High‐Altitude Polar NM With the New DAQ System as a Tool to Study Details of the Cosmic‐Ray Induced Nucleonic Cascade

Abstract

NMs record primarily the secondary nucleonic component (mostly neutrons) of the cosmic-ray induced atmospheric cascade with a small fraction of counts caused by muons. Its count rate is defined by the flux of primary (impinging on the top of the atmosphere) cosmic rays, as a combination of the cosmic-ray energy spectrum, detector's yield function and geomagnetic rigidity cutoff (Clem & Dorman, 2000;Mishev et al., 2020). The NM is an energy-integrating detector, with the effective energy ranging from about 12 GeV for polar NMs to 35 GeV for equatorial ones (Asvestari et al., 2017). The sensitivity of NMs to low-energy cosmic rays is highest in polar regions (low or no geomagnetic shielding) and high altitudes (lower atmospheric shielding) and decreases toward equatorial latitudes. The worldwide network of NMs can act as a giant spectrometer able to roughly estimate the spectrum of both galactic cosmic rays (e.g., Dorman, 2004) and relativistic solar protons (e.g., Duggal, 1979;Mishev et al., 2014). Along with the count rate, the multiplicity of NM counts (the average number of pulses within a short time interval) is sometimes studied (Balabin et al., 2011;Dorman, 2004) as a rough index of the spectral hardness of cosmic rays. Sometimes NM are accompanied by separate muon detectors to measure high-energy cosmic rays. This work is focused at two mini-NMs, DOMC and DOMB, located at the Concordia Antarctic Research station on top of Dome C, Central Antarctic plateau (75°06′S, 123°23′E, 3,233 m above sea level) (Poluianov et al., 2015). They are ones of the most sensitive NMs to lower energy cosmic rays (including solar energetic particles) thanks to the highly elevated polar location. Each NM has one BF 3 -filled detector surrounded by reflecting and moderating layers of polyethylene. In addition, DOMC has a layer of lead serving as a neutron Abstract A neutron monitor (NM) is, since the 1950s, a standard ground-based detector whose count rate reflects cosmic-ray variability. The worldwide network of NMs forms a rough spectrometer for cosmic rays. Recently, a brand-new data-acquisition (DAQ) system has been installed on the DOMC and DOMB NMs, located at the Concordia research station on the Central Antarctic plateau. The new DAQ system digitizes, at a 2-MHz sampling rate, and records all individual pulses corresponding to secondary particles in the detector. An analysis of the pulse characteristics (viz. shape, magnitude, duration, waiting time) has been performed, and several clearly distinguishable branches were identified: (A) corresponding to signal from individual secondary neutrons; (B) representing the detector's noise; (C) double pulses corresponding to the shortly separated nucleons of the same atmospheric cascades; (D) very-high multiple pulses which are likely caused by atmospheric muons; and (E) double pulses potentially caused by contamination of the neighboring detector. An analysis of the waiting-time distributions has revealed two clearly distinguishable peaks: peak (I) at about 1 ms being related to t...

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

“…The simulated time distribution of pulses in this range due to charged atmospheric secondary particles passing through the scintillator and PIN array is shown in Figure 3(c). These pulses are exclusively due to neutron capture by 10 B. Keeping in mind that the simulation does not include the uniform background of NM pulses that are unrelated to the charged-particle trigger, and the simulation has a different overall number of pulses than the experiment, the qualitative agreement between this simulated distribution and the experimental distribution in Figure 3(a) is quite close, except that the simulated distribution does not have a strong spike near t = 0.…”

“…We expect that the measured pulse height in the proportional counter corresponds proportionately to the simulated energy deposition. Based on a correspondence between the peak pulse height of observed NM pulses and the most probable energy release from 10 B fission (2.310 MeV), the high pulse height range of PH ≥ 1 V corresponds to energy deposition E d ≥ 1.36 MeV. The simulated time distribution of pulses in this range due to charged atmospheric secondary particles passing through the scintillator and PIN array is shown in Figure 3(c).…”

“…Our procedure also has the ability to detect pile-up and distinguish and individually record pulses overlapping by as little as 4 µs, though this does not frequently occur in the NM output. Note that overlapping pulses were also reported by [10], but they considered overlapping pulses as a single, long pulse. For consistency with pulse height distributions generated by standard NM electronics, in the case of pile-up between pulses that overlap, our procedure first records the total pulse height at the highest peak, not separating the pulses, and later records the pulse heights of the lower (remaining) peaks.…”

“…For this calculation, neutron production could result from the interaction of the charged atmospheric secondary particle (usually in the lead producer) or from subsequent interactions of such interaction products. Whenever a neutron is produced in the simulation, we record its energy and location and compare with those values when captured by 10 B (if such capture occurs). The peak of the distribution of the squared displacement is for a distance of 19.5 cm, slightly larger than the value of s = 18 cm obtained from Figure 6.…”

“…Recent work has shown that using neutron timing information from a single NM station can avoid such systematic errors and be used to provide information about spectral variations of galactic cosmic ray (GCR) ions over a rigidity range of ∼1-40 GV, from time scales as short as days to as long as the ∼11year solar cycle [3,4]. This method involves the use of specialized electronics to record distributions of the time delay (up to several ms) between successive neutrons detected by a NM counter tube [5,6,7,8,9,10], preferably on an hourly basis (or more frequently) to allow accurate correction for atmospheric pressure. As will be described in detail in the following section, a standard NM contains a lead producer in which atmospheric secondary particles from cosmic ray showers interact to produce neutrons that are then detected by neutron-sensitive proportional counters, which are more sensitive to thermal neutrons than neutrons of higher energy.…”