| dc.description.abstract | The propagation of high-energy cosmic rays is influenced by the time-varying heliospheric
magnetic field embedded in the solar wind, and by the geomagnetic field. To penetrate
through this geomagnetic field, they must have a rigidity that exceeds the geomagnetic cutoff
rigidity for a given position on the earth. In the atmosphere, the primary cosmic rays interact
with atmospheric nuclei, to form a cascade of secondary particles. Neutron monitors record
these secondary cosmic rays, mainly the neutrons, with energies about a decade higher than
detected by most spacecraft.
Since neutron monitors are integral detectors, each with its own detection efficiency, energy
spectra cannot readily be derived from their observations. One way to circumvent this is by
conducting latitudinal surveys with mobile neutron monitors. Another way is to use the
worldwide stationary neutron monitor network, but then the counting rates of these monitors
must be normalised sufficiently accurate against one another. For this reason two portable
calibration neutron monitors were built at the Potchefstroom campus of the North-West
University and completed in 2002.
To achieve sufficient calibration accuracy, several properties of the calibrator are
investigated in this work. Effects such as atmospheric pressure variations, diurnal variations,
short-term scintillations, and multiplicity, contribute to the fluctuations of the counting rate of a
neutron monitor. Due to these effects, the coefficient of variation of the calibrator is
determined to be -40% larger than the Poisson deviation. The energy response of the
calibrator over the cutoff rigidity interval from the poles to the equator is investigated, with the
result that it is almost 4% larger than that of a standard 3NM64 neutron monitor. It is also
determined that not only the calibrator, but also the stationary NM64 and IGY neutron
monitors, have fairly large instrumental temperature sensitivity, which must be accounted for
in calibration procedures. Furthermore, the calibrator has a large sensitivity to the type of
surface beneath it, influencing its counting rate by as much as 5%. This investigation is
incomplete and requires further experimentation before the calibration of the stationary
neutron monitors can start.
When calibrations of a significant number of the worldwide neutron monitors are done, their
intensity spectra as derived from differential response functions, will provide experimental
data for modulation studies at rigidities above 1 GV. | |
