ICRC2015

Relationship between the Neutron Time Delay Distribution and the Rigidity Spectrum of Primary Cosmic Rays up to 16.8GV

1 Aug 2015, 15:30

1h

Theater Foyer (World Forum)

Board: 16

Poster contribution SH-EX Poster 2 SH

Speaker

Dr Pierre-Simon MANGEARD (National Astronomical Research Institute of Thailand, Mahidol University)

Description

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 analysing 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 (PSNM) located in Thailand at a higher vertical cutoff of 16.8 GV have also been analysed 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. Supported in part by a postdoctoral fellowship from Mahidol University, the Thailand Research Fund, the United States National Science Foundation (OPP-0838839,PLR-1245939, and their predecessors) and the Australian Antarctic Division.