High Energy Neutron Detector Research Articles

Hydration state and abundance of zeolites on Mars and the water cycle

Recent experimental studies have suggested that zeolites, if present on the Martian surface, could undergo a strong diurnal cycle of hydration and dehydration with possible impact on the atmosphere. This study evaluates this possibility using a global model of hydration/dehydration of two different zeolites (clinoptilolite and chabazite) assuming actual diurnal, seasonal, and geographical temperature variations. If zeolites extensively cover the surface and undergo complete diurnal hydration/dehydration cycles as predicted from the water vapor adsorption isotherms, the resulting water content would be too high in both the surface material and the atmosphere, implying that the abundance of zeolites is low or that zeolites must exist in a more desiccated state. If the zeolite abundance is low with the same hydration behavior, the lowest mean surface water content would occur at the equator, which also does not agree with observations. If substantial hydration/dehydration occurs seasonally rather than diurnally, the latitudinal and seasonal variation of the water content becomes too large and the zeolite abundance would have to vary with season to match the observation, which is also unrealistic. The most realistic scenario is a diurnally and seasonally constant low hydration state of zeolites controlled by the annual maximum surface temperature, in addition to a low abundance. Using these assumptions, the global distribution of the water content in the near‐surface dry layer inferred from Mars Odyssey High‐Energy Neutron Detector (HEND) data can be roughly explained. The best estimate of the zeolite abundance in the surface material to account for the observed water content is, on global average, ∼30% for Ca‐clinoptilolite, ∼35% for Na‐clinoptilolite, ∼55% for K‐clinoptilolite, and ∼15% for chabazite if no other hydrated minerals are present. Putative regional confinement of zeolites to the dust‐rich regions generally worsens the correlation between the modeled and observed water content compared with the scenario with globally uniform zeolite distribution, implying that zeolites may be present in dust‐poor regions as well. In any case, our study shows that the diurnal atmospheric water cycle is unlikely to be affected by zeolites on the Martian surface.

Mars' time‐variable gravity and its determination: Simulated geodesy experiments

The seasonal carbon dioxide (CO2) cycle on Mars results in a time‐variable global redistribution of mass. These large‐scale variations are associated with changes in the gravity field, mainly in the two zonal gravity coefficients and , which have been recently evaluated from Doppler tracking data of the Mars Global Surveyor (MGS) spacecraft. In the present study, we calculated these variations from the mass redistribution obtained from outputs of two general circulation models (GCM) as well as from CO2 thickness measurements by the High Energy Neutron Detector (HEND) instrument on board the Mars Odyssey spacecraft and compared them to the observations. Tracking observations provide one of the most direct measures of the global‐scale atmospheric mass cycle. However, the associated uncertainties are relatively large, partly because the low‐degree zonals obtained from a single orbiter tracking analysis are contaminated by higher‐degree harmonics which are shown to have nonnegligible seasonal variations. Thus we investigated possibilities to improve the determination of the time‐variable gravity field by means of simulated geodesy experiments. Additional radio tracking of a second spacecraft with suitable orbital characteristics was shown to be able to separate the higher‐degree geodetic signatures. Radio links between landers on the Martian surface and a near‐polar orbiter can further better estimate especially the even zonals.

Modeling of Martian seasonal caps from HEND/ODYSSEY data

In this paper, we have analyzed neutron spectroscopy data gathered by the high energy neutron detector instrument onboard Mars Odyssey in order to study the behavior of CO 2 condensed mass at the polar regions of Mars. It is known that observation of the neutron albedo of Mars provides not only information on distribution of water ice/bounded water through Martian subsurface but is also sensitive to the condensation/sublimation of atmospheric CO 2 at the Martian poles. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the creation of multidimensional model of CO 2 deposition describing behavior of CO 2 condensed mass as a function of longitude, latitude and time. Based on this model, we have estimated the total masses of north seasonal cap ((3.8 ± 0.4) × 10 15 kg), south seasonal cap ((6.3 ± 0.6) × 10 15 kg) and compared our estimations with predictions of General Circulation Model. Furthermore, using the data gathered during second Martian year on the orbit, we have conducted preliminary comparison between seasonal cycles of two successive Martian years.

