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

Project Objectives

The Gamma Ray Spectrometer (GRS) suite 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 suite the Russian HEND instrument is designed to register high energy neutrons. The name HEND is an 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 rules on Mars, being the main cause for the surface upper layer formation in the previous ages as well as now.

The bedrock problem can be addressed partially on the basis of the 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 a wide energy range.

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

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

Continuous observations from the 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 tasks of the HEND experiment is to measure epithermal and fast neutrons flux in the energy range of 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 a 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 the hydrogen-bearing layer occurrence.

This task can be solved independently, using the 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 the hydrogen-bearing layer occurrence is calculated on the basis of comparative analysis of different energy ranges, as the maxima of epithermal and fast neutrons production are located at different depths. However, actual solution is far more complex and involves numerical methods to model the neutron flux on the 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 took about 7 months, which fell 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 the HEND experiment, that is

• temporal and spatial analysis of solar activity events during the 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 the 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 the HEND instrument register soft and hard gamma-radiation in the range of 60 keV to several MeV. Apart from studying solar flares, this range can be exploited to study 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

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 the 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 of stilbene C₁₄H₁₂ surrounded by active anticoincidence shield made from CsI(Tl) crystal.

Design and Mount

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 the elemental composition of Martian surface and search for water.
• THEMIS (the Thermal Emission Imaging System), main tasks of which are to study Martian surface in visible and infrared emission range.
• MARIE (the Mars Radiation Environment Experiment) dosimeter to measure the 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/)

The HEND's physical concept was chosen so as 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 with ³He, enclosed in 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 (top) 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 of 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 SC/ IN (Fig.3, bottom) is used in the 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 the 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 the 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 a stilbene scintillation detector Sc/IN with the anticoincidence detector Sc/OUT made from CsI(Tl).

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

Fig.5. General view of the 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 SC/ OUT made from CsI(Tl) to provide an anti-coincidence shield. The latter screens the SC/ IN stilbene detector in the direction of the sky, but leaves open the directions pointing to Mars surface. If a charged particle comes through the SC/ OUT outer detector, the instrument issues veto logic signal, thus suppressing pulse registration by the inner SC/ IN detector.

The 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

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, the ultimate goal of which 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.

The HEND's electronics was 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).

The HEND design was developed so that it is thermally isolated from the spacecraft by heat insulators installed between the HEND footprints and the spacecraft's science deck. These insulators exclude heat transfer due to thermal conductivity. Radiation exchange between the spacecraft and the 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 the open space. In stationary normal operation regime the HEND temperatures are around 12º C for the electronics and around 0º C for the housing.

Situations were anticipated during mission lifetime, when all scientific payloads are switched off. These were expected to be planned switch-offs, for example, during launch or insertion into orbit around Mars, or spontaneous shut-downs, triggered by the spacecraft switching to the 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 switched on and off to maintain the instrument's average power consumption ~ 3W and temperature of detectors and electronics in the range of −25ºC to −22ºC.

Sensitivity

The 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 the 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 the HEND detectors' efficiency are given in Fig.7. One can observe that the HEND sensitivity covers uniformly a wide energy range of 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.Fig.7. Sensitivity functions for different detectors comprised in the HEND instrument by colors: red — SD, green — MD, dark blue — LD, blue — stilbene.

Instrument results

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

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

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

Fig.8. Water ice distribution in the Mars northern hemisphere

Fig.9. Water ice distribution in Mars south hemisphere

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

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

Main Parameters

Developers and co-executors

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 of February 27, 2004 (R&D theme MSP-2001) and are included in the Federal Space Program for 2006–2016.

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