Origin of the low-energy gamma ray flux of the long-lasting thunderstorm ground enhancements

To identify the role of the gamma radiation from radon progenies in long-lasting thunderstorm ground enhancement (TGE) flux, the differential energy spectrum is measured with various spectrometers, including precise spectrometer of ORTEC firm built with $3”\times 3”$ inches sodium iodide NaI(Tl) crystal; full width at half maximum (FWHM) $\sim 7.7\%$ at 0.6 MeV. The measurements demonstrate that radon progeny radiation significantly contributes to the count rate enhancements measured in the winter of 2018–2019 in the energy range below 3 MeV. However, performed Monte Carlo simulations and observation of long-lasting TGEs with plastic scintillators of various thicknesses and energy thresholds show that TGEs originate in the intracloud electric fields. Radon progenies (mostly ${}^{214}\mathrm{Bi}$ spectral lines) contribute to count rate in the low-energy domain.

Figure 1

Indoor background gamma ray energy spectrum measured at Aragats and in Yerevan with various spectrometers: (a) NaI (Tl) and (b) HPGe. The ${}^{214}\mathrm{Bi}$ spectral line (peak) will be used for the investigation of diurnal variability of radon concentration at Aragats. We also consider the potassium ${}^{40}\mathrm{K}$ isotope as a stable spectral line used for the calibration of fast varying ${}^{214}\mathrm{Bi}$ spectral lines.

Figure 2

Illustration of the standard procedure of the background extraction and spectral peaks area calculation [(a), (b), and (c) are modified from 27] and routinely measured 1-hour histograms of the spectral line 0.609 MeV from ${}^{214}\mathrm{Bi}$ decay enumerated by the NaI (Tl) ORTEC spectrometer (d).

Figure 3

Diurnal variations of the intensity of the 0.609 MeV (${}^{214}\mathrm{Bi}$) spectral line measured indoors and in the open air, from measurements performed in December 2018.

Figure 4

Diurnal variations of the intensity of the 1.46 MeV (potassium-40) spectral line measured indoors and in the open air.

Figure 5

The scheme of electron acceleration modeling in the atmosphere.

Figure 6

The percentage of gamma rays reaching Earth’s surface after escaping from an electric field at different heights (related to the case when the electric field is prolonged to Earth’s surface). $\mathrm{Ez}=0.8\mathrm{kV}/\mathrm{cm}$ with an elongation of 1 km.

Figure 7

Four observed events of particle flux enhancement in winter 2018/2019. Disturbances of the near-surface electric field (black curves). 1-min time series of count rates of particle flux measured by the first NaI crystal located under the roof of the SKL experimental hall on Aragats (blue curves).

Figure 8

Differential energy spectra of the winter TGEs measured (a), (b) in open air and (c), (d) in the SKL experimental hall.

Figure 9

The TGE measured by (upper) the 1-cm-thick and $1\text{−}{\mathrm{m}}^2$-area plastic scintillator of the indoor CUBE detector and by (lower) the 5-cm-thick and $25\text{−}{\mathrm{m}}^2$-area plastic scintillators of the GAMMA experiment.

Figure 10

(a) The disturbances of the electric field and count rate measured by the large crystal of the NaI network. (b) Comparison of the count rate time series measured by the same crystal and two vertically stacked plastic scintillators of the STAND1 detector, with a thickness of 1 cm and an area of $1{\mathrm{m}}^2$, located outdoors near the MAKET experimental hall.

Figure 11

(a) The disturbances of the electric field and count rate measured by the large crystal of NaI network. (b) One-minute count rates of the same NaI crystal and count rates of the upper and middle of the vertically stacked scintillators of the STAND3 detector located indoors in the SKL experimental hall. The count rate in the panel (b) is shown in the number of standard deviations from the mean count rate measured before TGE to fit in one picture time series of two detectors that are significantly different in the count rates.

Figure 12

Differential energy spectra of (a) the winter enhancement that occurred on January 17, 2019, and (b) the TGE that occurred on March 4, 2016, during a strong thunderstorm. The energy release histograms were collected by four NaI crystals (see details in [35], Fig. 4) over 50 min.