(portions adapted from Simpson, J.A., "Cosmic-Radiation Neutron Intensity Monitor", in Annals of the IGY, 1955)
The purpose of the neutron monitor is to detect, deep within the
atmosphere, variations of intensity in the interplanetary cosmic ray
spectrum. Interactions of the primary
cosmic rays with the atmosphere produce, among other things, a
lower energy secondary nucleonic component consisting of nucleons (expand image at left), in
particular neutrons that are not slowed by ionization loss,
These secondaries fall in the energy range of a few hundred MeV
up to about one GeV. Because of the
falling energy spectrum of the primary cosmic rays, the neutron monitors
are most sensitive to the low energy (1-20 Gev) portion of the spectrum.These nucleons in turn produce further nuclear interactions, either in the atmosphere, or in lead target material surrounding the monitor. The interaction rate may be measured most conveniently and reliably by detecting the reaction product neutrons rather than by detecting the charged fragments directly.
John A. Simpson, at the University of Chicago, invented and developed the neutron monitor over the years 1948-50 and found that 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.
During the early 1950s John Simpson established a network of high-altitude 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. Our station at Climax, Colorado has been in continuous operation since 1950/51 and is the longest-operating station in the world-wide network of neutron monitors. Unfortunately, our Huancayo station (1951-1992) was recently forced to cease operation because of political unrest in Peru. These two stations, at ~3 GV and ~13 GV vertical cutoff rigidities, respectively, provided continuous coverage over four 11-year solar sunspot cycles, or two 22-year solar magnetic cycles.
The Haleakala Neutron Monitor Station, which consists of a standard 12-tube IGY monitor and an 18-tube NM-64 supermonitor (a design adopted in the early 1960s which used larger counters and produced much higher counting rates) was established in February 1991 to provide a long-term replacement for the IGY neutron monitor at Huancayo, Peru. The Haleakala site was chosen to provide a close match to the geomagnetic latitude and elevation of Huancayo. Twenty-one months of simultaneous operation of the Haleakala and Huancayo stations (Feb. 1991 through Oct. 1992) provided sufficient data to obtain a good intercalibration of the monitors.
More detail on the cross-calibration of the Huancayo and Haleakala monitors can be found here.