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Measurement of the Low-Energy Portion of the Primary Cosmic Ray Spectrum
Most cosmic rays come from the galaxy, though some are produced by eruptive events on the Sun’s surface. These high energy cosmic ray particles can affect satellites, causing single event upsets. Scientists have postulated that cosmic rays can affect the earth by causing changes in weather. Cosmic rays can cause clouds to form in the upper atmosphere, after the particles collide with other atmospheric particles in our troposphere. The process of a cosmic ray particle colliding with particles in our atmosphere and disintegrating into smaller pions, muons, and the like, is called a cosmic ray shower. These particles can be measured on the Earth’s surface by neutron monitors.Ground-based neutron monitors detect variations in the approximately 500 Mev to 20 GeV (low energy) portion of the primary cosmic ray spectrum. This class of cosmic ray detector is more sensitive in the approximate 500 Mev to 4 GeV portion of the cosmic ray spectrum than are cosmic ray muon detectors. The portion of the cosmic ray spectrum that reaches the Earth’s atmosphere is controlled by the geomagnetic cutoff which varies from a minimum (theoretically zero) at the magnetic poles to a vertical cosmic ray cutoff of about 15 GV (ranging from 13 to 17) in the equatorial regions. (Note: GeV is a unit of energy, GV is a unit of magnetic rigidity). The primary cosmic ray particles interact with the atmosphere and generate secondaries, some of which will reach the surface of the Earth. In high latitude regions of the Earth, where the geomagnetic cutoff is low, the lower threshold response of the neutron monitor is controlled by the atmospheric mass (about 1030 grams at sea level) which limits the response threshold of the neutron monitor to primary radiation of about 430 MeV. (At the South Pole, where the surface is 2820 M above sea level, the reduced atmospheric mass lowers the primary radiation detection threshold to about 300 MeV). At mid-latitudes or equatorial latitudes, the detection threshold is controlled by the geomagnetic cutoff. Neutron monitors at high altitudes have higher counting rates than neutron monitors at lower altitudes because of the atmospheric absorption of the cosmic ray secondaries generated near the top of the atmosphere. When the secondary cosmic rays interact in the monitor, (actually in lead surrounding the counters) they cause nuclear disintegrations, or stars. These stars are composed of charged fragments and neutrons typically in the energy range of tens to hundreds of MeV, even up to GeV energies. As a result of these high energy nuclear interactions, there will be more secondary fragments generated than incident particles and hence there is a multiplier effect for the counters. The neutrons are moderated and then counted using Boron tri-fluoride (BF3) proportional counters which are efficient thermal neutron detectors; hence the name neutron monitor. The original design is often designated as an IGY neutron monitor. A description of this type of instrument is given by Simpson, (Annals of the IGY, Vol. 4, pp. 351-373, 1957). The NM-64 or super neutron monitor was developed for the IQSY (International Quiet Sun Years 1964-65) when instruments with a higher counting capacity were required. A description of this type of neutron monitor is given by Carmichael (Annals of the IQSY, Vol. 1, pp. 178-197, 1968). Super neutron monitors are often designated as xx-NM-64 where xx is the number of tubes in the monitors. An 18-NM-64 at high latitude has a counting rate of approximately 1 million counts per hour or 0.1 percent statistics. The neutron monitor and supermonitor digital data for the time period 1953 to present are available at http://spidr.ngdc.noaa.gov and consist of over 100 stations’ hourly (UT) values of: (a) counting rates corrected for atmospheric pressure effects; (b) uncorrected counting rates; and (c) atmospheric pressure data. If it is not possible to have all three types of data, the most important is the counting rate corrected for atmospheric pressure.NGDC also holds paper archives for some cosmic ray Ionization Chamber monitors and some meson telescopes, mostly during the period of the International Geophysical Year (IGY) and International Quiet Sun Years (IQSY) 1957-1968.