 |
The Gamma-Ray Spectrometer.
The Gamma-ray Spectrometer (GRS) consists of a bismuth germanate (BGO) crystal placed within a borated plastic scintillator anti-coincidence (ACS) shield. Gamma-rays are measured when a gamma-ray produces a flash of light in the BGO crystal without producing a signal in the ACS. The amount of light detected from the BGO is a measure of the initial gamma-ray energy. The GRS also serves as a detector of fast and epithermal neutrons by using the boron content of the ACS. |
 |
Inside the Gamma-Ray Spectrometer.
This picture shows one of the photomultiplier tubes (PMT) used to measure light produced in either the BGO crystal or ACS. |
 |
Inside the Gamma-Ray Spectrometer.
This picture shows a schematic diagram of the GRS. The BGO crystal, which detects the gamma-rays, is housed within the plastic ACS (made of BC454 scintillator material). Light is measured separately from each scintillator by photomultiplier tubes (PMT) glued to the ends of the scintillators. Signals from each PMT are sent through pre-amp/HV bleeder boards to the spacecraft electronics where they are processed and telemetered to the ground. The entire GRS is enclosed in a graphite epoxy housing which is used to reduce background from scattering gamma-rays (figure courtesy of Ames Research Center). |
 |
Origin of the Lunar Gamma-Rays.
Gamma rays are made up of photons (or electromagnetic waves) at different energies. They result either from the interaction of cosmic rays with the moon surface, or they are naturally emitted by the radioactive materials of the surface. The gamma rays that the spectrometer will detect come from these two sources (figure and text courtesy of Ames Research Center).
|
 |
The Neutron Spectrometer.
The Neutron Spectrometer (NS) consists of two identical 5 cm diameter by 20 cm long 3He filled gas proportional counters. One is covered with a 0.75 mm thick cadmium shield, and so is only sensitive to epithermal neutrons having energies above 0.25 eV. The second counter is covered with a 0.75 mm thick tin shield and so is sensitive to both thermal and epithermal neutrons. The difference in their counting rates yield a measure of the flux of thermal neutrons (0 to 0.25 eV). |
 |
Origin of the Lunar Neutrons.
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, and other remain bounce to the moon on ballistic orbits. The Neutron Spectrometer looks at these neutrons that tells about the surface composition (figure and text courtesy of Ames Research Center). |
 |
The Alpha Particle Spectrometer
The Alpha Particle Spectrometer (APS) consists of five pairs of 3 cm by 3 cm square ion-implant silicon detectors, each pair placed on one face of a cube. The detectors are covered by thin, aluminum coated polypropylene foils to exclude sunlight, and collimated to a 90° field of view at half maxiumum. The combination of foil thickness, detector dead layer, and electronic noise gives each detector a spectral resolution of 100 keV at 5.5 MeV. |
 |
The Alpha Particle and Neutron Spectrometers
Both instruments are mounted together. |
