Calculated specific yield functions for neutron monitors

The world’s neutron monitor network was initiated about 50 years ago. It grew to a peak of almost 100 stations towards the end of the 1960s and at present about half of these are still active. Many of the original questions about the production of atmospheric secondaries, geomagnetic effects, and neutron monitor response in general, have been settled satisfactorily. Due to their long-term reliability and automated data acquisition, the remaining neutron monitors in the network are well suited for important future contributions in several areas. Amongst these are (a) spectral measurements, which require an optimal distribution along cutoff rigidities; (b) anisotropy studies, which require a set of neutron monitors with well-defined, narrow cones of acceptance for charged particles, covering all directions as evenly as possible; and (c) solar neutron measurements, which primarily require an even distribution in longitude, at high altitude and near the equator. Steps have already been taken to improve the network with these goals in mind, and to standardize methods. This contribution describes some of these steps and suggests further strategies to achieve the most optimal network.

Neutronic analysis of JET external neutron monitor response

Neutron monitors are a standard tool for high-precision monitoring of changes in the Galactic cosmic ray (GCR) flux that occur as a result of variations of the heliospheric conditions and solar storms. In Thailand, we have developed a mobile neutron monitor named “Changvan” based on the 3NM64 design, except that the middle counter lacks lead producer rings, so we call it a semileaded neutron monitor. From 2021 April to 2023 April, the Changvan operated at the Science and Technology Park, Mae Hia, Chiang Mai, Thailand, at an altitude of about 340 m above sea level, with a vertical cutoff rigidity of 16.7 GV, which is near the highest value observed globally. With a similar cutoff rigidity, the nearby Princess Sirindhorn Neutron Monitor is an 18NM64 at the summit of Doi Inthanon, the highest mountain in Thailand, at 2560 m above sea level. We examine how count rates at different altitudes with similar cutoffs, count rates from the unleaded versus leaded counters, and the leader fraction measured from time-delay histograms all reflect GCR spectral differences, thus providing a variety of techniques for tracking cosmic-ray spectral changes. Applied at high cutoff rigidity, these methods extend the reach of the worldwide neutron monitor network to even higher rigidity.

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.

A new reconstruction of the heliospheric modulation potential for galactic cosmic rays is presented for the neutron monitor era, since 1951. The new reconstruction is based on an updated methodology in comparison to previous reconstructions: (1) the use of the new‐generation neutron monitor yield function; (2) the use of the new model of the local interstellar spectrum, employing in particular direct data from the distant missions; and (3) the calibration of the neutron monitor responses to direct measurements of the cosmic ray spectrum performed by the Payload for Antimatter Matter Exploration and Light‐nuclei Astrophysics (PAMELA) spaceborne spectrometer over 47 time intervals during 2006–2010. The reconstruction is based on data from six standard NM64‐type neutron monitors (Apatity, Inuvik, Kergulen, Moscow, Newark, and Oulu) since 1965 and two International Geophysical Year‐type ground‐based detectors (Climax and Mount Washington) for 1951–1964. The new reconstruction, along with the estimated uncertainties is tabulated in the paper. The presented series forms a benchmark record of the cosmic ray variability (in the energy range between 1 and 30 GeV) for the last 60 years and can be used in long‐term studies in the fields of solar, heliospheric, and solar‐terrestrial physics.

A study of atmospheric processes based on neutron monitor data and Cherenkov counter measurements at high mountain altitude

Neutron monitors are essential instruments for studying cosmic rays, requiring accurate response modeling for interpreting flux variations. A new boron trifluoride (BF$_3$) proportional counter from LND Inc. matches the geometry, gas volume, and pressure of the standard NM64 BP28 counter but differs in anode and cathode thickness. At the Princess Sirindhorn Neutron Monitor (PSNM) in Chiang Mai, Thailand, an LND tube temporarily replaced one of three bare BP28 tubes between September 2022 and October 2023. This substitution enabled a direct comparison of their responses. Using Monte Carlo simulations with monthly solar modulation parameters from Usoskin et al.\ (2017), we modeled secondary particle interactions within these detectors. Simulated responses were benchmarked against actual count rates, confirming that the LND tube’s sensitivity was consistently about 0.93–0.95 relative to the BP28, aligning closely with laboratory calibrations. These findings validate the LND tube as a viable alternative for neutron monitor networks

