Gamma Ray Spectrometer - 2001 Mars Odyssey Lunar and Planetary Lab The University of Arizona

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Neutron Spectrometer

Figure 1. Neutron Spectrometer

The Mars Odyssey Neutron Spectrometer (NS) was built by the Los Alamos National Laboratory (LANL). The NS team is led by Dr. William Feldman at Los Alamos.


The NS instrument is designed to detect neutrons in three energy bands: thermal, epithermal and fast. Each energy class corresponds to the degree to which planetary neutrons have been "moderated", or been in contact with other planetary matter. These free neutrons are produced in collisions between galactic cosmic rays and planetary matter. As it turns out, hydrogen is a very good moderator of neutrons and hence the detector is quite sensitive to the presence of hydrogen on the surface (to a depth of about one meter) of Mars. Large concentrations of hydrogen are most likely indicative of water (H2O) in liquid or ice form.

The detector is box-shaped and segmented into four prism-shaped quadrants as shown in Figure 2 below. Each prism is a boron-loaded plastic scintillator. As the name implies, a scintillator emits flashes of light when struck by energetic particles. The prism segments are optically isolated from one another, and each is viewed by a separate photomultiplier tube (PMT). The PMT's amplify the light energy from the scintillators, sending a signal to the detector electronics.

Figure 3. Neutron Spectrometer

Both ends of the assembly are covered with a thick sheet of cadmium to shield all prisms from thermal neutrons coming from the end directions. In addition, one of the prisms (the downward looking prism in Figure 2) has its outward face covered by an identical cadmium sheet so that it only responds to neutrons having energies above the thermal energy range.

While in mapping, the downward (nadir) prism faces the planet, and thus will be sensitive to neutrons coming directly from Mars. The upward (zenith) prism provides a measure of the background neutron flux over the full energy range. The front (forward) prism faces in the direction of spacecraft motion. It runs into (or "scoops up") thermal neutrons, which travel more slowly than the spacecraft. By the same principle, the backward (aft) prism outruns thermal neutrons coming from Mars.

Using this simple but elegant combination of geometry and velocity, the NS can seperate out background and spacecraft neutrons from those emitted by Mars. The difference in counting rates between forward and aft prisms then yield a measure of the flux of thermal neutrons. The difference in the rates between the aft prism and the upward prism yeilds a fractional measure of the flux of epithermal neutrons. A second independent measure of epithermal neutrons is provided by the downward facing prism because it is shielded from the outside by sheets of cadmium and the other three prisms.

Figure 4. Cross Section Reaction Detail

Neutrons lose energy in the detector through multiple scattering collisions with the hydrogen and carbon nuclei that compose the scintillator. Because the neutron and the proton have the same mass, most of the energy is lost to proton recoils as a neutron scatters off a hydrogen nucleus.

As the recoil protons slow down in the scintillator, they produce multiple ion-electron pairs that eventually recombine to produce photons. Collection of these photons by the PMTs produces pulses of charge that are then amplified and digitized by the neutron detector analog electronics to generate histograms.

Figure three shows the signature reaction of a thermal (slow) neutron. An incoming neutron scatters multiple times off of protons. Finally it is captured by a 10B nucleus. (Boron with an atomic number of 10, 5 protons and 5 neutrons). This nucleus decays to 7Li*, an excited Lithium atom (3 protons, 4 neutrons). An alpha particle carries off the difference in atomic number of two protons and a neutron. The difference in atomic mass between the Boron atom and the Lithium plus the alpha particle is realized as energy of motion in the particles, plus a photon.

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