STUDY of RADIATION RELATED with ATMOSPHERIC PRECIPITATIONS 32nd International Cosmic Ray Conference, Beijing 2011 STUDY of RADIATION RELATED with ATMOSPHERIC PRECIPITATIONS E.V. Vashenyuk, Yu.V. Balabin, A.V. Germanenko, B.B. Gvozdevsky Polar Geophysical Institute, Apatity, Russia.

ABSTRACT Continuous measurements (monitoring) by X-ray spectrometer in the atmospheric surface layer of the arctic (Spitsbergen archipelago) and subarctic (Apatity) regions discovered systematic relationships between increases in the low-energy gamma (X-ray) background and precipitations as rains and snowfalls at a low and dense cloudiness. It is shown that these increases are not connected with any radioactivity. As the reason of X-ray increases the bremsstrahlung X-ray radiation produced by the secondary cosmic ray electrons, accelerated in electric fields inside rain clouds is suggested. The calculated X-ray spectrum obtained in the model assumptions, are in satisfactory agreement with measurements.

OUTLINE Instrumentation for study of X-rays during precipitations Examples of X-radiation during precipitations Additional experiments clearing up a nature of observed effects Proofs of absence of influence of a radioactivity on discussed effects A model of generation of X-radiation by CR electrons accelerated in an electric field of clouds

Fig.2. Block-diagram of scintillation x-ray spectrometer Gamma-spectrometer for monitoring of x-ray background in the lower atmosphere Fig. 1. The detecting block of the scintillation gamma spectrometer including crystal NaI(Tl), the photomultiplier, a high-voltage supply and the amplifier. The detector is made by the instrument used for balloon research of auroral X-rays [Lazutin, 1982]. Fig.2. Block-diagram of scintillation x-ray spectrometer

Energy resolution of gamma-spectrometer The spectrometers were tested with the help of a reference gamma source of Am241, having a weak line of 26 KeV and a basic one of 60 KeV. The relative energy resolution (which is also known as pulse height resolution and energy resolution), defined as the full-width at half-maximum (FWHM) divided by the value of peak centroid. The measured energy resolution of the instruments makes ~20 %. Spectrum of reference gamma source (Am241)

Detector of charged particles Charged particles are detected by Geiger-Mueller counters (2 rows by 8 tubes). The upper and lower counter layers are sensitive to electrons >0.2 MeV, protons >5 MeV. The coincidence signal detects electrons with energies >5 MeV, protons >30 MeV and muons with energies >20 MeV. G-M counter has the very low effectiveness (~1%) of detecting γ-rays and ~ 100% of detecting charged particles Figure 2. Detector of charged particles

Precipitation detector Infrared (IR) The instrument is not calibrated and allows identifying only the presence of precipitation and qualitative estimating its intensity. Therefore, the precipitation intensity is given in arbitrary units.

X-ray increases related to precipitations, Apatity station Event of 26 July, 2009 Event of 30 August, 2009 The characteristic increase profiles of gamma (X-rays) in the channel > 20 KeV during a rain for station Apatity. Clear correlation between rain and gamma (X-rays) increases at ground level is seen.

Fig. 4. Examples of X-ray events related to precipitations in Apatity for the 4 seasons.

X-ray increases related to precipitations, Barentsburg station (Spitsbergen) Event of 12 November, 2009 Event of 10 December, 2009 The characteristic lateral views of increase of a gamma (X-rays) > 20 KeV and precipitations (snowfall) for station Barentsburg. Data on precipitates (3 hour averages) are obtained from the meteorological observatory of Barentsburg.

Characteristic spectrum of X-ray increases Fig.8. The X-ray > 20 KeV increase event in Apatity on 8.03.2010 Fig.9. X-ray energetic spectrum of the increase X-ray spectrum in a maximum of increase of 8.03.2010 and the background spectrum measured after event, accordingly red and dark lines. Spectra were obtained with the 4096-channel pulse height analyzer. The spectra do not show gamma lines, characteristic for radionuclides. Other proofs of absence of connection of a radioactivity with observable increases are obtained from the analysis of precipitations (rain water and snow).

Upper limit of energy Figure 7. The open X-ray detector shows an increase during precipitations. The second detector covered by lead bricks does not show an increase. The lead shield cuts off photons of energies <5 MeV.

There is no radiation in rainwater Figure 8. The rainwater had been collected and placed over the covered detector The detector did not show an increase. This is a clear evidence of the lack of any radionuclide in rainwater.

