Simulation of 500 MeV neutrons by using NaCl doped Water Cherenkov detector
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 eV to 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 Cl based compound as an additive to the water, mainly because this isotope has a high cross Section (43.840.17 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 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 H and the three lines from 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.
Section snippets
Geant4 simulation scheme
We performed a detailed simulation of all the particle production physics and the detector geometry using Geant4 Allison et al. (2016). The detector is composed of a 96-cm-diameter by 133-cm-tall cylinder made of a of 0.5-m-thick stainless steel sheet. Depending on the selected configuration, the cylinder is filled with pure water or water with different concentrations of NaCl. Located inside at the top and center of the detector, there is a spherical section of h = 5 cm and 10.16 cm radii that
Results and discussion
Simulations in Geant4 were performed including all the relevant parameters of the detector (listed in Section 2) injecting 150000 neutrons. First we aimed to understand the absorption process of the neutrons, comparing H2O and including NaCl in different concentrations. In Table 2 we show the fraction of neutron captures within the active volume for the case of pure water and water with NaCl additive and the maximum absorption distance.
After a moderation process of the neutron in the water, it
Conclusions
This work introduces results from simulations that shows in detail the physics of a WCD when high energetic neutrons enter into the detector active volume. We conclude that using a WCD with pure water as active volume, and a single PMT, 500 MeV neutrons can be detected. In addition, we have also shown that adding NaCl as dopant to the water the signal intensity increase, thus enhancing the capability for measure high energy neutrons. Being NaCl, cheap, non-toxic and easily accessible material,
Acknowledgment
The authors would like to acknowledge the full support by CONICET and CNEA. We are very thankful to the technicians in our lab that help to set up the detectors: P. D’avanzo, A. Mansilla, F. Y. Moreira, G. Anibal. This work has been done thanks to the following grants: PICT ANPCyT 2015-1644, PICT ANPCyT 2016-2096, PIP CONICET 2011 0552, UNCuyo Proy. Cod. 06/C483 and 06/C594.
References (16)
- et al.
Properties and performance of the prototype instrument for the Pierre Auger Observatory
Nucl. Instrum. Methods Phys. Res., Sect. A
(2004) - et al.
Recent developments in Geant4
Nucl. Instrum. Methods Phys. Res., Sect. A
(2016) - et al.
Spectral-directional reflectivity of Tyvek immersed in water
Nucl. Instrum. Methods Phys. Res., Sect. A
(1999) Status of the Super-Kamiokande gadolinium project
Nucl. Instrum. Methods Phys. Res., Sect. A
(2013)- et al.
First study of neutron tagging with a water cherenkov detector
Astropart. Phys.
(2009) The discovery of cherenkov radiation and its use in the detection of extensive air showers
Nucl. Phys. B Proc.Suppl.
(2011)- et al.
Preliminary results from the latin american giant observatory space weather simulation chain
Space Weather
(2018) - et al.
Modeling the LAGO’s detectors response to secondary particles at ground level from the Antarctic to Mexico
PoS ICRC2019
(2019)
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