Elsevier

Radiation Measurements

Volume 68, September 2014, Pages 42-48
Radiation Measurements

Radon contribution to single particle counts of the ARGO-YBJ detector

https://doi.org/10.1016/j.radmeas.2014.07.006Get rights and content

Highlights

  • The ARGO-YBJ experiment is an air shower detector for gamma ray astronomy of very large area.

  • The ARGO-YBJ detector can work into two modes: single particle mode and shower mode.

  • The work shows how natural radioactivity can influence the single particle counting.

  • The paper shows how to evidence (and correct) the radon influence on the detector counting.

Abstract

The ARGO-YBJ experiment is an air shower detector for gamma ray astronomy and cosmic ray studies with an energy threshold of ∼500 GeV. Working in “single particle mode”, i.e. counting the single particles hitting the detector at fixed time intervals, ARGO-YBJ can monitor cosmic ray and gamma ray transients at energies of a few GeV.

The single particle counting rate is modulated by the atmospheric pressure and temperature, and is affected by the local radioactivity from soil and air. Among the radioactive elements, radon gas is of particular importance since its concentration in air can vary significantly, according to environmental conditions. In this paper we evaluate the contribution of the radon daughter gamma ray emitters to the single particle counting rate measured by ARGO-YBJ. According to our analysis, the radon gas contribution is roughly 1–2%, producing a counting rate modulation of the same order of magnitude of the atmospheric effects.

Introduction

The ARGO-YBJ experiment, located at the YangBaJing Cosmic Ray Laboratory (Tibet, P.R. China, 4300 m a.s.l.), is a full coverage air shower detector devoted to gamma ray astronomy and cosmic ray studies, working with an energy threshold of ∼500 GeV. Besides the shower triggered detection, ARGO-YBJ operates in “single particle mode”, or “scaler mode”, i.e. counting the single particles hitting the detector in fixed time intervals (Aielli et al., 2008). This technique allows to study temporal variations of the cosmic ray flux due to solar events like Ground Level Enhancements and Forbush decreases, and to search for short time duration excesses from Gamma Ray Bursts at primary energies above a few GeV (Aielli et al., 2009). In such a measurement, it is important to identify all the possible causes of counting rate variations due to local sources.

The single particle counting rate is modulated by the atmospheric pressure (that affects the shower propagation in the atmosphere) and the ambient temperature (that affects the detector efficiency). Beside these known effects, that in principle can be corrected, a fraction of the counting rate is due to gamma rays emitted by natural radionuclides (Cattaneo et al., 2009): a constant contribution from soil (and concrete floor) radioactivity under the ARGO-YBJ hall and a variable and less predictable one due to radon concentration in air.

In this paper we evaluate the effect of gamma rays from the decay of radon daughters. The data of radon measurements performed with different techniques at the experiment site will be correlated with the ARGO-YBJ counting rate and compared to the rate expected by simulating the radon activity in the ARGO-YBJ hall.

Section snippets

The ARGO-YBJ detector

The ARGO-YBJ detector consists of a 74 × 78 m2 carpet made of Resistive Plate Chambers (RPCs) operated in streamer mode, with 92% of active area, surrounded by a partially instrumented (20%) area up to 100 × 110 m2. The detector is divided into 153 logical units, called clusters, of area 5.7 × 7.6 m2, each composed of 12 RPCs. The experiment layout is shown in Fig. 1, where the position of the radon monitors and the clusters used in this analysis are also marked. Details about the detector and

Radon gas concentration at the ARGO-YBJ site

Radon (222Rn) is a noble gas belonging to the uranium (238U) radioactive family. It is produced via the decay of 226Ra and emanates from soil. It enters into buildings because it is an inert gas with a half-life of 3.82 days. It produces radionuclides emitting gamma rays able to influence the detector counting rate: 214Bi and 214Pb (BIPM, 2004). Because the radon progeny activity is approximately proportional to that of radon in air, in this work we study the influence of the radon

Monte Carlo simulations

Searching for a possible radon influence on the ARGO-YBJ lowest multiplicity channel counts, we first simulate the response of the RPC detectors to the gamma rays emitted by the 222Rn daughters, using the FLUKA code (Battistoni et al., 2007, Fassò et al., 2005). The detector efficiency has been evaluated simulating the gamma ray interaction with air and with the RPC components structure and assuming any particles entering the detector gas with an energy higher than the argon ionization

Results and discussion

The data analysed have been collected in different seasons of 2010 (from January 1 to February 28, from June 2 to 15, from October 1 to December 31) and on various clusters located in different positions inside the experiment hall (clusters 4 and 32 on the North side, clusters 104 and 108 at the center, clusters 208 and 228 on the South side).

The analysis is based on the following time series of experimental data x(t): radon concentration in air CRn, atmospheric pressure P, temperature of the

Summary and conclusions

The ARGO-YBJ experiment works in two detection modes: shower mode and scaler mode. In the scaler mode operation, the study of the four channels counting rates points out a different behaviour of the C1 channel with respect to the higher multiplicity ones (C2, C3 and C4). We ascribe this behaviour to the presence of the radon gas emanating from soil and diffusing inside the hall hosting the ARGO-YBJ detector. The radon decay produces radionuclides emitting gamma rays able to influence the

Acknowledgements

This work is supported in China by the NSFC (No. 10120130794, No. 11205165), the Chinese Ministry of Science and Technology, the Chinese Academy of Sciences, the Key Laboratory of Particle Astrophysics, CAS, and in Italy by the Istituto Nazionale di Fisica Nucleare (INFN). We also acknowledge the essential support of W.Y. Chen, G. Yang, X.F. Yuan, C.Y. Zhao, R. Assiro, B. Biondo, S. Bricola, F. Budano, A. Corvaglia, B. D'Aquino, R. Esposito, A. Innocente, A. Mangano, E. Pastori, C. Pinto, E.

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