Detection Concept for MeV Gamma Rays

         The MeV gamma-ray measurements require accurate reconstruction of the photon energy and direction. While the pair-production process dominates for gamma rays with energies above 10 MeV, gamma rays below 10 MeV tend to Compton-scatter. A gamma ray may undergo many Compton scatterings before being photo-absorbed or before escaping from the detector. The incident gamma-ray energy E and cone angle θ can be reconstructed based on the Compton equation by accurately determining the position and energy of the Compton-electron(s) and photo-absorption. The measured cone angle defines a Compton “event circle” for each event. The intersection of three or more such “event circles” pinpoints the direction of the gamma-ray source.



Detection Concept for Antimatter

         The GRAMS antimatter survey involves capturing an antiparticle in a target argon atom with the subsequent formation and decay of an exotic atom. The time-of-flight plastic scintillators measure the velocity and angle of the incoming antiparticle. The charged antiparticle slows down as it enters the LArTPC (through ionization energy loss) and stops, forming an exotic atom with a target argon atom. The exotic atom emits atomic X-rays as it de-excites, with energies that are unique to the mass and the atomic number of the antiparticle and the target atom. At the end of the atomic cascade, the antiparticle subsequently annihilates on the nucleus, resulting in the emission of several hadrons (pions and protons). Identification of incoming antiparticles based on the atomic X-rays and the hadronic multiplicity, as well as the stopping range and energy deposition inside the LArTPC, can provide background rejection to the level of 0.01 background events for three GRAMS LDB flights (105 days of observation time).

Physics of the Cosmo (PhysCOS)


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