FREND: Fine-Resolution Epithermal Neutron Detector for ExoMars Project


Introduction Вверх

FREND (Fine-Resolution Epithermal Neutron Detector) for ExoMars 2016 Trace Gas Orbiter spacecraft shall map neutron fluxes coming from Mars with high spatial resolution. Neutrons coming from the planet's surface are good indicator of hydrogen abundance in the shallow layer (up to 1 m deep) of the regolith. Such maps are therefore of great importance for future martian landing missions, since this information will help to choose landing sites more discriminatively.

FREND contains four 3He counters for neutrons with energies from 0.4 keV to 500 keV, and stilbene-based scintillator for high-energy neutrons (up to 10 MeV). All of them have narrow field of view (FOV) with pixel size corresponding to 40 km on the Martian surface.

The instrument also includes dosimetry module, which will constantly monitor radiation environment in orbit around the planet.

FREND is built within joint Roscomos-ESA ExoMars project to be placed onboard Trace Gas Orbiter spacecraft, due to launch in 2016.

ExoMars mission Вверх

ExoMars is a joint project of Mars exploration implemented under bilateral Agreement between European Space Agency (ESA) and Federal Space Agency (Roscosmos). Two missions and four spacecraft are foreseen within the ExoMars programme to be launched in 2016 and 2018 by two Proton launchers.

The first mission consists of Trace Gas Orbiter (TGO) spacecraft, built by ESA, with European and Russian instruments, plus an Entry, Descent and Landing Demonstrator Module (EDM, recently dubbed Schiaparelly), built by ESA. TGO shall stay in orbit and map planet's surface for at least 2 years. EDM will perform demonstration landing. Thereupon it will make some essential climatic measurements and will be switched off in few days after landing.

The second mission, in 2018, features a landing platform, built by Roscosmos, to deliver a rover to the surface of Mars. After this part of the mission is complete, the landing platform will deploy its own scientific payload. The rover, named Pasteur, is built by ESA and hosts European and Russian instruments, Both of the spacecraft are expected to work on Mars over a long period of time.

Legacy Вверх

In 2001 Russian instrument HEND (High Energy Neutron Detector) started operating onboard Mars Odyssey mission (NASA). It bears similar set of detectors (namely, three 3He counters). After 13 years of operation, detailed maps of neutron fluxes were built, corresponding to hydrogen content in the surface's upper layer. HEND's detectors have omnidirectional FOV, and therefore its spatial resolution is the distance 'from horizon to horizon', which in case of Mars Odyssey's orbit is about 300 km. Such resolution is insufficient to study hydrogen distribution in upper layers in details and hampers the choice of landing sites for new missions.

To narrow a neutron detector's FOV, it should be put inside a collimator, which can limit FOV to a small spot on the surface under the spacecraft. This collimator should be opaque for neutrons, coming outside nadir.

Up until now collimator of this kind, which narrows a neutron detector's FOV, has been used only once, in LEND (Lunar Exploration Neutron Detector) instrument onboard Lunar Reconnaissance Orbiter mission (NASA). The scheme has proved its efficiency. LEND data was used to map neutron fluxes coming from the Moon with spatial resolution of 5 km.

FREND's design uses the legacy of its predecessors, HEND and LEND, and exploits the technologies, which have already been tested and proved.

Goals and objectives Вверх

Main scientific goals of FREND instrument are:

  • to map epithermal and fast neutrons from Martian surface with spatial resolution of 40 km;
  • to map hydrogen content in the upper layer (with depth to 1.5 m) of Martian regolith with spatial resolution of 40 km;
  • to compare neutron fluxes from Martian surface emitted in various seasons;
  • to compare orbital data with those of similar instruments, both orbital and Mars-based;
  • to monitor neutron and charged particles fluxes during periods of high and low solar activity;
  • to monitor radiation environment in orbit around Mars, which includes heliosphere regions on distances of 1 up to 1.5 AU from the Sun and in orbit around Mars, to study contributions of electrons, protons, and HZE-particles to the total radiation dose in the martian orbit.

Technical Features Вверх

Mapping with high spatial resolution

To map the fluxes of neutrons (epithermal, thermal, and fast ones) and, thereafter, hydrogen distribution, FREND is equipped with a set of four 3He detectors and one stilbene-based scintillation crystal.

3He detectors are proportional counters filled with helium-3 under the pressure of 6 atmospheres and placed inside four openings of the collimator. Each of them counts neutrons independently, so that count statistics is higher (and, thus, maps are statistically more reliable) and instrument is more resistant to failures.