2001 Mars Odyssey Mission Summary

The 2001 Mars Odyssey spacecraft, now in orbit at Mars, will observe the Martian surface at infrared and visible wavelengths to determine surface mineralogy and morphology, acquire global gamma ray and neutron observations for a full Martian year, and study the Mars radiation environment from orbit. The science objectives of this mission are to: (1) globally map the elemental composition of the surface, (2) determine the abundance of hydrogen in the shallow subsurface, (3) acquire high spatial and spectral resolution images of the surface mineralogy, (4) provide information on the morphology of the surface, and (5) characterize the Martian near-space radiation environment as related to radiation-induced risk to human explorers. To accomplish these objectives, the 2001 Mars Odyssey science payload includes a Gamma Ray Spectrometer (GRS), a multi-spectral Thermal Emission Imaging System (THEMIS), and a radiation detector, the Martian Radiation Environment Experiment (MARIE). THEMIS and MARIE are mounted on the spacecraft with THEMIS pointed at nadir. GRS is a suite of three instruments: a Gamma Subsystem (GSS), a Neutron Spectrometer (NS) and a High-Energy Neutron Detector (HEND). The HEND and NS instruments are mounted on the spacecraft body while the GSS is on a 6-m boom. Some science data were collected during the cruise and aerobraking phases of the mission before the prime mission started. THEMIS acquired infrared and visible images of the Earth-Moon system and of the southern hemisphere of Mars. MARIE monitored the radiation environment during cruise. The GRS collected calibration data during cruise and aerobraking. Early GRS observations in Mars orbit indicated a hydrogen-rich layer in the upper meter of the subsurface in the Southern Hemisphere. Also, atmospheric densities, scale heights, temperatures, and pressures were observed by spacecraft accelerometers during aerobraking as the spacecraft skimmed the upper portions of the Martian atmosphere. This provided the first in-situ evidence of winter polar warming in the Mars upper atmosphere. The prime mission for 2001 Mars Odyssey began in February 2002 and will continue until August 2004. During this prime mission, the 2001 Mars Odyssey spacecraft will also provide radio relays for the National Aeronautics and Space Administration (NASA) and European landers in early 2004. Science data from 2001 Mars Odyssey instruments will be provided to the science community via NASA’s Planetary Data System (PDS). The first PDS release of Odyssey data was in October 2002; subsequent releases occur every 3 months.

The Mars Odyssey Gamma-Ray Spectrometer Instrument Suite

The Mars Odyssey Gamma-Ray Spectrometer is a suite of three different instruments, a gamma subsystem (GS), a neutron spectrometer, and a high-energy neutron detector, working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The instruments are complimentary in that the neutron instruments have greater sensitivity to low amounts of hydrogen, but their signals saturate as the hydrogen content gets high. The hydrogen signal in the GS, on the other hand, does not saturate at high hydrogen contents and is sensitive to small differences in hydrogen content even when hydrogen is very abundant. The hydrogen signal in the neutron instruments and the GS have a different dependence on depth, and thus by combining both data sets we can infer not only the amount of hydrogen, but constrain its distribution with depth. In addition to hydrogen, the GS determines the abundances of several other elements. The instruments, the basis of the technique, and the data processing requirements are described as are some expected applications of the data to scientific problems.

Regions of Potential Existence of Free Water (Ice) in the Near-Surface Martian Ground: Results from the Mars Odyssey High-Energy Neutron Detector (HEND)

We report results of the analysis of the data on global mapping of neutron fluxes from the Martian surface, which have been obtained during the first ten months of measurements carried out by the Russian high-energy neutron detector HEND mounted aboard the AmericanMars Odysseyorbiter. This analysis allowed us to separate regions where free water (in ice form) prevailed in the surface layer (with a thickness of up to 2 m) of the Martian ground from regions where physically and chemically bound ground water was most likely to be the dominant form of water. The global mapping of regions with increased ice content in the ground-surface layer revealed a direct correlation with regions of polygonal terrains morphologically similar to terrestrial polygonal forms of permafrost origin. The potential content of bound water forms in the ground of circumpolar areas of the planet is also estimated.

CO2 snow depth and subsurface water-ice abundance in the northern hemisphere of Mars.

Observations of seasonal variations of neutron flux from the high-energy neutron detector (HEND) on Mars Odyssey combined with direct measurements of the thickness of condensed carbon dioxide by the Mars Orbiter Laser Altimeter (MOLA) on Mars Global Surveyor show a latitudinal dependence of northern winter deposition of carbon dioxide. The observations are also consistent with a shallow substrate consisting of a layer with water ice overlain by a layer of drier soil. The lower ice-rich layer contains between 50 and 75 weight % water, indicating that the shallow subsurface at northern polar latitudes on Mars is even more water rich than that in the south.

