Detecting single and multiple atmospheric secondaries in an 18NM64

Abstract

We present an update on the analysis of cross-counter time-delay histograms with 5 µs bins as recorded since late 2015 in the Princess Sirindhorn Neutron Monitor, an 18NM64 (with 18 counter tubes in a continuous row) at 2560 m altitude in Doi Inthanon, Thailand. A cross-counter relative leader fraction Li j can be defined as the fraction of pulses measured in counter number i that are uncorrelated in time with the most recent pulse in counter j. For low tube separation, Li j rapidly increases with increasing tube separation |i− j|, as expected from the spatial spreading of neutrons produced by the same atmospheric secondary particle inside the neutron monitor. At high tube separation, Li j increases slowly with increasing tube separation and remains different from unity, which we interpret as due to multiple atmospheric secondaries arriving in rapid succession after originating from the same primary cosmic ray. We study temporal variations of the cross-counter relative leader fraction at large tube separation (|i− j| > 6, or roughly more than 3 m) and analyze the possible contribution of atmospheric effects to the data.