A neutron monitor is a ground-based detector designed to measure the number of high-energy charged particles striking the Earth's atmosphere from outer space. For historical reasons the incoming particles are called "cosmic rays", but in fact they are particles, predominantly protons and Helium nuclei. Most of the time, a neutron monitor records galactic cosmic rays and their variation with the 11-year sunspot cycle and 22-year magnetic cycle. Occasionally the Sun emits cosmic rays of sufficient energy and intensity to raise radiation levels on Earth's surface to the degree that they are readily detected by neutron monitors. They are termed "ground level enhancements" (GLE).
The neutron monitor was invented by University of Chicago Professor John A. Simpson in 1948.[1] The "18-tube" NM64 monitor, which today is the international standard, is a large instrument weighing about 36 tons.