A ground-based neutron monitor detects atmospheric secondary particles from cosmic ray
showers that interact in the monitor to produce neutrons, which can propagate to and be
detected by neighboring neutron monitor counters. To study the propagation time
distribution, the arrival time and position of atmospheric secondaries are needed. In this work,
we used a plastic scintillator bar with two silicon photomultipliers attached at opposite ends
to track the position of charged secondary particle passage. Triggering together with a small
scintillator above that scintillator bar, we determined the trajectories of charged secondary
particles and traced them to the lead producer of the Princess Sirindhorn Neutron Monitor
(PSNM). With such timing and position data, we can study the propagation time distribution
measured by various neutron monitor counters and compare that with the two-dimensional
diffusion-absorption model proposed in previous work.