A calibration neutron monitor: Energy response and instrumental temperature sensitivity

Neutron monitors are integral detectors of secondary cosmic rays. Since each of them has its own detection efficiency, energy spectra cannot readily be derived from their observations. To circumvent this problem, latitudinal surveys have been conducted for many years with mobile neutron monitors to derive such spectra. Another way, however, is to use the worldwide stationary neutron monitor network, but then the counting rates of these monitors must be normalized sufficiently accurately against one another. For this reason, two portable calibration neutron monitors were built at the Potchefstroom campus of the North-West University, South Africa. When calibrations of an adequate number of the worldwide neutron monitors have been done, this paper demonstrates that the differential response functions derived from them will provide experimental data for modulation studies in the rigidity range of 1 to 15 GV. Several properties of these calibrators were investigated, in order to achieve sufficient calibration accuracy. The energy response over the cutoff rigidity range from the poles to the equator, as well as the instrumental temperature sensitivity, are described in this paper. The main conclusion is that the calibrator has a difference of almost 4% in its energy response over the cutoff rigidity range 0-16 GV in comparison with a standard 3NM64 neutron monitor. Furthermore, it is shown that not only the calibrator, but also the NM64 and IGY monitors, have fairly large instrumental temperature sensitivities. Correction coefficients for these effects are given. Copyright 2008 by the American Geophysical Union.