NSSDCA ID: 2007-039A-08
Mission Name: KaguyaThe Gamma-Ray Spectrometer (GRS) will use a cooled germanium (Ge) detector to observe the spectrum of lunar gamma rays from 0.1 to 12 MeV for the one year nominal mission with the objective of mapping the chemical composition of the surface and the global distribution of uranium, thorium, and potassium. The uranium potassium, and thorium are detected directly by gamma rays emitted due to radioactive decay. Other elements, such as iron, titanium, oxygen, silicon, aluminum, magnesium, and calcium, emit gamma rays when hit by cosmic rays or solar wind particles.
The GRS consists of three subsystems: the Gamma-Ray Detector (GRD), the Cooler Driving Unit (CDU), and the Gamma-ray and Particle detector Electronics (GPE). The GRD is mounted on the lunar-facing side of the mission module. The detector is a high-purity n-type Ge crystal surrounded by a scintillator shield comprising bismuth germanate (BGO) and plastic scintillators. The detector is 6.5 cm in diameter and 7.7 cm long and is hermetically sealed in a cylindrical aluminum cannister at a pressure of 10^-8 torr. The axis of the cylinder is prependicular to the nadir direction. The Ge detector is connected to a pre-amplifier and these signals are fed to a shaping amplifier, a base-line restorer, and finally to two amplifiers, one for low energy (0.1-4.0 MeV) gamma rays and one for high energy (0.1-12 MeV).
The BGO shield is open in the nadir direction with a 5 mm plastic scintillator cover to reject signals from energetic ions. Two photomultiplier tubes (PMTs) are attached to the BGO and one to the plastic scintillator at the end of the cylinder. The PMT outputs are fed to preamplifiers and then to the GPE. The GPE contains anti-coincidence logic, signal analysis, pulse height processing, and data handling electronics. The GPE is shared with the Charged particle Spectrometer experiment. The CDU is 55 W power supply which drives a Stirling cryocooler at 52 Hz. Along with a radiator and cryostat in the Ge detector, the cryocooler can maintain the temperature below 90K, giving an energy resolution of 3.0 keV (FWHM) at 1.33 MeV. The cannister can be cycled between roughly 20 and 100 degrees C in order to anneal any radiation damage.
Questions and comments about this experiment can be directed to: Dr. David R. Williams
Name | Role | Original Affiliation | |
---|---|---|---|
Dr. Nobuyuki Hasebe | Principal Investigator | Ehime University |