Darrell M. Drake

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.

Science applications of the Mars Observer gamma ray spectrometer

The Mars Observer gamma ray spectrometer will return data related to the elemental composition of Mars. The instrument has both a gamma ray spectrometer and several neutron detectors. The gamma ray spectrometer will return a spectrum nominally every 20 s from Mars permitting a map of the elemental abundances to be made. The gamma rays are emitted from nuclei involved in radioactive decay, from nuclei formed by capture of a thermal neutron, and from nuclei put in an excited state by a fast-neutron interaction. The gamma rays come from an average depth of the order of a few tens of centimeters. The spectrum will show sharp emission lines whose intensity determines the concentration of the element and whose energy identifies the element. The neutron detectors, using the fact that the orbital velocity of the Mars Observer spacecraft is similar to the velocity of thermal neutrons, determine both the thermal and epithermal neutron flux. These parameters are particularly sensitive to the concentration of hydrogen in the upper meter of the surface. By combining the results from both techniques it is possible to map the depth dependence of hydrogen in the upper meter as well. These data permit a variety of Martian geoscience problems to be addressed including the crust and mantle composition, weathering processes, volcanism, and the volatile reservoirs and processes. In addition, the instrument is also sensitive to gamma ray and particle fluxes from non-Martian sources and will be able to address problems of astrophysical interest including gamma ray bursts, the extragalactic background, and solar processes.

New electronically black neutron detectors

Two neutron detectors are described that can function in a continuous radiation background. Both detectors identify neutrons by recording a proton recoil pulse followed by a characteristic capture pulse. This peculiar signature indicates that the neutron has lost all its energy in the scintillator. Resolutions and efficiencies have been measured for both detectors.

A Doppler filter technique to measure the hydrogen content of planetary surfaces

Abstract A new technique to measure the hydrogen content of planetary surfaces is suggested. It relies on the Doppler energy shift of planetary albedo neutrons in the frame of a planar neutron sensor aboard a low-altitude orbiting spacecraft. It can be realized by a small modification to the current design of the anticoincidence shield enclosing a possible Mars Observer gamma-ray spectrometer. The resultant sensitivity to hydrogen near the surface of the Moon is estimated to be nearly three orders of magnitude greater than the sensitivity inherent in the detection of 2.223 MeV neutron capture γ-rays by the spectrometer.

Bismuth germanate scintillators as detectors for high-energy gamma radiation☆

Abstract The properties of bismuth germanate (BGO) relevant to the detection of high-energy gamma-rays are discussed on the basis of a comparison between a 76 mm diameter by 76 mm long BGO scintillator and a 152 mm diameter by 254 mm long NaI(Tl) crystal. The two detectors were exposed to monoenergetic gamma rays from the 12 C(p, p′γ) 12 C reaction at the proton energy 17 MeV ( E γ = 4.44, 12.71 and 15.11 MeV) and from the 1 H(t, γ) 4 He reaction at the triton energy 9 MeV( E γ = MeV). The bismuth germanate detector has several properties which are important for the detection of high-energy gamma radiation.

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.

Seasonal Carbon Dioxide Depositions on the Martian Surface as Revealed from Neutron Measurements by the HEND Instrument Onboard the 2001 Mars Odyssey Spacecraft

We present the results of eighteen months of observations of the seasonal caps of Mars based on data from the neutron spectroscopy of the surface by the Russian HEND Instrument mounted aboard the NASA 2001 Mars Odyssey spacecraft. A four-dimensional model of the Martian seasonal caps was developed on the basis of these observation data. The model shows how the thickness of the frozen carbon dioxide changes in different surface regions. Using the results of the model, we estimated the total mass of the seasonal caps for the period of maximal accumulation of seasonal depositions and the rates of condensation and sublimation of the seasonal cover.

The contribution of carbon interactions to the neutron counting efficiency of organic scintillators

Abstract The neutron counting efficiency of an NE-213 detector was measured for neutron energies up to 15 MeV using biases at 0.3 and 2.0 MeV neutron energies. For the lower bias, nonelastic carbon interactions contribute appreciably to the efficiency at energies as low as 5.5 MeV.

Theoretical fluxes of gamma rays from the Martian surface

[1] Theoretical fluxes of gamma rays escaping the surface of Mars were calculated. These and other calculated fluxes are needed to model the counting rates in the Mars Odyssey gamma ray spectrometer that are used to determine elemental compositions and other results using these measurements. Cross sections for the formation of gamma rays by both thermal and fast neutrons were compiled and evaluated. These evaluated cross sections were used with neutron fluxes calculated with the Monte Carlo N Particle Extended (MCNPX) code to get gamma ray production rates as a function of depth in the Martian surface. The fluxes of these gamma rays as a function of angle at the Martian surface were then calculated using gamma ray attenuation coefficients.

Experimental evaluation of the BDD-I dosimeter for the Global Positioning System

Abstract The Burst Detection Dosimeter I (BDD-I) is a multipurpose silicon detector system that has flown on two Block I satellites of the Global Positioning System (GPS) series. These spacecraft occupy circular orbits about 20 000 km above the surface of the Earth. BDD-I measures the radiation dose received by the GPS spacecraft, primarily from energetic electrons trapped in the Earths radiation belt, but also from solar energetic particles and galactic cosmic rays outside the trapped radiation zone (when the spacecraft is near the Earths magnetic poles). Absorbers, located in front of four separate silicon sensors, determine energy thresholds for measuring incident particle fluxes, and the magnitude of energy loss in each sensor provides an imperfect but useful separation between protons and electrons over a wide range of energies. This article describes the important mechanical and electronic features of this detector. Relations are given for converting the observed counting rate in the detector to parameters representing the incident flux of charged particles. The overall uncertainty in this determination of the integral electron flux is about 30%. The detector response was well characterized by the use of electrons with energies below 3 MeV. Monte Carlo simulations agree with measured responses to within the experimental uncertainty. The simulation results also enable assessments of the sensitivity of the experimental measurements to systematic effects such as an angular divergence in the incident electron beam. The article presents some sample data measured with the detector.