Ground Calibration of the Helios ENA Rapid MApper (HERMA) for Heliospheric Boundary Exploration

Abstract The Helios ENA Rapid MApper (HERMA) is a new imaging instrument that features a novel concept and technical principles for detecting energetic neutral atoms (ENAs) from the heliospheric boundary. Based on a 256‐pixel semiconductor detector and one‐dimensional spatial modulation (coded aperture) imaging technology, this mapper enables all‐sky imaging of ENAs with high spatial and temporal resolution, which is of high significance for understanding scientific issues such as the interaction between the solar wind and the local interstellar medium. This paper describes its design and calibration experiments. During the experiments, the energy scale equations for all 256 pixels were obtained using 133 Ba and 241 Am radioactive sources. The mapper's energy and angular responses were systematically tested using proton and neutral hydrogen beams generated by a highly charged‐ion platform. The calibration results demonstrate that the energy resolutions of the 256‐pixel mapper for 100 keV hydrogen atoms range from 4.0% to 8.9%, its angular coverage in the modulation direction exceeds 45°, and its angular resolutions are 0.22°–0.60° in that direction (1.52° orthogonally, in simulation). Additionally, the integrated electrostatic deflection system in front of the detector is capable of deflecting 100 keV protons effectively. All measured performance parameters were in accordance with the design requirements. These calibration experiments verify that the instrument can effectively measure ENA fluxes and is suitable for the imaging detection of ENAs originating from the outer boundary of the heliosphere. The calibration methods and results obtained can serve as references for the future development of similar space detection payloads.