Angular scattering of protons through ultrathin graphene foils: Application for time-of-flight instrumentation

Abstract

Space plasma instruments often rely on ultrathin carbon foils for incident ion detection, time-of-flight (TOF) mass spectrometry, and ionization of energetic neutral atoms. Angular scattering and energy loss of ions or neutral atoms in the foil can degrade instrument performance, including sensitivity and mass resolution; thus, there is an ongoing effort to manufacture thinner foils. Using new 3-layer graphene foils manufactured at the Los Alamos National Laboratory, we demonstrate that these are the thinnest foils reported to date and discuss future testing required for application in space instrumentation. We characterize the angular scattering distribution for 3–30 keV protons through the foils, which is used as a proxy for the foil thickness. We show that these foils are ∼2.5–4.5 times thinner than the state-of-the-art carbon foils and ∼1.6 times thinner than other graphene foils described in the literature. We find that the inverse relationship between angular scattering and energy no longer holds, reaffirming that this may indicate a new domain of beam–foil interactions for ultrathin (few-layer) graphene foils.

Precision Measurement of the Lamb Shift in Muonium

“…Graphene foils [51] are 5-10 times thinner than our currently used foil. As the scattering half angle is proportional to the foil thickness [52], our statistics could be increased by a factor 15-25. Therefore, the main systematic uncertainty, originating from a possible contamination of the M 2S beam of higher excited states, would become comparable with the statistical one.…”

Precision measurement of the Lamb shift in Muonium

“…These are 5-10 times thinner than our currently used foil. As the scattering half angle is proportional to the foil thickness [48], our statistics could be increased by a factor 15-25. With an order of magnitude more events, the main systematic uncertainty of the experiment, originating from a possible contamination of the M 2S beam of higher excited states, would become comparable with the statistical one.…”

Understanding Mass Resolution of Foil‐Based Time‐of‐Flight Mass Spectrometry

“…Angular scattering distributions are conventionally parameterized in terms of the scattering half‐angle ( ψ 1/2 ), which is defined as the half‐width at half maximum (HWHM) of the scatter distribution (Allegrini et al, 2016; Funsten et al, 1992, 1993, 1994; Funsten & Shappirio, 1997; Högberg & Norden, 1970). In laboratory experiments, the scatter distribution through a carbon foil is typically measured on an MCP (Ebert et al, 2014; Funsten et al, 1993; Vira et al, 2020). The measured distribution is then fitted to a cylindrically symmetric Gaussian, which is the expected shape of the distribution (Hanle & Kleinpoppen, 1979), to determine the scattering half‐angle.…”

“…The angular scattering distribution and the associated scattering half‐angle are useful metrics for characterizing the thickness of a foil (Fernandes et al, 2019; Funsten et al, 1992; Vira et al, 2020) but are not adequate for quantifying how angular scattering degrades the TOF measurement. To quantify the impact of the path length uncertainty on the instrument response, the probability distribution of ions exiting the foil must be calculated.…”

“…In Equation , the energy loss and straggling out of the carbon foil contributes to δE f /E f whereas the angular scattering of ions affects δd/d . Employing thinner foils minimizes these foil effects and enhances the measurement precision of the incident particle energy or speed (Allegrini et al, 2006, 2014; Ebert et al, 2014; Funsten et al, 1993; McComas et al, 1990; Vira et al, 2020). However, limited effort has gone into quantifying the extent to which carbon foils degrade the mass resolution of space‐based instruments that employ this TOF technique.…”