Neutron time delay analysis for the princess sirindhorn neutron monitor at Doi Inthanon, Thailand

Abstract

The Princess Sirindhorn Neutron Monitor (PSNM) has operated as an 18-NM-64 since December, 2007 at the summit of Doi Inthanon, Thailand's highest mountain (2565 m altitude). PSNM records the flux of galactic cosmic rays with the world's highest vertical cutoff rigidity for a fixed station, 16.8 GV. In addition to monitoring the count rate, PSNM has special electronics, as previously deployed in a latitude survey, to record the time delay of each neutron from the previous one in the same tube. We accumulate and collect hourly time delay histograms for individual tubes (with over 50,000 counts), which show an exponential tail at long times ( > 1 ms) due to chance coincidences, i.e., counts associated with independent atmospheric nucleons. Shorter time delays, however, are dominated by counts from the same interaction between an atmospheric nucleon and a nucleus (typically Pb) in the neutron monitor, thus containing information about the energy distribution of atmospheric shower particles. Time delay analysis has a goal similar to multiplicity analysis, with the advantage that we examine and remove the effects of chance coincidences to derive the leader fraction, L, i.e., the fraction of neutron counts not associated with a previous neutron count in the same tube from the same nuclear interaction. While time variations in PSNM multiplicity are dominated by "contamination" from variations in chance coincidences according to the count rate (uncorrected for pressure), this is not evident for L. We report on variations of L with time and their dependence on atmospheric pressure.