Russian neutron detector HEND for the NASA space mission 2001 Mars Odyssey

ЗProject Objectives Вверх

The Gamma Ray Spectrometer (GRS) suit is a part of scientific payload of the 2001 Mars Odyssey spacecraft. It consists of a set of independent instruments for studying Martian soil composition by neutron and gamma-ray spectroscopy methods. As a part of the suit Russian HEND instrument is designed to register high energy neutrons. The name HEND is abbreviation for 'High Energy Neutron Detector'.

Data obtained with nuclear spectroscopy of the Martian subsurface can pertain to several branches of Martian science. The main task was to determine chemical composition of the Martian subsurface. Gamma-rays are born at the depth of several tens of centimeters, thus allowing to estimate the abundances of main rock-forming elements, such as H, O, Mg, Al, Si, S, Cl, K, Ca, Mn, Fe, Th, U, in the subsurface upper layers, as well as in dust, stones, and rock debris on the surface. Although this deposited layer is rather thin and does not reflect the composition of the bedrock, it still holds important information about climatic conditions and erosion which have been ruling and still rule on Mars, being the main cause for surface upper layer formation in the previous ages as well as now.

The bedrock problem can be addressed partially on the basis of analysis of ancient volcanoes and meteoritic craters. Here the upper layers of Martian soil contain substantial amount of volcanic rocks or bedrock debris ejected due to volcanic activity or meteoritic impact.

Gamma spectroscopy provides data on the processes, which led to the formation of planet crust from the ocean of molten magma. GRS instrument's sensitivity and resolution are sufficient to determine the abundance of natural radioactive elements with long half-lives of billions of years, such as potassium (K), thorium (Th) and uranium (U). Their relative abundances are holding a clue to the processes having occurred in the very beginning of Martian evolution.

Absolute concentration of chemical elements building up the subsurface of Mars is derived from the intensity of nuclear lines, which in turn depends on spectral density of neutron flux that interacts with the nuclei of main rock-forming elements. To derive the latter, the neutron spectroscopy is used, which allows to estimate the neutron albedo of Mars in wide energy range.

Thus follows the first scientific objective of the HEND experiment, that is:

  • to estimate the absolute value of fast neutron flux from Mars in various spectral ranges.

Continuous observations from circular polar orbit gives an opportunity to solve the task for different regions of Mars, that is to make a map of neutron flux spectral density from Mars. One of the task of the HEND experiment is to measure epithermal and fast neutrons flux in energy range 0.4 eV — 15 MeV. Hence, HEND data can be used to derive the intensity of gamma-rays lines, which are born through nuclear reactions of neutron inelastic scattering.

Neutron spectroscopy is highly sensitive to the presence of light nuclei in the subsurface upper layers, especially to hydrogen-bearing compounds. Many years of Mars research have shown that it is hydrologically active planet. In the past, rivers were flowing over its surface, and, probably, vast territories were occupied by oceans. Even though today climatic conditions do not favor liquid water existence on the Martian surface, still, water ice can be present underneath. The most favorable conditions are in polar and circumpolar regions of the planet, where permafrost depth is minimal.

Consequently, the second objective of the HEND experiment is:

  • to map the hydrogen-abundant regions of the Martian surface and to determine mass fraction of water ice and chemically-bound water, as well as to estimate the depth of hydrogen-bearing layer occurrence.

This task can be solved independently, using HEND data only. Basically, water abundance in the Martian subsurface can be determined from the magnitude of decrease of epithermal and fast neutron fluxes. The depth of hydrogen-bearing layer occurrence is calculated on the basis of comparative analysis of different energy range, as the maxima of epithermal and fast neutrons production are located on different depths. However, actual solution is far more complex and involves numerical methods to model the neutron flux at Martian orbit.

