No abstract

No abstract

7 Be fallout measurements performed around Samarkand in 2002-2009 included in this research covers the ascending part of the 23rd solar activity cycle; this part of the cycle corresponded to a decrease of the monthly average value of the Wolf number from 60 to <5. The linear correlation observed between the fallout and amount of wet precipitation indicates that the latter plays the main role in the removal of 7 Be from the troposphere. The average seasonal 7 Be fallout is 223 ± 46 Bq/m 2 during winter, 324 ± 66 Bq/m 2 during spring, 75 ± 29 Bq/m 2 during summer, and 189 ± 43 Bq/m 2 during fall. These measurements make it possible to evaluate the contribution of precipitation in the form of dust and dew to the total 7 Be fallout during the summer. The correlation between the solar radiation and 7 Be fallout is confirmed.Cosmogenic 7 Be (T 1/2 = 54 days) is formed in the upper layers of the Earth's atmosphere in cosmic-ray induced fission of nitrogen and oxygen nuclei. The amount of 7 Be formed and reaching the Earth's surface depends on the intensity of the cosmic radiation [1], exchange processes occurring in the upper and lower layers of the atmosphere [2], and local climatic conditions. The possibility of detection in natural objects by γ-spectrometry makes cosmogenic 7 Be an indicator of processes occurring in the atmosphere [3][4][5] and surface layers of the soil (erosion and increase of deposits in water reservoirs). For effective use as an indicator of natural processes, the main regularities of local 7 Be fallout must be determined.This article presents measurements of 7 Be fallout performed around Samarkand (39°39′ N, 65°57′ E, 702 m above sea level) in 2002-2009. The calendar period included in this research covers the ascending part of the 23rd solar cycle, corresponding to the average-monthly value of the Wolf number -an index of solar activity -decreasing from 60 to <5 [6] (Fig. 1).Measurement Procedure. The 7 Be fallout occurring together with atmospheric precipitation was collected on gauze with area 1 m 2 permeated beforehand with glycerin and placed in a vessel of appropriate size. An open area located 8 m above ground was chosen for the exposure. 7 Be was collected in parallel in two vessels: one was a stationary vessel placed at the center of the area and the other one was moved periodically. The exposed samples were removed once a month. Prior to the measurements the gauze was dried and packed in a Marinelli vessel. Wet precipitation in the form of water drops or snow remaining in the vessels at the moment of sampling were evaporated at 70-80°C to a dry state and then included in the measurements.The 7 Be activity in the samples was determined from measurements of the intensity of the 478 keV γ-line performed with a 63 × 63 mm NaI(Tl) scintillator and a semiconductor ultrapure germanium γ-spectrometer with energy resolution

No 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.