Seasonal Neutron-Flux Variations in the Polar Caps of Mars as Revealed by the Russian HEND Instrument Onboard the NASA 2001 Mars Odyssey Spacecraft

We analyze the flux of epithermal neutrons from the Martian surface recorded by the Russian High-Energy Neutron Detector (HEND) from February 19 through December 19, 2002. The HEND was installed onboard the NASA 2001 Mars Odyssey spacecraft and is designed to measure neutron fluxes with energies above 1 eV. Over the period of observations, statistically significant variations in the flux of epithermal (10–100 keV) neutrons were found in the northern and southern polar caps. The largest neutron-flux variations were found at subpolar latitudes, where the relative difference between the summer and winter values can reach severalfold. This correlation becomes weaker with increasing distance from the poles. Thus, the relative change in the neutron flux near the 60° parallel is slightly more than 10%. We assume that the detected variations result from the global circulation of atmospheric carbon dioxide in subpolar Martian regions. To additionally test this assumption, we compared the HEND neutron measurements onboard 2001 Mars Odyssey and the seasonal variations in the CO2-layer thickness as observed by the Mars Orbital Laser Altimeter (MOLA) onboard Mars Global Surveyor (MGS).

Search for Traces of Chemically Bound Water in the Martian Surface Layer Based on HEND Measurements onboard the 2001 Mars Odyssey Spacecraft

Analysis of the distribution of the epithermal and fast neutron fluxes from the Martian surface within the ±60° latitude zone measured by the High-Energy Neutron Detector (HEND) from mid-February through mid-June 2002 has revealed regional neutron-flux variations outside the zones of climatic effects, which appear to be attributable to the presence of chemically bound water. With the exception of the epithermal neutron fluxes in Arabia and southwest of Olympus Mons (Medusae Fossae), these variations show no correlation with the geologic structure of the terrain at the level of global geologic maps. The lack of such a correlation probably implies that to the formation depth of the epithermal neutron flux (1–2 m), let alone the fast neutron flux (20–30 cm), much of Mars is covered by a surface material that bears little relation in composition to local bedrocks. Clearly, this is an aeolian cover whose fine-grain component was mixed by dust storms in the geologic time on the scale of large regions. The decrease in the flux of epithermal neutrons in Arabia and southwest of Olympus Mons (Medusae Fossae) appears to be attributable to an enhanced concentration of materials containing chemically bound water (clay minerals, palagonite, hydroxides, and hydrosalts) in the surface layers of these regions.

Search for Water in Martian Soil Using Global Neutron Mapping by the Russian HEND Instrument Onboard the US 2001 Mars Odyssey Spacecraft

We present the first results of the global neutron mapping of Mars by the Russian High-Energy Neutron Detector (HEND) onboard the US 2001 Mars Odyssey spacecraft. Global neutron maps of Mars in various spectral ranges allow the content of water ice and adsorbed and bound water in a near-surface layer of the planet 1 to 2 m in thickness to be estimated. Huge regions of permafrost with a high (several tens of percent by weight) content of water ice are shown to be present in the north and the south of Mars. The continuous observations of Mars for 12 months, from February 18, 2002, through February 8, 2003, are indicative of significant seasonal variations on Mars where the transition from northern winter to northern summer occurred.

Maps of subsurface hydrogen from the high energy neutron detector, Mars Odyssey.

After 55 days of mapping by the High Energy Neutron Detector onboard Mars Odyssey, we found deficits of high-energy neutrons in the southern highlands and northern lowlands of Mars. These deficits indicate that hydrogen is concentrated in the subsurface. Modeling suggests that water ice-rich layers that are tens of centimeters in thickness provide one possible fit to the data.

EOSCOR III, a High Energy Solar Neutron Detector

A light weight high energy neutron detector has been developed for the observation of 20-150 MeV solar flare neutrons on a long duration Sky Anchor balloon flight. It consists of two 1 m2 plastic scintillators separated by a 1 m time-of-flight path to detect the protons from n-p scattering and 12C(n, pX) interactions in the upper scintillator. Adiabatic isochronous light pipes view the scintillators. Time-of-flight and pulse height analysis are used to measure the proton velocity, to discriminate against albedo events, and to separate the slower moving protons from Compton electrons. The Monte Carlo efficiency calculations are compared with accelerator calibration results using mono-energetic neutrons. Some results from short duration high altitude balloon observations are presented.

