Measurement and simulation of neutron monitor count rate dependence on surrounding structure

Abstract

Neutron monitors are the premier instruments for precise measurements of time variations (e.g., of solar origin) in the galactic cosmic ray (GCR) flux in the range of ∼1–100 GeV. However, it has proven challenging to accurately determine the yield function (effective area) versus rigidity in order to relate a neutron monitor's count rate with those of other monitors worldwide and the underlying GCR spectrum. Monte Carlo simulations of the yield function have been developed, but there have been few opportunities to validate these observationally, especially regarding the particular environment surrounding each monitor. Here we have precisely measured the count rate of a calibration neutron monitor near the Princess Sirindhorn Neutron Monitor (PSNM) at Doi Inthanon, Thailand (18.59∘N, 98.49∘E, 2560 m altitude), which provides a basis for comparison with count rates of other neutron monitors worldwide that are similarly calibrated. We directly measured the effect of surrounding structure by operating the calibrator outside and inside the building. Using Monte Carlo simulations, we clarify differences in response of the calibrator and PSNM, as well as the calibrator outside and inside the building. The dependence of the calibrator count rate on surrounding structure can be attributed to its sensitivity to neutrons of 0.5–10 MeV and a shift of sensitivity to nucleons of higher energy when placed inside the building. Simulated calibrator to PSNM count rate ratios inside and outside agree with observations within a few percent, providing useful validation and improving confidence in our ability to model the yield function for a neutron monitor station.

Monte Carlo simulation of the neutron monitor yield function

“…Moreover, the model does not take into account moist air or seasonal variation. In order to account for those effects, the atmospheric profile of the present work (and also Aiemsa‐ad et al []) combines information from two models: the Global Data Assimilation System database (GDAS, http://ready.arl.noaa.gov/gdas1.php) at low altitude (which includes moist air), and the Naval Research Laboratory Mass Spectrometer, Incoherent Scatter Radar Extended model (NRLMSISE‐00) [ Picone et al , ] for dry air at higher altitude. Details of our calculation of the profiles are presented in Appendix .…”

“…For vertical incidence, the beam is centered over the building and started at 6 m above the floor with a rectangular area of 34 × 30 m 2 . For nonvertical incidence, the beam area is expanded depending on the incidence angle so that the beam covers the detector well, as described in Aiemsa‐ad et al []. We define a count in the detector as a 10 B disintegration (due to neutron capture) in one of the neutron counter tubes.…”

Dependence of the neutron monitor count rate and time delay distribution on the rigidity spectrum of primary cosmic rays

“…The area is expanded depending on the incidence angle in order for the beam to cover the detector well [Aiemsa-ad et al, 2015]. The beam is centered and located at 6 m over the MM with a rectangular area of 100 m 2 for vertical incidence.…”

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

“…Combining data from multiple NMs can in theory indicate variations of the cosmic ray spectrum. However, such analysis requires an inter-calibration to ∼ 0.2% of the yield functions of different detectors and achieving this accuracy proves to be very challenging both experimentally [3,10] and computationally via Monte Carlo simulation [7]. Recording and analyzing the time between successive counts may allow us to track the variations of the secondary particle spectrum using a single neutron monitor, thus avoiding such systematic uncertainties.…”