1. Introduction

The intense electric field inside a storm cloud (of the order of 10⁵ V/m) can accelerate free electrons in the atmosphere to relativistic velocities. The electron avalanche resulting from this process may also start from intense electric field generated by lightning discharges. However, the type of radiation created by this mechanism is still unknown. Experiments with detectors placed in aircraft and also in stratospheric balloons registered an increase of gamma radiation during the production of lightning[1]. Detectors located in high mountains recorded events in which gamma rays with energies above 1 MeV were also produced during lightning[2]. Rocket-borne detectors also recorded increases in the intensity of X- and gamma radiation during lightning [3,4]. Based on observations made by satellites, Fishman reported that gamma-ray emissions coincided with the location of storms[5]. In 2007, a group of researchers measured gamma ray events with energies up to 1.0 MeV during storms and lightning events[6]. In Brazil, near Cachoeira Paulista, SP, increases in gamma ray count rate during storms and intense rainfall were also observed[7].

2. Method

Figure 1. View of (A) ACA Tower in São José dos Campos, SP, Brazil, (B) instrument trailer, and (C) meteorological station

In this study we analyzed data collected between November 2012 to April of 2013 (one-minute intervals) at the Atmospheric Phenomena Observation Tower, (Figure 1), of the Institute of Aeronautics and Space (IAE-ACA), in São José dos Campos, SP, Brazil. Measurements of X- and gamma radiation in the range of energies from 30 keV to 10 MeV (counts/minute) and atmospheric electric field were recorded simultaneously and compared with lightning strike records. Measurements of X- and gamma radiation, were carried out using a scintillator crystal of sodium iodide activated with thallium[NaI(Tl)] coupled to a photomultiplier, with energy resolution of 15% (Figure 2). Records of lightning strikes were provided by the Brazilian Lightning Detection Network (BrasilDat) operated by National Institute for Space Research (INPE). Rainfall, atmospheric pressure and temperature, and relative humidity were recorded (one-minute intervals) with sensors (Figure 2) coupled to a five-channel specific data logger[8].

Figure 2. Trailer and instruments inside the trailer at the ACA Tower. Note the pluviometer at the top of the pole attached to the side of the container

3. Results

During the study period, we observed that there was a strong temporal correlation between the increase in the magnitude of the atmospheric electric field due to the presence of storm clouds and increase in X- and gamma count rates (Figure 3).

Variations in the electric field caused by the presence of electrified clouds over the study region coincided with an increase in radiation counts. Although it was not possible to quantify the contribution of lightning discharges, increases in gamma radiation counts were recorded during the occurrence of storms (Figure 4).

To clarify this problem, it will be necessary to collect additional data on atmospheric electric field variations and the spectra of gamma radiation resulting from the radioactive decay of radon gas before, during and after rainfall since radon is an important source of environmental radiation. Preliminary studies of the profiles of the peaks of gamma radiation as a function of time indicate that there is a difference between the profiles that were recorded during rainfall and the occurrence of lightning compared with periods when there is no rain. During rainfall without lightning, the profiles of the lines tend to have a symmetric Gaussian shape. During lightning storms the profiles of the peaks are describe by an asymmetric double sigmoid function (Figure 5). It was also found that some of the peaks have shapes that can be fit by a Voigt function.

Figure 3. X- and gamma rays measurements and rainfall data from 12/24/2012 to 01/24/2013. Sixty-minute smoothed average of the count rates is shown in red

Figure 4. Atmospheric electric field data and X- and gamma count rates measured from 02/19/2013 to 04/01/2013 at the ACA Tower. Sixty-minute smoothed averages of the raw data are shown in red

Figure 5. Profile of a radiation peak (black) fit with a sigmoid function (red)

Figure 6 shows the X- and gamma count rates during a rainy period (03/10/2013 to 04/25/2013). The background count rate remained around 40000 counts/minute, but during rainfall it peaks to values above 44000 counts/minute. Measurements around 60000 minutes coincided with the occurrence of a very intense rainfall that resulted in the production of X- and gamma rays with count rates 40 % above the background level of the region.

Figure 6. X - and gamma ray count rates during a rainy period (03/10 to 04/25). Sixty-minute smoothed average of the count rates is shown in red

The energy spectrum of X and gamma radiation in the region of São José dos Campos, SP, Brazil measured on December 30, 2013 during the 24 hours that day, between 30 keV and 3.0 MeV is shown in Figure 7. The day was rainy.

The intensity of the peaks ²¹⁴Bi appearing in 0.609, 1.12 and 1.76 MeV energies, the ²⁰⁸Tl appear at 0.59 and 2.61 MeV, while the ⁴⁰K has peak intensity at 1.46 MeV. On the day of measure of the spectrum, there was rain and the peak of ²¹⁴Bi, who hail from radon gas also coming from rain, are very evident, but it was not possible to separate the contribution that extent of photons gamma from the rain and the background site without rain.

Figure 7. Energy spectrum of X and gamma radiation, measured in (12/30/2012) showing the largest peak intensities emitted by radionuclide’s 238U, 40K, 232Th and radon gas 222Rn

4. Conclusions

During the study period, we observed that variations in the atmospheric electric field caused by the presence of electrified clouds over the study region were correlated with increases in the X- and gamma ray count rates. This result is in agreement with those previously reported in the literature. The correlation between variations in the X- and gamma ray counts and changes in the atmospheric electric field as well as changes in the profiles of gamma ray count peaks in the presence and absence of lightning activity associated with storms suggests that lightning probably play an important role in the production of X- and gamma radiation during of storms. We also observed an important contribution due to the radioactive decay of radon gas to the X- and gamma radiation count rates in the study region.