Measurement and simulation of the neutron propagation time distribution inside a neutron monitor

Abstract

Using a setup for testing a prototype for a satellite-borne cosmic-ray ion detector, we have operated a stack of scintillator and silicon detectors on top of the Princess Sirindhorn Neutron Monitor (PSNM), an NM64 detector at 2560-m altitude at Doi Inthanon, Thailand (18.59 • N, 98.49 • E). Monte Carlo simulations have indicated that about 15% of the neutron counts by PSNM are due to interactions (mostly in the lead producer) of GeV-range protons among the atmospheric secondary particles from cosmic ray showers, which can be detected by the scintillator and silicon detectors. Those detectors can provide a timing trigger for measurement of the propagation time distribution of such neutrons as they scatter and propagate through the NM64, processes that are similar whether the interaction was initiated by an energetic proton (for 15% of the count rate) or neutron (for 80% of the count rate). This propagation time distribution underlies the time delay distribution between successive neutron counts, from which we can determine the leader fraction (inverse multiplicity), which has been used to monitor Galactic cosmic ray spectral variations over ∼1-40 GV. Here we have measured and characterized the propagation time distribution from both the experimental setup and Monte Carlo simulations of atmospheric secondary particle detection. We confirm a known propagation time distribution with a peak (at ≈70 µs) and tail over a few ms, dominated by neutron counts. We fit this distribution using an analytic model of neutron diffusion and absorption, for both experimental and Monte Carlo results. In addition we identify a group of prompt neutron monitor pulses that arrive within 20 µs of the charged-particle trigger, of which a substantial fraction can be attributed to charged-particle ionization in a proportional counter, according to both experimental and Monte Carlo results. Prompt pulses, either due to neutrons or charged-particle ionization, are associated with much higher mean multiplicity than typical pulses. These results validate and point the way to some improvements in Monte Carlo simulations and the resulting yield functions used to interpret the neutron monitor count rate and leader fraction.

Neutron Propagation Time Distribution Measured by Various Neutron Monitor Counters Relative to Direction-Tracked Charged Atmospheric Secondaries

“…We observed some background counts at all times from unrelated secondary particles from other cosmic ray showers. The spike from 𝑡 = 0 to 20 μs includes prompt pulses from charged particle ionization in the proportional counters, which was also observed in previous measurements and Monte Carlo simulations [10]. For simplicity we still refer to all signals as "neutron signals".…”

“…In the diffusion-absorption model of Chaiwongkhot (2021) [10], the distribution of the propagation time 𝑡 of slow neutrons inside the neutron monitor is related to the distance 𝑟 between neutron production and detection:…”

“…Neutron propagation from direction-tracked charged atmospheric secondaries where 𝛼 is the rate of neutron absorption by materials inside the neutron monitor, including capture by 10 B as detected by a proportional counter. The rise time depends on the distance 𝑟 as 𝑇 𝑟 ≡ (𝑥 2 + 𝑦 2 )/(4𝐷), where (𝑥, 𝑦) is the 2D displacement in the plane perpendicular to the proportional counter wire, and 𝐷 is a neutron diffusion coefficient.…”

“…In our previous work [10], we studied the propagation time distribution of neutrons associated with charged particles entering a neutron monitor. A small scintillator was placed on top of PSNM for non-destructive detection of charged secondary particles, which provided a timing trigger for neutron propagation time measurement.…”

Measurement of sparse vs. dense atmospheric secondary particles from cosmic ray showers using coincident signals on various counters in a neutron monitor

“…In a neutron monitor, atmospheric secondary particles from cosmic ray showers interact in lead to produce low-energy tertiary neutrons that are detected in a proportional counter. Studying the timing along the complicated path of a neutron inside the neutron monitor [1] is important for Monte Carlo simulations and studying the tertiary neutron multiplicity for which the inverse is referred to here as the "leader fraction" [2].…”