Cosmic Radiation

A significant component of detector background arises from the secondary radiations produced by cosmic-ray interactions in the earth atmosphere.  The primary cosmic radiation, which can be either of galactic or solar origin, is made up of charged particles and heavy ions with extremely high kinetic energies.  In their interaction with the atmosphere, a large assortment of secondary particles is produced, including pi and mu mesons, electrons, protons, neutrons, and electromagnetic photons, with energies that extend into the hundreds of MeV range.   Many of these radiations reach the earth's surface and can create background pulses in many types of detector.

Because of their very high kinetic energy, the cosmic primary and secondary particles have a relatively low spesific energy loss (-dE/dx) comparable with that of fast electrons.  Thus, the corresponding pulse amplitude is small in "thin" detectors design to stop typical charged particles but not electrons.  However, in solid detectors a larged thickness, such as NaI(TI) scintillators or germanium detectors, the deposited energy may be many MeV and the corresponding pulses can then be large compare with typical signal pulses.

The various secondary components differ in their hardness or ability to penetrate matter, and some gain are achieved even with modest amount of shielding.  Other component persist through many meters of common materials.  At the earth's surface, the cosmic secondary radiations are directed primarily downward, so that shielding against cosmic background is most effective when located above the detector.

Fast neutrons from cosmic interactions (or any other source) can also create secondary gamma rays liberated when a neutron is moderated and absorbed.  When the shield contains hydrogen (e.g., as in concrete), the capture gamma ray at 2.22 MeV can sometimes be identified in the background spectrum.