ELENA (Emitted Low-Energy Neutral Atoms)

ELENA is devoted to the detection of Energetic Neutral Particles (ENA) in the energy range from 10 eV – 5 keV with the discrimination of the incoming direction. Neutral particles can be detected by a 1-dimensional array composed by MCPs and a discrete anodes set corresponding to a Field of View of 10°x76°, allowing the reconstruction of the direction of the incoming events. The spacecraft footprint track will provide the second dimension. The FoV is nadir directed. The Flight Model is mounted on MPO and the instument has been succsefull calibrated.

ELENA Flight Model

The main science objectives of the ELENA instrument are the following:

- Surface emission rate and release processes
- Particle loss rate from Mercury's environment
- Remote sensing of the surface properties
- ENA imaging applications for comparative solar-planetary relationship.

ELENA is a TOF (Time-of-Flight) sensor, based on the state-of-the- art of ultra-sonic oscillating shutter (operated at frequencies above 20 kHz and up to 50 kHz), mechanical gratings and MCP (Micro-Channel Plate) detectors. The purpose of the shuttering system is to digitize space and time when tagging the incoming particles without introducing "disturbing" detector elements, which may affect the particle's trajectory or the energy. This is particularly important in this case, in which neutrals of energies of a few tens of eVs must be detected.

ELENA Sensor

The ELENA sensor concept is showed in the figure above. The entrance of the start section (an aperture of about 1 cm2) allows the impinging neutral particles to enter through the shuttering system with a definite timing. Particles are then flown into a TOF chamber, and finally detected by a 1-dimensional array composed by MCP and a discrete set of anodes corresponding to a FOV of 4.5° x 76°, allowing the reconstruction of both, velocity and direction of the incoming particle events. The spacecraft footprint track will provide the second dimension.
The composite radiation made by neutrals, ions and light impinges onto the ELENA sensor entrance. A grid system placed between the main ELENA entrance and the shutter minimizes the ion and electron background radiation before the shuttering system. The internal ion deflector is a stack of particle cross-track grids which introduce a transversal E-field able to filter out the bulk of the charge particles of both signs. The unit defines intensity and direction of the incoming particles.
The energetic neutral particles that can be detected by ELENA are:
- Back-scattered and Sputtered neutrals. Precipitating protons reflected by the surface and neutralized during the reflection togheter with high energy sputtered particles.
- Charge exchange neutrals. Primarily H-ENAs of energy of 1 keV (detectable at apoherm, when a small part of the ELENA FoV looks tangent to the planet)

Performance parameters of ELENA