Figure 2. Spectrograms of the natural gamma radiation responsible for the enhancement of the count rate at low energies. The spectrograms are observed by an ORTEC precise spectrometer with high resolution (FWHH ∼7.7%).
In the second column of Table I, we show the enhancement of radiation relative to background radiation measured at fair weather before and after the event. In the following columns we show the enhancement for each identified spectral line and its percent relative to overall enhancement.. In the last two columns, we summarize the characteristics of the NGR enhancement: the share of the continuous spectrum (CR +Compton scattered) and the sum of the peaks correspondent to radio-nuclides spectral lines. Energies of most of the NGR emitters measured by the NaI spectrometer are below 1.2 MeV (85%). And most of the gamma radiation below 1.2 MeV comes from Radon and Thoron chain isotopes (75%).