Neutron monitor

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.

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.

When a high-energy particle from outer space ("primary" cosmic ray) encounters Earth, its first interaction is usually with an air molecule at an altitude of 30 km or so.

The exceptions to this are a few specific nuclei (most notably 10B and 3He) that quickly absorb extremely low energy neutrons, then disintegrate releasing very energetic charged particles.

The main advantage of the neutron monitor is its long-term stability making them suitable for studied of cosmic-ray variability through decades.

As this matter moves through the solar system, it suppresses the intensity of Galactic cosmic rays.

Approximately 10-15 times per decade, the Sun emits particles of sufficient energy and intensity to raise radiation levels on Earth's surface.