ComPair Gamma-Ray Balloon Mission

ComPair is designed to detect gamma rays with energies between 200,000 and 20 million electron volts. (For comparison, the energy of visible light is 2 to 3 electron volts.) Supernovae and gamma-ray bursts, the most powerful explosions in the cosmos, glow brightest in this range, as do the most massive and distant active galaxies, which are powered by supermassive black holes.

ComPair gets its name from the two ways it detects and measures gamma rays: Compton scattering and pair production. Compton scattering occurs when light hits a particle, such an electron, and transfers some energy to it. Pair production happens when a gamma ray grazes the nucleus of an atom. The interaction converts the gamma ray into a pair of particles – an electron and its antimatter counterpart, a positron.

The ComPair instrument has four major components:

1. A tracker containing 10 layers of silicon detectors that determine the position of incoming gamma rays.

2. A high-resolution calorimeter that precisely measures the lower-energy Compton-scattered gamma rays.

3. Another calorimeter that measures the higher energies of electron-positron pairs.

4. An anticoincidence detector that detects the entry of high-energy charged particles called cosmic rays, allowing ComPair's other instruments to ignore them.

ComPair is a collaboration among Goddard, the Naval Research Laboratory in Washington, Brookhaven National Laboratory in Upton, New York, and Los Alamos National Laboratory in New Mexico.