It is known that Martian atmosphere goes through seasonal variations, with about quarter of all atmospheric carbon dioxide condensing on the surface of Martian polar regions during autumn and winter. The thickness of precipitated CO2 can vary from several tens of centimeters to about one meter. Neutron spectroscopy can be used to observe seasonal cycles on Mars and to determine the thickness of seasonal cover layer made of precipitated CO2. Hence, the third scientific objective to be addressed with the help of HEND data is:

  • to look for seasonal variations of Martian neutron albedo over polar regions. By applying model-dependent approach, to determine surface density and composition of seasonal polar caps (as seasonal deposits of carbon dioxide may contain admixtures of water and dust).

Successful completion of the objective shall supply us with plethora of new information about patterns in Martian climate as well as allow us to map the distribution of seasonal deposits in polar regions and to understand the details of volatiles (such as CO2 and water vapor) transfer through the Martian atmosphere.

Main priority in the HEND scientific program is given to the investigation of the Martian surface. Still, mission parameters and instrument's characteristics provide an opportunity to address several additional and not less significant scientific objectives.

The 2001 Mars Odyssey interplanetary flight takes about 7 months, which falls on the period of increased solar activity. During solar flares the spacecraft is exposed to intense flux of high energy charged particles, which in turn increases the background radiation from the spacecraft registered by HEND detectors. Using time profile and spectral hardness of induced radiation, one can study solar activity at the great distance from Earth. If the angle between Earth and Mars is great enough, the stereoscopic image of a solar flare can be derived. Feasibility of such observations leads to the fourth scientific task of HEND experiment, that is

  • temporal and spatial analysis of solar activity events during cruise to Mars as well as on the orbit around Mars and stereoscopic imaging of separate solar events.

A number of severe solar events were registered during the cruise phase and even after the commencement of the main mission, aimed at planetary surface mapping by HEND instrument. During several of these events, the signal exceeds the background by several orders of magnitude. Important information complementing Earth-based observations was obtained.

Along with neutrons the detectors comprising HEND instrument register soft and hard gamma-radiation in the range from 60 keV to several MeV. Apart from studying solar flares, this range can be exploited to study the cosmic gamma-ray bursts (GRB). Although HEND's sensitivity allows to register powerful GRBs only, this is sufficient for getting useful information on time profiles and spectral characteristics of GRBs. The remoteness of the spacecraft from Earth is advantageous, as the data on gamma-radiation can be used for GRBs' interplanetary triangulation. Then, an additional scientific task of HEND experiment can be defined:

  • GRB registration and participation in the international triangulation network (IPN) for transient sources positioning determination.

Several hundred GRBs were registered during HEND's operating. Sources' coordinates were defined and X-ray and optical afterglows were registered for some of them.

Operation Principle Вверх

HEND uses registration of secondary neutrons coming from the Mars, which are born in the shallow layer of Martian subsurface within the depth of 1-2 m, that is constantly bombarded by cosmic rays. High energy neutrons born in the subsurface are moderated and captured by main rock-forming nuclei through the reactions of inelastic scattering and capture. The neutron flux emanating from the soil depends on soil composition, and first of all, on presence of hydrogen or hydrogen-bearing compounds. Having collided with a hydrogen nucleus, a neutron immediately loses half of its energy, thus leading to quick thermalization and hence to significant increase of thermal neutron and decrease of epithermal neutron fluxes. It is possible to make conclusion about Hydrogen concentration in the soil using measurements of neutron flux spectral density from Martian orbit.

The instrument is a spectrometer with four independent neutron detectors. Three detectors of epithermal neutrons (SD, MD, LD) are proportional gas counters filled with 3He, while the fourth detector of high-energy neutrons registration (SC) is made from stilbene C14H12 surrounded by active anticoincidence shield made from CsI(Tl) crystal.