The original design by J. A. Simpson of the neutron monitor enabled continuous monitoring of the primary cosmic-ray flux by ground-based recordings of the nucleonic component with only a rather simple correction for atmospheric effects. Simpson (1957) extended the original pile to the 12 counter IGY neutron monitor which was deployed in a world wide network during the International Geophysical Year 1957/8. The desirability for monitors with higher counting rates became evident soon afterwards. Subsequently the NM64 super neutron monitor was designed by H. Carmichael for deployment in time for the International Quiet Sun Year 1964. Using unusually large 10BF3 proportional counters made at Chalk River, Hatton and Carmichael (1964) studied comprehensively the experimental design of the NM64. Consequently the efficiency of neutron counters to record evaporation neutrons produced in the lead of a monitor increased from 1.9% for the IGY to 5.7% for the NM64, an increase of 3.3 times the counting rate per unit area of lead producer. During the years much attention was given to the neutron multiplicity spectrum in neutron monitors. This spectrum is related to the energy spectrum of the nucleonic component incident on the neutron monitor, but is only weakly dependent on the spectrum of galactic cosmic rays at the top of the atmosphere. Contrary to galactic cosmic rays, solar flare protons and neutrons are observed predominantly as single counts per interaction, in multiplicity 1, because of the softness of solar flare particle energy spectra. Neutron monitors have also been specially designed to record solar neutrons with increased sensitivity. Newly developed 3He counters with a largely reduced thermal neutron absorption mean free path should lead to improved efficiency in recording primary cosmic radiation. Design criteria are discussed.

Evidence for interplanetary acceleration of cosmic rays : The GLE of 4 August 1972

Neutron monitors are integral detectors of secondary cosmic rays. Since each of them has its own detection efficiency, energy spectra cannot readily be derived from their observations. To circumvent this problem, latitudinal surveys have been conducted for many years with mobile neutron monitors to derive such spectra. Another way, however, is to use the worldwide stationary neutron monitor network, but then the counting rates of these monitors must be normalized sufficiently accurately against one another. For this reason, two portable calibration neutron monitors were built at the Potchefstroom campus of the North‐West University, South Africa. When calibrations of an adequate number of the worldwide neutron monitors have been done, this paper demonstrates that the differential response functions derived from them will provide experimental data for modulation studies in the rigidity range of 1 to 15 GV. Several properties of these calibrators were investigated, in order to achieve sufficient calibration accuracy. The energy response over the cutoff rigidity range from the poles to the equator, as well as the instrumental temperature sensitivity, are described in this paper. The main conclusion is that the calibrator has a difference of almost 4% in its energy response over the cutoff rigidity range 0–16 GV in comparison with a standard 3NM64 neutron monitor. Furthermore, it is shown that not only the calibrator, but also the NM64 and IGY monitors, have fairly large instrumental temperature sensitivities. Correction coefficients for these effects are given.

<p>A Forbush decrease (FD) is the decrease in Galactic cosmic ray (GCR) flux, e.g., as observed by a neutron monitor count rate, in association with a coronal mass ejection (CME) and/or its shock. The FD amplitude is known to decrease at higher cutoff rigidity. During Solar Cycle 24, the Mawson neutron monitor in Antarctica, with a low (atmosphere-limited) cutoff rigidity of ~1 GV, observed numerous FDs, while the Princess Sirindhorn Neutron Monitor (PSNM) located near the Earth's equator at Doi Inthanon, Thailand, with the world’s highest geomagnetic cutoff rigidity (17 GV), observed only a fraction of these as FDs.  Instead, we find that the shock arrival is often followed by repeated dips in the PSNM count rate at only certain times of day, while the GCR flux from other directions remains near the pre-shock level.  We refer to decreases of this type as diurnal dips.  In this work, we have surveyed FDs and diurnal dip events observed by PSNM and the dependence of their maximum amplitude on the solar wind speed, ICME speed, and interplanetary magnetic field.  We acknowledge logistical support from Australia's Antarctic Program for operating the Mawson NM and support from the National Astronomical Research Institute of Thailand and grant RTA6280002 from Thailand Science Research and Innovation.</p>

