Simulation of 500 MeV neutrons by using NaCl doped Water Cherenkov detector

Abstract

In this work we show the capabilities of a water Cherenkov detector (WCD) to detect high energy neutrons. We present the simulation of the response of a doped WCD to 500 MeV monochromatic neutrons using Geant4. To do this, a detailed model of the WCD has been implemented. The active volume of the detector is composed of pure water and different concentrations of a ${}^{35}$Cl based additive. The addition of this dopant shows an enhancement in the detection of high energetic neutrons. The sensitivity of this detector to neutrons achieved in our simulations is a relevant result for cosmic rays and space weather studies.

Introduction

Water Cherenkov detectors (WCD) of large volumes using ultra pure water as active volume, are sensitive to charged relativistic particles and high energy photons. These detectors are used in a variety of implementations: from a diversity of astrophysical studies to the detection of Special Nuclear Material (SNM) for homeland security. WCD are usually chosen because they are cheap, robust and have a large solid angle. These detector have been widely used for cosmic ray detection since the late sixties, Watson (2011). The LAGO Suárez-Durán et al., 2016, Asorey et al., 2018, Sidelnik, 2016, Auger Abraham et al. (2004) and HAWC Joshi et al. (2018) collaborations use nowadays WCDs to measure changes in the flux of cosmic rays and relate them with solar activity indicators studying, for instance, Forbush decreases Suárez-Durán et al. (2016). This could point out that a ground based space weather oriented experiment, using neutron monitors to study low energy cosmic ray flux variations, can use WCD as an alternative detector. There has also been a growing interest in the measurement of neutrons with additives into the water to increase the response of these detectors. The Super Kamiokande collaboration, for example, has been testing different approaches to the use of gadolinium (Gd) in their very large WCD (50,000 ton) Mori, 2013, Watanabe et al., 2009.

There are studies that found that the major change in low energy cosmic rays are produced by neutrons as a part of secondary showers produced by cosmic ray particles Asorey et al., 2018, Calderón-Ardila et al., 2019. CORSIKA simulations performed by the LAGO Collaboration shows that for cosmic rays that have its primary energy in the ${10}^{11}$ eV to ${10}^{15}$ eV range, and for different altitudes¹, the flux of secondaries is dominated by neutrons, with a spectrum that has a peak at 500 MeV Asorey et al., 2018, Calderón-Ardila et al., 2019, extending beyond the GeV. For this reason we choose the primary neutron energy of 500 MeV to run the present study.

Our choice is to use a ${}^{35}$Cl based compound as an additive to the water, mainly because this isotope has a high cross Section (43.84$\pm$0.17 barns) for neutron absorption and a gamma prompt emission in cascade with a maximum of $~$8578.6 keV Firestone et al. (2007). For this reason NaCl salt was selected to be used in the water Cherenkov detector, not only for the mentioned advantages of the Cl, but it is also a compound that can be easily dissolved in water, is cheap, and it is not contaminant if spilled, unlike Gd. WCD results ideal to detect high energy neutrons because of the neutron moderation capability of water.

In Table 1 the main elements used in the active volume of the detector are showed, as long as the corresponding abundance and neutron absorption cross sections with the most energetic prompt gamma line. The intensity of the prompt gamma lines emitted are determined by the neutron absorption probability as well as the line yield. In order to take into account both values simultaneously, a k₀ value is computed as the product of the line yield and the neutron absorption cross section. The k₀ value for a gamma ray emitted from each isotope is defined relative to that of ${}^1$H, see details in Firestone et al. (2007). In order to take into account both magnitudes a k₀ is commonly used. Table 1 shows the most intense value of k₀ for each isotope,

It can be seen in Table 1 that the most intense lines are the 2223 keV from ${}^1$H and the three lines from ${}^{35}$Cl. It must be noted that the k₀ value reported are nuclear parameters. In each simulated configuration the Geant4 code takes into account not only these values, but also the number of nuclei per unit volume.

Our group has been working in neutron detection with WCD that uses pure water Sidelnik et al. (2017) and with NaCl as additive Sidelnik et al. (2019) using neutron sources that reaches energies of $~$15 MeV. In this work we present the study of a WCD of 1 m³ for neutron detection comparing the use of pure water and a non contaminant additive (NaCl) with a single PMT for 500 MeV impinging neutrons.

In Section 2 we describe the characteristics of the simulation performed with Geant4. Section 3 shows the results obtained, and finally, the conclusions are presented in Section 4.