Design and Mount Вверх

2001 Mars Odyssey scientific payload:

  • GRS instrument suite includes the gamma-ray spectrometer (GRS) itself, thermal neutron detector (NS) and high-energy neutron detector (HEND) developed at IKI RAS under the contract with Rosaviakosmos (now Federal Space Agency of Russian Federation ). The instrument is designed to study elemental composition of Martian surface and search for water.
  • THEMIS (the Thermal Emission Imaging System), which main tasks are to study Martian surface in visible and infrared emission range.
  • MARIE (the Mars Radiation Environment Experiment) dosimeter to measure radiation level in open space and further analysis of its hazardous effects upon humans.

Fig.1. 2001 Mars Odyssey spacecraft. (Photo courtesy NASA - http://mars.jpl.nasa.gov/odyssey/mission/spacecraft/)

HEND's physical concept was chosen so that to cover maximum neutron energy range from 0.4 eV to 15 MeV within mass limit (no more than 4 kg) with sensitivity sufficient for unambiguous interpretation of data.

Fig. 2. Thermal neutrons spectrum registered by the proportional counter filled with 3He

Three detectors of the HEND instrument: SD, MD, and LD (LD — Large Detector, MD — Medium Detector, SD — Small Detector) — are built using neutron proportional counters filled by 3He, enclosed by various polyethylene moderator layers (Fig.3 and 4). These counters register thermal neutrons by neutron-capture reactions with 3He nuclei, which results in birth of tritium and proton.

3He + n = 3H + p + 765keV

Fig.3. Chart of LD detector (up) and scintillation module (bottom)

The industrial 3He proportional neutron counters LND 2517 were used in the instrument. When neutrons from outside get into the detector, they are moderated in the polyethylene layer down to thermal energy and are registered by proportional counters. The moderation efficiency in the polyethylene depends on its thickness, so that LD detector with the thickest moderator layer about 30 mm is the most sensitive to neutrons with energies 10 eV—1 MeV. MD detector with the moderator layer 14 mm thick registers neutrons with energies 10 eV—100 keV. SD detector with the thinnest moderator layer (3 mm) registers mostly neutrons with energies from the 'Cadmium threshold' ~0.4 eV up to 1 keV. Thus, the set of three detectors SD, MD, and LD covers wide range of neutron energies from 0.4 eV up to 1 MeV.

The scintillation detector IN/SC (Fig.3, bottom) is used in HEND instrument to register neutrons with energies higher than 1 MeV. It is based on organic stilbene scintillator. This scintillator is able to register high energy neutrons by light flashes from recoil protons, knocked out from crystal lattice. However, in space environment stilbene would detect not only recoil protons, but primary protons of cosmic rays as well. Moreover, both cosmic rays electrons and secondary electrons born by gamma-photons will be registered.

To discriminate between pulses from protons and pulses from electrons, special shape separation circuit was developed. This circuit uses the differences between signal shapes from optical flashes for different particles. During HEND testing it was shown that the circuit provides sufficiently effective discrimination between proton and electron pulses. The probability of false registration of electron's signal as proton's corresponds to the probability at the level 5*10-4.

Fig.4. HEND scheme with SD, MD, and LD detectors based on 3He proportional counters and stilbene scintillation detector Sc/IN with the anticoincidence detector Sc/OUT made from CsI(Tl).

Fig. 5 shows the general view of HEND flight unit №2.

Fig.5. General view of HEND flight unit №2 with technological feet as seen from the X- and Y-axes intersection (the ruler's scale in cm)

To separate recoil protons from primary protons of cosmic rays, we used additional scintillation detector OUT/SC made from CsI(Tl) to provide an anti-coincidence shield. The latter screens the IN/SC stilbene detector in the direction of the sky, but leaves open the directions pointing to Mars surface. If a charged particle comes through the OUT/SC outer detector, the instrument issues veto logic signal, thus suppressing pulse registration by the inner IN/SC detector.

HEND's construction implies that all its 4 detectors are spatially separated and turned relatively to each other, for them to have the best fields of view from the circular polar orbit with the altitude of approximately 400 km.

