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An introduction to the neutron spectrometer


Neutron Spectrometer
Image courtesy of Los Alamos National Laboratory

The Neutron Spectrometer (NS) may be the "star of the show" in the first few weeks of the Lunar Prospector mission. This instrument will search for water ice -- specifically, by detecting the element hydrogen on the Moon's surface. Since every water molecule contains two atoms of hydrogen and one of oxygen, hydrogen is a good marker for water. Traces of hydrogen could also be implanted in the Moon's surface by the solar wind. But if water is present in usable quantities, the amount of hydrogen locked up in water molecules will dwarf the amount that is present for other reasons. In 1994, the Department of Defense satellite Clementine probed the Moon's surface with radar. The radar signals that bounced back from the deep, permanently shadowed craters near the Moon's poles were consistent with water ice being there. However, there could be other explanations for Clementine's observations. The NS will give a clear and definite answer to the question of whether water exists in usable quantities at the poles. Even if only 0.5 percent of the surface material at a given location is water, Lunar Prospector will be able to find it. To put it another way, the NS could detect a cup of water in a cubic yard of soil.

The NS will not detect hydrogen directly, since it will be 63 miles (100 km) above the Moon's surface. Instead, it will look for what scientists call "cool" neutrons -- neutrons that have bounced off a hydrogen atom somewhere on the lunar surface. When cosmic rays collide with atoms in the crust, they violently dislodge neutrons and other subatomic particles, such as gamma rays. Some of the neutrons escape directly to space, as hot or "fast" neutrons. Other neutrons shoot off into the crust, where they collide with other atoms, bouncing around like pinballs. If they only run into heavy atoms, they do not lose very much energy in the collisions, and are still traveling at close to their original speed when they finally bounce off into outer space. They will still be warm (or "epithermal") when they reach Lunar Prospector and are recorded by the NS.

The only effective way to slow down a speeding neutron is to have it collide with something its own size. (The same is true with a pinball: if it runs head on into a stationary pinball, it will come to a dead stop. If the collision is not quite head on, it will slow down dramatically.) There's only one atom the same size as a neutron: hydrogen, the lightest of all the elements. If the Moon's crust contains a lot of hydrogen at a certain location -- say, a crater with water in it -- any neutron that bounces around in the crust before heading out to space will cool off rapidly. When Lunar Prospector flies over such a crater, the NS will detect a surge in the number of cool ("thermal") neutrons, and a dropoff in the number of warm ("epithermal") neutrons.

The NS has two different counters -- a cadmium-wrapped canister of helium-3 and a tin-wrapped canister of helium-3. When a neutron collides with an atom of helium-3, a nuclear reaction takes place, producing a triton, a proton, and lots of energy. That burst of energy tells scientists that they have detected a neutron.

Except for the outside wrapping, the two counters are nearly identical. The cadmium-wrapped counter detects only the epithermal neutrons, because cadmium is good at screening away the slow-moving thermal neutrons, whereas the tin-wrapped counter lets all of the neutrons through. Since the two counters are otherwise identical, any difference between the two must be attributable to thermal neutrons. Lunar Prospector will measure epithermal neutron flux with a separate instrument (its gamma ray spectrometer). The respective count rates of the different types of neutron fluxes are an indicator of hydrogen content, and hence the presence of water ice, embedded within the lunar soil. Soil containing ice, for instance, should yield a higher thermal neutron flux and a lower epithermal neutron flux than soil devoid of ice.

As scientists receive data from the spacecraft, their screens will be updated with new neutron counts twice per second, and new energy spectra (showing the speed of the captured neutrons) will appear every 32 seconds. Since the data will have a lot of random noise in it, several passes over the surface and careful statistical analysis may be required to reduce the uncertainty to the point where scientists can be sure they have found water. However, there is one factor working in the scientists' favor: the spacecraft will pass over the poles every orbit, while it passes over any given region on the equator only a few times a month. Thus the NS will produce more accurate data in the polar regions -- precisely where the water is thought to be. If water is present in the amounts suggested by the Clementine mission, the NS should be able to detect it, possibly within a month.

* The Neutron Spectrometer weighs 8.5 pounds (3.9 kilograms), consumes 2.5 watts of power and produces 49 bits of data per second. It will be deployed together with the Alpha Particle Spectrometer

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