EOSCOR: A light weight, microprocessor controlled solar neutron detector

A light weight high energy neutron detector with vertical detection efficiency of 0.005 at 40 MeV and 1.4 m 2 sensitive area has been developed for long duration super-pressure balloon flight observations of solar neutrons and gamma rays. It consists of two sets of four plastic scintillator hodoscopes separated by a 1 m time-of-flight path to observe n−p, C(n, p), and C(n, d) interactions. The neutrons interactions are separated from gamma ray events through TOF measurements. For a large flare, the signal from solar neutrons is expected to be an order of magnitude greater than that of the atmospheric background. The microprocessor controls the data acquisition, accumulation of histograms, and the encoding of data for the telemetry systems. A test flight of the detector was made with a zero-pressure balloon. The expected many-week duration of a super-pressure balloon flight would significantly increase the probability of observing 20–150 MeV neutrons from a medium or large flare.

An apparatus of the NBC type and the physics results obtained

A nonbubble chamber (NBC) setup, especially designed to perform a series of experiments, is described. The setup consists of a neutron missing-mass spectrometer coupled with a system of large electromagnetic shower detectors. In spite of its large dimensions (the sensitive surface and volume are 2.16 × 10 4 cm 2 and 7.8 × 10 5 cm 3, respectively), the neutron spectrometer has a time resolution of ±0.70 nsec FWHM, with ∼25% detection efficiency in the range (70–390 MeV) neutron kinetic energy. The time equalization between the various components of the neutron spectrometer has been established to within ±0.1 nsec. At present this is the most powerful and the most accurate high-energy neutron detector. The electromagnetic shower detector is based on the principle of simultaneous measurements of the spatial development of the electromagnetic cascade and of its energy release. This is obtained with nine elements of lead foil — spark chamber — plastic scintillator, all sandwiched together. The sensitive surface and volume of the electromagnetic shower detector are 1.45 × 10 4 cm 2 and 7.2 × 10 5 cm 3, respectively. The setup has been used to perform a series of 14 experimental investigations. Ten of these have already produced conclusive results which are either published or submitted for publication: 1. 1. Evidence against the existence of the S 0 meson; 2. 2. an accurate measurement of the branching ratio η → neutrals η → total ; 3. 3. an accurate determination of the branching ratio ω → neutrals ω → total ; 4. 4. study of φ production in π − + p interactions at 2.1 GeV/ c; 5. 5. evidence for the new decay mode φ → e + e − and measurement of the cross section σ[ π − + p → ( φ → e + e −) + n]; 6. 6. evidence for the new decay mode ω → e + e − and measurement of the cross section σ[ π − + p → ( ω → e + e −) + n]; 7. 7. points (5) and (6) above allowed the first direct determination of the ( ω − φ) mixing angle; 8. 8. evidence for the new decay mode X 0 → γγ; 9. 9. cross section measurements for X 0 production and value of the branching ratio X 0 → neutrals X 0 → total ; 10. 10. evidence for a structure in the neutral A 2 mass region. The data of the following three experiments are being analysed: 11. 11. measurement of the branching ratio X 0 → γγ X 0 → total ; 12. 12. study of the ϱ − ω interference in the reaction π − + p → π + + π − + n at 1.67GeV/ c incident pion momentum; 13. 13. study of the ( π 0 π 0) system in the invariant mass region from 400–950 MeV. 14. 14. Finally, the last experiment is still under way: the apparatus is at present running for the detection of the decay mode φ → η + γ and for the measurement of the branching ratio φ → η + γ φ → total . The setup described and the experimental results obtained show that it is possible, in the field of NBC physics, to reduce the cost and the time needed for an experiment provided that multipurpose NBC setups are used.

The detection efficiency of a plastic scintillator for neutrons in the energy range of 21–29 MeV

A method of calibrating the efficiency of a high-energy neutron detector is described. Results are given for a plastic scintillator block, in the region 21–29 MeV. Comparison with results from other laboratories shows that, with a proton energy detection threshold of about 4 MeV, and for linear dimensions not too much smaller than the range of the maximum energy protons, the efficiency is rather independent of the size or shape of the scintillator and equal to nearly 2% per centimetre thickness in the range 10–30 MeV.