Design Features Вверх

Main requirements to HEND's design concerned mass, power and temperature limits, as well as its mechanical strength.

The instrument's mass should not have exceeded 4 kg (including power and data transfer cables), its power consumption should have been less than 6 W (in case the instrument is switched off, its average heater power should not exceed 3 W). Besides, one had to consider strict limitations on heat transfer between HEND and the spacecraft's science deck, where the instrument is mounted. Mechanical strength was an important factor, as during the launch and insertion to the orbit the spacecraft would be exposed to significant vibration and shock load.

A special procedure has been implemented to optimize the detectors' masses. It included both selection of detectors among domestic and foreign manufacturers on the basis of sensitivity-to-mass ratio and multistep (trial-and-error) process of harmonization, which ultimate goal was to get the most light and compact construction within the acceptable limits for mechanical strength. Eventually, after two years a compromise was found between the detectors' sensitivity and mass requirements put forth by the project.

HEND's electronics were developed within the same framework of mass and radiation resistance limitations, as well as the amount of emitted heat. The instrument's electronics includes analog units (primary signal processing, Fig. 6a), high-voltage boards (high-voltage feed to the detectors, Fig. 6b) and digital electronics (digital signal processing, Fig. 6c), which were implemented on the basis of FPGA Actel chip (unreprogrammable). Power consumption was divided equally between high-voltage and digital units: half of the power supplies digital units, while the other half — high-voltage boards.

Fig. 6. Analog board (a); High-voltage boards (b); Digital electronic boards with Actel chip (c); General view of detectors and electronics boards (disassembled) (d).

HEND design was developed so that it is thermally isolated from the spacecraft by heat insulators installed between HEND footprints and spacecraft's science deck. These insulators exclude heat transfer due to thermal conductivity. Radiation exchange between the spacecraft and HEND instrument was minimized by surrounding multilayer thermal insulation (MLI). Thermal regimes are maintained by compensation of internal heat generation (~6 W in operation regime) and radiation losses through passive radiative cooler directed at open space. In stationary normal operation regime HEND temperatures for are around 12º C for the electronics and around 0º C for the housing.

Situations were anticipated during mission lifetime, when all scientific payloads would be switched off. These were expected to be planned switch-offs, for example, during launch or insertion to orbit around Mars, or spontaneous shut-downs, triggered by the spacecraft switching to so-called safe mode (idle mode, during which all scientific payload is turned off). To prevent scientific instruments from excessive cooling, a standby power line was provided, which could be used to heat the instruments. The project requirements stated that average instrument's power consumption during these episodes should not exceed 3 W. To satisfy this requirement, a special heater (maximal heat power ~4.5 W) was installed inside HEND. It switches on at the temperature of −25º C and switches off at −22º C. Thus, as long as the main power supply is turned off and standby power is applied to heat the onboard scientific payload, this heater inside the HEND instrument is periodically switch on and off to maintain the instrument's average power consumption ~ 3W and temperature of detectors and electronics in range from −25ºC to −22ºC.

Sensitivity Вверх

HEND detectors' response functions were obtained during Earth-based calibrations and numerical simulations.

HEND was calibrated at various Russian nuclear centers, including Joint Institute for Nuclear Research (Dubna, Moscow district), Kurchatov Institute (Moscow) and Arzamas-16 (Nizhny Novgorod). Standard calibrated neutron sources with known spectrum and flux (californium and plutonium-berillium) where used for calibrations. The instrument was also calibrated at accelerating facilities with monochromatic neutron beams of given energy.

Along with direct measurements, we employed numerical simulations based on Monte-Carlo methods, including both proprietary programs (developed by Ioffe Physical Technical Institute, St.Petersburg) and standard programs for nuclear processes computation (MCNPX, Los Alamos National Laboratory, the USA).

