A device used to detect and measure radiation characteristically emitted in atomic, molecular, or nuclear processes, including photons (light, including visible light, gamma rays, and x-rays), lightweight charged particles (electrons or positrons), nuclear constituents (neutrons, protons, and heavier ions), energetic neutral particles (fast-moving atoms, for example), and subnuclear constituents such as mesons. The device can also be known as a radiation detector. Since human senses generally do not respond to these types of radiation (with the obvious exception of visible light), detectors are essential tools for the discovery of radioactive minerals, for all studies of the structure of matter at the atomic, nuclear, and subnuclear levels, and for protection from the effects of radiation. They have also become important practical tools in the analysis of materials using the techniques of neutron activation and x-ray fluorescence analysis.