The BepiColombo SERENA/ELENA unit development: a new technique to detect sputtered neutral atoms escaping from Mercury surface

Abstract

ELENA (Emitted Low-Energy Neutral Atoms) is one of the four units of the SERENA experiment for the ESAcornerstone BepiColombo mission to Mercury. It is primarily devoted to understanding of Ion Sputtering processesand emission from planetary surfaces, particle back-scattering and Charge Exchange via neutral atoms detectionsin the energy range ? 20 eV - 5 keVELENA instrument is the first attempt of a new design techniques approached for the neutral particles identificationin the low energy range. It is a Time-of-Flight system based on a peculiar Start section: an oscillating shutter(operated at frequencies up to a 100 kHz) and mechanical grating (two self-standing silicon nitride (Si3N4)membranes, patterned with arrays of long and narrow openings) that allows to identify the start time of theparticles entering in the Time-of-Flight chamber. The Stop section at the end of the pattern is a 1-dimensionalarray composed by MCPs detector with discrete anodes corresponding to a Field of View of 4,5?x76?. This systemallows having the determination of velocity and direction of the incoming particles.The instrument has a good capability to reject UV photons with the start section and to reject charged particle witha deflector system.In this paper the crucial parts of the instrument and test results will be described: the nano-structure membranesmanufacturing, the shuttering system, the position encoder, the optical propriety of the membranes, the photonand particle test, the electronic box