Relationship between the neutron time delay distribution and the rigidity spectrum of primary cosmic rays up to 16.8 GV
2016
Neutron monitors are the premier instruments for precisely tracking time variations in the Galactic
cosmic ray flux at GeV-range energies above the geomagnetic cutoff at the location of measurement.
In addition to the count rate, recording and analyzing the time delays between successive
counts allows us to infer variations in the cosmic ray spectrum as well. In particular, we can
determine the leader fraction L, defined as the fraction of neutrons that did not follow a previous
neutron detection in the same tube from the same nuclear interaction, from time delay histograms.
By analyzing data taken during 2001–2007 by a ship-borne neutron monitor latitude survey we
confirm a strong dependence of L on the geomagnetic cutoff up to 16 GV. The data of the Princess
Sirindhorn Neutron Monitor located in Thailand at a higher vertical cutoff of 16.8 GV have also
been analyzed for the period 2007–2014. We have developed Monte Carlo simulations of cosmic
ray interactions in the atmosphere and in both neutron monitors. Both data and simulations show
that the absolute value of L depends significantly of the configuration of the detector and that
experimental conditions such as the electronic dead time must be well monitored. The simulation
results show a change in L with the geomagnetic cutoff as observed by the latitude survey, confirming
that this change in L can reasonably be attributed to changes in the cosmic ray spectrum.
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