ExoMars Trace Gas Orbiter Instruments
FREND - Fine Resolution Epithermal Neutron Detector
The Fine Resolution Epithermal Neutron Detector (FREND) is part of the instrument payload on the ExoMars Trace Gas Orbiter (TGO), launched in March 2016.
FREND will be a neutron detector with a collimation module that significantly narrows the field of view of the instrument, thus allowing the creation of higher-resolution maps of hydrogen-abundant regions on Mars.
FREND will measure the flux of neutrons from the Martian surface; these are produced by the continuous cosmic ray bombardment that interacts with the first few metres of rock and regolith. The cosmic rays are sufficiently energetic to break apart atoms in the top one or two metres of the planetary surface, releasing high-energy neutrons that are then slowed down and absorbed by the nuclei of elements in the surrounding material. Not all the neutrons are captured – many escape, creating a leakage flux of neutrons that the FREND instrument will observe.
The distribution of neutron velocities, which depends upon how much they were slowed down before escaping, can reveal much about the surface material since it depends on the composition of that material – primarily on its hydrogen content. Even small quantities of hydrogen are known to cause a measurable change in the neutron velocity distribution from the surface of celestial bodies with thin or no atmospheres. This measured hydrogen content can be related to the presence of water.
Epithermal neutrons with energies in the 0.4 eV - 500 keV range will be detected by 3He counters, while neutrons with energies from 0.5 MeV - 10 MeV will be detected using a stylbene scintillator crystal.
FREND also contains a dosimeter module to monitor the local radiation environment. This helps to increase the accuracy of the neutron measurements by providing information about other radiation fluctuations (including space weather driven by the Sun) that can have an impact on the signals from the 3He and stylbene detectors.
FREND is designed to:
FREND builds on previous and current flight heritage, most notably the High Energy Neutron Detector (HEND) on Mars Odyssey; the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter; the Mercury Gamma and Neutron Spectrometer on BepiColombo; and DAN on MSL.