Calibrating instruments to analyze energetic neutral atoms in space

Energetic neutral atoms (ENAs), such as hydrogen and oxygen, move at high velocities through interplanetary space. They can arise from neutralized solar wind particles or physical processes on planetary surfaces, or they can originate outside the heliosphere and fall inward toward the solar system.

Because ENAs are electrically neutral, they are not affected by electric or magnetic fields. This makes the atoms pristine observables that carry key information about their sources. However, detecting ENAs at low energies is inefficient and requires thorough calibration. Gasser et al. developed an absolute beam monitor to calibrate a low-energy ENA imaging instrument.

At the device, a beam of neutral atoms passes through a defined aperture hole onto a tungsten crystal surface. There, the beam is reflected and releases electrons from the surface. The monitor records the rate of released electrons, the rate of reflected neutrals, and the rate at which these events coincide. By measuring the time difference in coincidence events, the researchers obtained the kinetic energy of the ENAs.

“The Absolute Beam Monitor is the first ever device that is capable of determining the flux of a neutral atom beam at energies below 500 eV in a calibration-independent manner,” said author Jonathan Gasser.

In a dedicated test facility at the University of Bern, the device will calibrate the neutral beam source, allowing tests of preflight instruments for space research. Current missions with ENA instruments include the Mars Express, Venus Express, Chandrayaan-1 lunar mission, and Mercury Plasma/Particle Experiment on BepiColombo, among others.

The existing beam monitor is a prototype, and the authors plan to further refine the calibration and enable real-time monitoring of neutral beam intensity.

Source: “Absolute beam monitor: A novel laboratory device for neutral beam calibration,” by Jonathan Gasser, André Galli, and Peter Wurz, Review of Scientific Instruments (2022). The article can be accessed at https://doi.org/10.1063/5.0092065 .