Effect of natural factors on 7Be activity in atmospheric precipitation in 2007 in Samarkand

Abstract

Cosmogenic 7 Be (half-life T 1.2 = 54 days, E γ = 478 keV) is formed in upper layers of the atmosphere in reactions where cosmic radiation splits the atoms in the air. The 7 Be atoms formed stick to aerosol particles and together with dry particles (dust, dew, hoarfrost, frost) and wet precipitation (rain, snow) fall onto the Earth's surface.The 7 Be concentration in near-Earth space is largely determined by the solar component of the cosmic radiation, moving from the upper to the lower layers of the atmosphere -geophysical processes, fallout on the Earth's surface -dry and wet precipitation, the content in objects of the environment -soil erosion and formation of bottom deposits.The present work is a continuation of the investigations started in 2002 [1-3] of 7 Be activity in monthly atmospheric precipitation in Samarkand.The samples of atmospheric fallout were obtained by the method of monthly exposure of 100 × 100 cm and 10 cm high cell placed at height 8 m. The bottom of the cell was covered with a layer of gauze swabbed with glycerin. The method of preparing samples was described in [3].The samples were packed in 1-liter Marinelli vessels, whose radiation was measured using a gamma spectrometer with a 63 × 63 mm NaI(Tl) crystal and resolution ~10% on the 1332 keV line of 60 Co. The γ-radiation detection efficiency was determined and the γ spectra were analyzed using 226 Ra, 232 Th, and 40 K sources from the OMACH set. The 7 Be activity in the samples was determined according to the area of the 478 keV photopeak, separated from the experimental spectrum of the 7 Be component (Fig. 1). The methods used for measuring and analyzing the spectra are examined in [4].It has been established that the specific activity of 7 Be in the samples varies, depending on the sampling time, in the range A = 10-295 Bq/m 2 (see Table 1). The contribution of the other natural radionuclides (uranium-thorium families and 40 K) does not exceed ~15 Bq/m 2 .The 7 Be activity in the samples was compared with the following indicators:• solar activity, for which the inverse average monthly Wolf numbers (number of sun spots) W i were used [5];• exchange processes between the upper and lower layers of the atmosphere -monthly sums of daytime geomagnetic indices D i of the perturbations of the Earth's magnetic field [5]; and • dry and wet precipitations, average monthly temperature T i and moisture content V i of air, monthly amounts of precipitation H i (according to the data from the Hydrometeorological Bureau for the Samarkand Region). For convenience of making comparisons, the reduced (dimensionless) values of the quantities normalized to the corresponding average yearly values are used: 7 Be activity a i = A i /101 Bq/m 2 , Wolf number w i = W i /6.5, geomagnetic index d i = D i /399 nT, temperature t i = T i /15 °C, moisture content v i = V i /60.3 %, and amount of precipitation h i = H i /30.8 mm.