclear fusion can retain atmospheric helium unless pretreated before an experiment. Understanding helium outgassing is important for accurate diagnostics in experiments surrounding nuclear fusion. The presence of helium is often cited as the primary evidence that a nuclear reaction has occurred, so it is imperative that known sources of helium are mitigated prior to proceeding with novel nuclear experiments. It is also necessary to ensure hermiticity when transferring gas aliquots from an experiment to a mass spectrometer. In this article, we present studies of detecting helium leak rates in systems used in novel nuclear experiments. We also present studies of helium retention in materials subjected to various heating profiles and atmospheric concentrations. Without pretreatment, stainless-steel 316 retains between 15 $\unicode{x2013}$ 240 pmol of $^{ 4}$He or an areal outgassing amount of 0.07 $\unicode{x2013}$ 1.20 pmol/$cm^{ 2}$. It also may reabsorb $^{ 4}$He from the atmosphere in time. These studies also demonstrate that it is necessary to pretreat most materials prior to performing experiments where the presence of $^{ 4}$He is being used as an indicator for novel nuclear reactions.
Comment: 10 pages, 4 figures, 5 tables