First Observations of Cosmic Ray Neutrons by a Mini Neutron Monitor at Riyadh, Saudi Arabia

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 m 2 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.


Introduction
Cosmic rays (CRs) are energetic charged particles originating from deep spaces that reach the top of our atmosphere about 30 km above the Earth's surface.
They come from a variety of sources including our own Sun, stars and distant objects such as black holes and active galaxies. Most CRs are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table, as well as, high energy electrons, positrons, and other subatomic particles. Cosmic ray studies are linked to many branches of physics and astrophysics [1] [2]. Cosmic ray experiments allow high-energy physics researchers to extend their interaction models to super-accelerator energies, leading to the discovery of many elementary particles [3] [4]. Cosmic ray observations provide information for astrophysicists about the interstellar medium, magnetic fields and the nature of their sources [5] [6] [7]. Cosmic rays studies have been found to be a valuable tool to search for large disturbed phenomena in the heliosphere [2] [8]. Additionally, recent studies have shown that CRs, due to their capacity to cause ionization, influence various physio-chemical processes, which in turn affect the global weather and climate [9] [10] [11] [12]. Solar disturbances change the energy output from the sun, which affects the interplanetary medium and the terrestrial environment [13]. These variations affect the propagation of the high-energy CR in the heliosphere, and their rate at the top of the atmosphere is modulated [14] [15].
Knowledge of CR flux is important to understand the effect of the solar activity and CR on the physical and chemical properties of the Earth's atmosphere, e.g. [9].
Since their discovery by Viktor Hess in 1912, different types of space-based and ground-level CR detectors have been developed to study the CR modulations in different time scales [16] [17]. These detectors were sensitive to various components of CRs.
Because of its high cut off rigidity and unique location [18] [19] [20] [21], monitoring CR variations in Saudi Arabia is of an incredible significance for the research community. CR monitoring program began in Saudi Arabia in 2002 by installing a single channel detector for CR muon observations [22]. In 2017, as a part of our radiation detector laboratory activities [22], we have installed a mini-neutron monitor in Riyadh, Saudi Arabia (Rc = 14.4 GV), aiming to monitor and explore the variations of CR neutrons at various time scales. This detector was designed and built by the Centre of Space Research (North-West University, Potchefstroom, South Africa) and has been installed in a few locations around the world and demonstrates its usefulness with accuracy as comparable as the standard NM [23]- [29].
The paper is organized as follows: brief description about this detector will be given in Section 2. The obtained results will be presented and discussed in Section 3, whereas the conclusions and future work will be summarized in the last section.

Concept of Neutron Monitors
Neutron monitors are the standard (and most commonly use) ground-based in-International Journal of Astronomy and Astrophysics strument and most effective tool for monitoring the variations of the low energy (500 Mev to 20 GeV) part of the CR spectrum [30].
A standard neutron monitor, as developed by Simpson and his colleagues in 1953 [31], is designed as a counter surrounded with consecutive layers of a light-element material and lead ( Figure 1).
The outer layer reflector shields the detector from external thermal neutrons and is transparent to cascade neutrons, which are multiplied in the lead producer (producing additional low energy neutrons via nuclear interactions to increase the monitor's detection efficiency). The inner moderator slows nuclei down in order to enhance their registration probability. The multiplied and decelerated neutrons are then counted via electrical pulses produced in a proportional counter tube, traditionally filled with either BF3 or 3He gas [32].
The design of the basic NM has changed slightly over the years [17]  After several years of operations in different places, these types of detectors have shown excellent performance and comparable data with the standard NMs, which are then named mini NMs [37]. Since their first development, the mini-NMs design has been constantly optimized and the counting electronics updated. The detector used in this work is the up to dated version of the developed mini NM [36].