Radiochemical control of precipitations In several cases of considerable X-ray increases the samples of precipitations in the form of rain and snow have been collected and analyzed with radiochemical methods in the regional laboratory of radiochemical control. This laboratory carries out regular measurements of all radionuclides, both natural, and anthropogenic origin for the last few years. For instance, see the results of the test of the collected rain-water in the event on October10, 2010, when increase of about 25 % was observed. The gamma-spectrometric analysis of 2 liters collected rain water has shown usual presence of the natural and induced radio nuclides. Namely: the trace amount is registered of natural radionuclides of series Th-232 and U-238, daughter products of their decay (Ra-226, Pb-212,214, Bi-212,214, etc.), K-40, Ве-7 and anthropogenic radionuclide Cs-137. By ‘trace’ we mean that it is at the limit of sensitivity of the method. And it is much less, than normalized value for potable water. And it is quite unlikely that it could cause the X-ray increases registered. The detected total specific alpha-activity, 0.0002 Bk/l, is more than 2 orders of magnitude lower than the normalized value for potable water (0.2 Bk/l). The total specific beta-activity is an order of magnitude lower than the normalized value for potable water (1 Bk/l).

Another hypothesis about nature of the X-ray increases was related with electric field existing inside clouds. The capability of thunderstorm clouds to accelerate particles with their strong electric fields is well known (Lidvansky&Khaerdinov, Makhmutov et al., 2010, Chilingaryan et al, 2010). In the Arctic (Spitsbergen) and subarctic (Apatity) regions thunderstorms are very rare. But we observe Increases of X-rays during each intense precipitations, with dense and low altitude cloudiness (200-600 meter the cloud base ). Such type of clouds has a title nimbostratus. Data on cloudiness and their base altitudes we take at [http://rp5.ru/1122/ru]. The significant electric fields were discovered inside the nimbostratus clouds: Rust, W.D. and Trapp, R.J, Initial balloon soundings of the electric field in winter nimbostratus clouds in the USA. Geophys. Res. Lett. V.29, No 20, 1959, 2002.

Two examples of electric field measurements from the paper ( Rust & Trapp, 2002) Vertical (blue) and Horizontal (red) components of electric field in nimbostratus clouds measured by balloons ( Rust & Trapp, 2002) Left: the maximal vertical field is 10 kV/m and horizontal one 27 kV/m Right: volume charge

Thus, in the rain (snow) clouds the electric field gradient is rather high and may reach even tens of kV/m. But this is not enough due to the strong ionizing energy losses to accelerate thermal electrons to necessary energies for generating observable fluxes of X-rays. However, secondary cosmic ray electrons having energies of several units/tens of MeV after additional acceleration in the electric field of nimbostratus clouds are capable to generate such fluxes of X-rays. As our observations show the X-ray increase events are observed only for clouds with a base not higher than 300-600 m. It probably is a consequence of strong absorption of Х-rays in air, that also proves to be true by accounts. The major contribution to the spectrum of X-rays gives the low energy (< 1 MeV) part of electronic spectrum. It can be approximated by the exponential law:

In its motion in the atmosphere accelerated electrons generate X-ray bremsstrahlung (XRB). Efficiency of this process is spotted by differential cross section XRB Q (hν, E), depending on energy of electrons E and X-ray quanta hν. Completely expression for Q, is: where

Due to the strong absorption of electrons in the air, we shall consider only those X-rays, which were generated at the lower edge of a cloud (l meters above the earth's surface). Based on the calculations of (Lazutin, 1982) as well as assumptions about the linearity of the absorption coefficient of X-rays in the energy range 100-1000 KeV, we obtained an expression that describes a spectrum of X-ray radiation at the ground base after accounting for losses due to absorption of both electrons and gamma-rays: where l is altitude of generation of accelerated electrons, reaching the base of clouds μ is the linear attenuation factor of gamma-radiation Q (E, hν) is differential cross-section of a braking radiation k is energy losses of electrons in air (220 KeV/m)

Comparison of modeling spectra (color lines) with the observed spectrum (dark) Figure shows the results of X-ray spectra modeling under the formula given above. Spectra of bremsstrahlung quanta at a surface of the earth, born by the accelerated electrons in an atmosphere layer at the altitude l for various values of this parameter from 450 to 650 m. Thick curve is the spectrum of quanta measured in event on March, 8, 2010. It is seen that the measured spectrum in the best way fits the modeling, gained for height of generation of 550 m. The results of modeling have rather weak dependence on the form of electron energy spectrum.

Results The continuous measuring (monitoring) by X-ray spectrometers in a ground stratum of atmosphere of arctic (Spitsbergen) and subarctic (Apatity) regions has found out systematic relationships of increases of low-energy gamma (X-rays) background with atmospheric precipitations as rain and snow at a low and dense cloudiness. It is shown that these increases are not connected with any radioactivity, including radon. As the reason of increases the bremsstrahlung X-rays yielded by cosmic ray electrons, accelerated in electric fields in the nimbostratus clouds is suggested. The calculated X-ray spectrum obtained in the model assumptions, are in satisfactory agreement with measurements.

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