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 $3''\ifmmode\times\else\texttimes\fi{}3''$ inches sodium iodide NaI(Tl) crystal; full width at half maximum (FWHM) $\ensuremath{\sim}7.7%$ 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.