KACST Mini Neutron Monitor
Brief description of the installed detector will be presented below and detailed [38] [39].
The detector ( Figure 2) has a dimension of 85 cm long and 60 cm of radius which is around 1/3 of the original size of the typical full size NM64 neutron monitor. However, the count rate of this monitor is similar to a standard NM64 detector [36]. The utilized counter is LND2043 type and filled by BF3 gas with a pressure of 933 hPa. The pressure in the standard NM64 detectors is 300 mbar.
The high pressure used in the mini NM compensates for the reduced efficiency of the mini-NM as compared to the much larger full-size NM64.
The detector comprises of 63 cm long counter encompassed by a 2 cm polyethylene moderator which was surrounded by a 5 cm neutron producer made of lead. The lead ring is surrounded by a 9.5 cm polyethylene reflector. High voltage unit incorporated with the low-noise pre-amplifier was utilized to generate the high voltage (2300 V for the BF3 detector) for the detector tube. Atmospheric pressure was measured by Vaisala pressure transducer.
The detector has a GPS module for location determination. A PIC32 microcontroller was implemented to control the entire activity of the detector. A data acquisition unit was developed for logging the data, interfacing with the electronics, and storing the data. The electronics make data records with 1-second resolution.
The detector is situated in the radiation detector laboratory at KACST and has been in operation since April 2017. During this period, the detector went through periods of downtime due to technical issues which caused some periods of missing data. Thus, data for the period between April 2017-January 2018 were considered for the purpose of this study.
Also, cosmic ray data acquired from KACST muon detector installed at a similar site were utilized for correlations with those obtained from the mini neutron monitor for a similar timeframe. The muon detector is a single channel 1 m 2 plastic scintillator detector detects high energy CR muons. Details regarding this detector are given in several research articles [11] [40].

Pressure Corrections
To properly consider the impact of the solar activities on the primary CR, atmospheric effects on the secondary CR must be evaluated. Atmospheric pressure is the most effective atmospheric variable influencing the flux of the secondary CR particles (e.g., Dorman 2004).
I, is the cosmic ray rate at pressure P, I 0 is the rate at the mean atmospheric pressure at the site altitude (here 940 mbar), and α is the barometric coefficient. α can be determined experimentally by using a simple linear regression between the atmospheric pressure and the secondary cosmic ray rate. For instance, De Mendonca et al., [43] used data from the Global Muon Detector Network (GMDN) to study the atmospheric impacts on the cosmic ray muons and found that the barometric coefficient values range between 0.114%/mbar and 0.168%/mbar.
The obtained coefficients, from the neutron and muon data, were utilized to International Journal of Astronomy and Astrophysics eliminate the effect of the atmospheric pressure from the collected data using Equation (1). When the correction for local pressure effect has been made, result variations in CR rate reflect the condition of the heliosphere and the solar wind, which modulates the intensity of the primary CR flux. Figure 6 demonstrates the consistencies between the behaviour of the pressure corrected CR counts from the mini NM and the muon detector for a period of two months.

Diurnal Variations
The daily variations of the CR are because of the daily rotation of the earth around the sun, which results recording CR with greater and lesser intensities.   The daily variations of the muon data described by a gradual increase in rate

Short Term Periodicities
Power spectral investigations have been carried out using Fourier transform technique to investigate short term periodicities in the CR data gathered by the mini NM and muon detectors during the study period. Figure 9 is a power spectral density demonstrating the significant peak of different amplitudes and strengths over every one of the frequencies for the (a) neutron monitor and (b) muon detector.
The strength and the amplitudes of the peaks are diverse for the two detectors and generally found in the range run between 0 -0.05 1/h. The neutron monitor has a few significant peaks (higher than 99% significant level were considered), for example, the 15.

Conclusions
Solar disturbances affect the propagation of the primary high-energy cosmic rays in the heliosphere, and their rate at the top of the atmosphere is modulated. The effect of the atmospheric pressure on the mini neutron monitor data was studied and the required coefficient was determined. It was found that the atmospheric pressure is anti-correlated with the CR data. The barometric coefficient for this analysis was 0.61%/mbar ± 0.01%/mbar with a 0.65 correlation coefficient and was close to those reported by several investigators. The detectability of the mini neutron monitor was tested against the 1 m 2 muon detectors and showed comparable behaviours. These include the daily variations of the CR particles which presented diurnal behaviour similar to those previously reported.
Fourier transform analyses were carried out to the collected data to investigate the short term periodicities exhibited by CR particles. Some short-term periodicities, for example, the 11 -12 days, 7 days, 5 -6 days, and 4.3 days were identified. The obtained periodicities are similar to those reported by different researchers.
Despite the fact that the installed mini NM has a small size in comparisons with the standard neutron monitors, it indicated high abilities to record low-energy cosmic rays and a strong response of their variability.
With its unique position in this part of the world, long and short term data from this detector can be utilized to supplement CR data from neutron monitor and muon detector observations at different places around the world to explore the cosmic ray variations beyond the energy ranges of extant cosmic ray detectors.