Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

"Physics o fAuroral Phenomena ", Proc. XXXIVAnnual Seminar, Apatity, pp. 185-188 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute STUDY OF RAD IAT ION R E LA T ED W ITH ATMOSPHERIC PR EC IP ITAT ION S A.V. Germanenko1, Yu.V. Balabin1, B.B. Gvozdevsky1, E.V. Vashenyuk1, N.A. Melnik2 1. Polar Geophysical Institute KSC RAS, Apatity, Russia; E-mail: 2. I. V. Tananaev Institute o f Chemistry and Technology o f Rare Elements and Minerals, Apatity, Russia; E-mail: Abstract. We suggest that increases during precipitations are caused by the bremsstrahlung X-rays, created by the electrons accelerated in an electric field of clouds. It is necessary to emphasize that our results differ from similar results of works related with thunderstorm clouds. In the arctic (Spitsbergen) and subarctic (Apatity) regions where observations are carried out, thunder-storms are almost not observed. The Nal(Tl) detector is sensitive not only to the gamma and X-ray radiation, but also to the charged particles. Besides, the instrument can register a gamma radiation from the atmospheric radionuclides scavenged by precipitations. We carried out additional studies to estimate the contribution of hindering factors (charged particles, radioactivity) in the data of Nal(Tl) detector for deriving true data on X-rays. Simultaneous with Nal(Tl) detector measurements by the instrument on the basis of Geiger counter tubes showed the insignificant contribution of charged particles to composition of the radiation caused the increases. The radiochemical analysis of precipitations collected during a number of great increase events showed only trace amounts of the natural and induced radio nuclides. And anyway they could not cause the X-ray increases registered. A possible mechanism of the electron acceleration and generation of X-ray photons is suggested and simulations of the X-ray spectra compatible with observations have been carried out. 1. Introduction The existence of excess radiations associated with thunderstorms is a known fact [1-3]. It was shown that the main cause of the excess radiation during thunderstorms is the particles accelerated by strong electric fields within thunderclouds [1,2]. Another very interesting phenomenon, related with generation of energetic particles by thunderstorm clouds, is “Terrestrial gamma-flashes” or TGF events [4]. Short bursts of energetic gamma-ray quanta are observed above thunderstorm clouds during discharges between a cloud and an ionosphere. TGF events are interesting to us in connection with the developed theory of acceleration of charged particles in a discharge [5] at the presence of so- called runaway electrons [6]. However, energetic photons studied in the given work are bom not in a thunderstorm, but in "quiet" rain clouds. We organized monitoring of low-energy gamma (X-ray) radiations on the ground level and recorded increases, usually associated with precipitations. It should be noted that in the subarctic (Apatity, Murmansk region) and arctic (Barentsburg, Spitsbergen) areas, where observations were made, thunderstorms are observed extremely seldom. Nevertheless, as the cause of the increases associated with precipitations, we assume the electric field of the clouds (though not as strong as in thunderstorm clouds), which accelerates electrons and creates the bremsstrahlung radiation penetrating to the ground level. 2. Instrumentation To monitor gamma (X-ray)-background at the ground level in Apatity and Barentsburg, we used the scintillation spectrometer based on the Nal(Tl) crystal of 6 cm in diameter and 2 cm thick. The signal after the photomultiplier and the amplifier is continuously recorded in 4 integral channels with a threshold photon energies > 20, > 60, > 100 and > 200 KeV. Detection in integral channels allows us continuous estimation of the integral spectrum of high energy photons. Besides it, the spectrometer in Apatity is equipped with the 4096-channel pulse-height analyzer V4K-SATSP-USB, based on the high-speed spectrometric ADC. It allows us to produce accurate measurements of energy spectrum. The instrumental complex in Apatity includes also the second Nal(Tl) spectrometer for the check experiments, a detector of charged particles, temperature and pressure sensors, and a precipitation gauge. All these instruments are installed at the loft right under the 1 mm thick iron sheet roof. The precipitation gauge allows us to evaluate the intensity of precipitation in the form of rain and snow. The principle of its operation includes measuring of intensity of the scattered back on particles of precipitations radiation from the infrared source. The detector of charged particles is a telescope on G-M counter tubes and permits detecting charged component of the secondary cosmic rays. 185