These detectors measure neutrons with energies from 0.4 to 500 keV. Scintillation counter, which uses stilbene crystal, measures fast neutrons with energies 0.5–10 MeV. It is also placed inside the collimator. Scintillation module includes anti-coincidence shielding, to discriminate between signals from high-energy charged particles and neutrons.

FREND's collimation module is a passive element, encasing all five detectors. The collimator consists of two layers, the outer one made of high-density polyethylene, the inner one of enriched boron powder (B10). Neutrons, hitting the collimator's sides, are slowed down by polyethylene, with large number of hydrogen atoms. So thermalized neutrons then pass through and get into B10 layer, which absorbs them.

Collimator's opening angle narrows FOV of the detectors to the spot with diameter of 40 km on the Martian surface, when seen from the circular orbit with 400 km altitude, allocated for TGO. Thus, spatial resolution of the neutron maps of Mars will be enhanced 7.5 times compared to HEND maps.

Radiation control

Liulin-MO dosimetry module measures the flux, absorbed dose, and dose intensity from cosmic rays charged particles, as well as spectra of their ionization losses onboard TGO. Ionization losses spectra are then converted into linear energy transfer (LET) spectra in human tissues, to estimate equivalent dose of cosmic radiation.

Main objectives of the dosimeter are:

  • radiation probing of Earth-Mars cruise trajectory, orbit around Mars, and Martian surface, with respect to radiation safety for future spacecraft;
  • flight tests of the system to detect flux, dose, and cosmic radiation LET spectra. Similar systems then can be used in radiation control sets onboard future spacecraft;
  • further refining of our knowledge of radiation environment in interplanetary space and on surface of Mars, to estimate exposure levels of spacecraft and maintain radiation safety of the crew during manned interplanetary flights.

Liulin-MO measures the flux, absorbed dose, and dose intensity from cosmic charged particles, as well as ionization losses spectra and energy release spectra in the detectors of the instrument. Liulin-MO's detection system consists of two telescopes with 2 semiconductor detectors (effective area of 2 cm2) each. Dosimeter's energy resolution is not lower than 100 keV in 100 keV÷8MeV energy release range (in the detector) and not lower than 350 keV in 8÷70 MeV range.

Design Вверх

Figure 1 gives general view of FREND instrument.

Fig.1. FREND. General view

The instrument consists of electronics and dosimetry modules (located separately, in the upper part of the figure), collimator module (several composite sections), which shields the detectors from radiation coming outside nadir direction, and five detectors (not seen in the figure).

The instrument is mounted via feet, as seen in the bottom part of the figure. On the opposite side of the instrument there is a radiator, which maintains thermal regime for the instrument.

The instrument's profile and inner architecture of the collimator and detectors are presented in Figure 2.

Fig.2. FREND's inner architecture

Main parameters Вверх

Mass : 36 kg
Power consumption : 14 W
Dimensions : 465 x 380 x 370 mm
Energy range : 0.4–500 keV (neutrons)
0.5–10 MeV (charged particles)
Time resolution : from 1 sec
Surface resolution : ~40 m
Depth resolution : ~1 m
Telemetry : 50 Mbit per day

Developers and co-executors Вверх

Funding organization — Federal Space Agency of Russian Federation.

Primary contractor — Space Research Institute of the Russian Academy of Sciences (IKI RAS).

FREND Principal Investigator — Dr. Igor Mitrofanov.

Works on FREND project are led under the State contract №025-5120/13/446 from December 23, 2013 (R&D theme ExoMars-YaF).

Works on the FREND project are planned for 2012–2016 (development, tests, assembling, and instrument delivery) and 2016–2017 (operating and data processing).

Space Research and Technology Institute of the Bulgarian Academy of Sciences
Liulin-MO dosimetry module development.
Federal State Unitary Enterprise 'Fedorovsky All-Russia Science and Research Institute for Mineral Resources' of Ministry of Natural Resources and Environment of Russian Federation
(VIMS, Moscow)
Scintillator module development.
Joint Stock Company "State Scientific Center Research Institute of Atomic Reactors"
(Dimitrovgrad-10, Ulyanovsk distr.)
Enriched B10 recovery, filling the collimator with B10 powder.
A.A. Blagonravov Institute for Engineering Science of the Russian Academy of Sciences
(IMASH, Moscow)
Design verification and mechanical testing.
Joint Institute for Nuclear Research
(Dubna, Moscow district)
Physical calibrations and modelling.
Institute of Biomedical Problems of the Russian Academy of Sciences
Dosimetry module development and testing support.