Portions adapted from:
John A. Simpson," The Cosmic Ray Nucleonic Component: The Invention And Scientific Uses Of Neutron Monitor"
Space Science Reviews 93, 1-20, 2000
Frank B. McDonald, "Integration Of Neutron Monitor Data With Spacecraft Observation: A Historical Perspective"
Space Science Reviews 93, 239-258, 2000
Galactic and solar cosmic ray particles entering the earth's atmosphere with energy above 0.5 GeV undergo nuclear interactions, producing secondary whose effects can be extended down to the sea level. The development of the neutron monitor by J. A. Simpson (Simpson,1957) provided an improved method of detecting low-energy neutron secondary that are not slowed by ionization loss. These secondary fall in the energy range of a few hundred MeV up to about one GeV. So the neutron monitors are most sensitive to the low energy (1-20 GeV) portion of the spectrum. Neutron monitors with their reliability and basic simplicity offered a means of studying the longer-term temporal variations while their sensitivity and high counting rates made possible the observation of short term intensity changes as well.
In the 1960's Carmichael (1968) in preparation for the year of Quiet Sun (IQSY) in 1964-1965 developed the IQSY neutron monitor with a statistical accuracy of 0.1% for hourly data. This larger monitor called the "super" monitor or the NM-64 utilized more modern data collection techniques. The data from these monitors which have proven to be extremely stable detectors, are particularly valuable for long term cosmic radiation studies. Nowdays there are about 59 Neutron monitors station in operation around the world with a wide range of geomagnetic cutoffs, altitudes and viewing directions.
The energy of cosmic ray particles that can penetrate to the top of the atmosphere is a function of the geomagnetic field. The Earth's magnetic field could be used as a spectrometer to allow measurements of the cosmic-ray spectrum down to low primary energies. The magnetic latitude of a particular neutron monitor determines the lowest magnetic rigidity of a primary that can reach the monitor , the so called 'cut off rigidity'. The station's altitude determines the amount of absorbing atmosphere above the station and hence the amount of absorption of the secondary cosmic rays (the higher the station, the higher the counting rate}. By using a combination of lead ( to produce local interactions}, paraffin or polyethylene ( to moderate or slow down the neutron component) and multiple slow neutron counters, Simpson greatly increased the counting rate in his monitor design.
At first Simpson established a network of high-latitude neutron monitor stations over a wide range of geomagnetic latitudes. This neutron monitor design (IGY) was standardized for use in the International Geophysical Year (1956/1957) and dozens were put into operation around the world. Their station at Climax Colorado has been in continuous operation since 1950/51 and is the longest operating station in the worldwide network of Neutron Monitors.