Resulting functions for HEND detectors' efficiency are given in the Fig.7. One can observe that HEND sensitivity covers uniformly wide energy range from 0.4 eV to 10–15 MeV.

Fig.7. Sensitivity functions for different detectors comprising HEND instrument by colors: red — SD, green — MD, dark blue — LD, blue — stilbene.

Instrument results Вверх

The science mission began on 7 April, 2001. On the day NASA interplanetary spacecraft 2001 Mars Odyssey was launched from Cape Canaveral. 7 months later (on 24 October 2001) it was successfully inserted into the orbit around Mars.

For acquisition and analysis of data streaming from 2001 Mars Odyssey spacecraft, HEND Scientific Team was formed. Both Russian and U.S. scientists are participating in this Team.

Main results of HEND experiment are presented below, which include high-latitude maps of water ice distribution in the north (Fig. 8) and south (Fig.9) hemispheres, as well as observations of Martian polar caps (Fig. 10–11).

Fig.8. Water ice distribution in Mars north hemisphere

Fig.9. Water ice distribution in Mars south hemisphere

Fig.10. Variations of Martian Northern Polar Cap mass as derived from HEND data in comparison with Global Climate Model predictions

Fig.11 Variations of Martian Southern Polar Cap mass as derived from HEND data in comparison with Global Climate Model predictions

Main Parameters Вверх

МMass (with cables) 3695 g
Total energy consumption 5.7 W by supply voltage 28 V
Detectors on the basis of proportional 3He counters Small detector (SD) with thin polyethylene moderator and external cadmium shield. Middle detector (MD) with polyethylene moderator of medium thickness and external cadmium shield. Large detector (LD) with thick polyethylene moderator and internal cadmium shield
Scintillation module Internal scintillation detector on the basis of stilbene crystal (SC/IN)
External scintillation detector on the basis of CsI(Tl) crystal for anticoincidence (SC/OUT).
Thermal parameters The instrument is supplied with separate system for thermal regime maintaining and heating to keep the instrument temperature in the range from −30°C to −40°C in all operational modes.
Working modes Heating mode
Standby mode
Mapping mode
Measurement mode with bursts registration
Telemetry For each synchronization signal from GRS gamma-spectrometer with period from 12 s to 1 h HEND send to the telemetry system the following data:
Service information frame, 92 bytes, in Standby mode
Standard frame with 6 spectra, 212 bytes, in Mapping mode
Profile frame with 6 spectra and 2 profiles, 512 bytes, in Observing mode with bursts registration

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)

HEND Principal Investigator ― Prof. Igor Mitrofanov

Works on the HEND project are led under the theme MSP-2001 on the basis of the State contract №025-5452/04 from February 27, 2004 (R&D theme MSP-2001) and are included in the Federal Space Program for 2006–2016.

Works on HEND project are planned for 2002–2004 (development, tests, assembling, and instrument delivery) and 2002–2014 (operating and data processing).

 
Joint Institute for Nuclear Research
(Dubna, Moscow district)
Numerical modeling of HEND sensitivity; participation in the development of HEND physical scheme; preparation for and calibrations of the instrument with natural and artificial neutron sources on test bed.
A.A. Blagonravov Institute for Engineering Science of the Russian Academy of Sciences
(IMASH, Moscow)
Development of mathematical model of instrument mechanical structure; participation in the development of testing facilities for HEND instrument in compliance with NASA requirements; development of mechanical test program; support of the instrument mechanical testing.
Federal State Unitary Enterprise 'Fedorovsky All-Russia Science and Research Institute for Mineral Resources' of Ministry of Natural Resources and Environment of Russian Federation
(Moscow)
Packaging of the scintillation neutron and gamma-materials with photomultipliers in compliance with requirements to space technics.
Joint-Stock-Company «Specialized scientific research institute for instrumentation engineering»
(Moscow)
Circuitry development for fast neutrons scintillation registration electronics.
Яндекс.Метрика

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