In April 2017, a mini neutron monitor (NM) was installed at King Abdulaziz City for Science and Technology (KACST) central Saudi Arabia (Riyadh; cut-off rigidity, Rc = 14.4 Gv) for continuous observation of the cosmic ray (CR) neutrons. The detector was built as a major aspect of the international scientific joint effort between the Centre of Space Research (North-West University, Potchefstroom, South Africa) and KACST. The recorded data correspond to low energy neutrons that primarily have energies lower than 20 GeV. In this paper, a brief description about the mini NM detector will be given. The influence of atmospheric pressure on the recorded CR neutrons was studied and the barometric coefficient was calculated and used to eliminate the pressure effects from the measured data. The obtained coefficient was consistent with those previously obtained by several investigators. The daily variation of the CR neutron was studied and characterized. Short-term CR periodicities, such as the 27-day period, and its two harmonics, were identified. The obtained periodicities are in agreement with those reported by different researchers. The obtained results from this detector have been compared to the existing 1 m2 scintillator detector showing comparable results. Long-term data from this detector will be of incredible significance to the research community to investigate several types of CR variations resulting from solar activity at such high cut off rigidity site.

This report describes a general yield function for anisotropic elastoplastic materials written in a quadratic form in terms of the stress components. The function contains 28 independent coefficients that must be determined experimentally. A set of inequality constraint conditions imposed on these coefficients is derived from the convexity requirement for all yield functions. A number of previously derived yield functions are recognized here as special cases of the general quadratic yield function, and the convexity condition for each of them is also provided. These inequality conditions should aid experimental projects for material identification.

The pressure corrected hourly data from the global network of cosmic ray detectors, measurements of the interplanetary magnetic field (IMF) intensity (B) at Earth’s orbit and its components B x , B y , B z (in the geocentric solar ecliptic coordinates) are used to conduct a comprehensive study of the galactic cosmic ray (GCR) intensity fluctuations caused by the halo coronal mass ejection of 13 May 2005. Distinct differences exist in GCR timelines recorded by neutron monitors (NMs) and multidirectional muon telescopes (MTs), the latter respond to the high rigidity portion of the GCR differential rigidity spectrum. The Forbush decrease (FD) onset in MTs is delayed (∼5 h) with respect to the onset of a geomagnetic storm sudden commencement (SSC) and a large pre-increase is present in MT data before, during, and after the SSC onset, of unknown origin. The rigidity spectrum, for a range of GCR rigidities (≤200 GV), is a power law in rigidity (R) with a negative exponent (γ=−1.05) at GCR minimum intensity, leading us to infer that the quasi-linear theory of modulation is inconsistent with observations at high rigidities (>1 GV); the results support the force field theory of modulation. At present, we do not have a comprehensive model for the FD explaining quantitatively all the observational features but we present a preliminary model listing physical processes that may contribute to a FD timeline. We explored the connections between different phases of the FD and the power spectra of IMF components but did not find a sustained relationship.

The intercalibration of 9 European monitors of the cosmic-ray micleonic component from Rome to Bergen (geomagnetic rigidity thresholds from 6.4 to 1 GV) was carried out in 1963 by means of a standard IGY neutron monitor mounted on a lorry. At each station the ratio of the fixed to the mobile monitor counting rate was measured along with the differences in the simultaneous readings of the « official » fixed barometer and of the mercury barometer of the expedition. Using these parameters the daily intensity data from each station were normalized and reduced to a single pressure level to draw daily sea-level latitude curves for the period June to October 1963. The curves obtained show the interest the experiment offers for the study of short-term perturbations of the primary spectrum at low rigidities. In fact a displacement of the latitude « knee » towards high threshold rigidities during Forbush decreases is observed, similar to the one already known to take place in the long-term perturbation.