In the second part of this research we have studied the natural and artifi cial radioactivity in the soils of the Siro Negri forest. The analysis with high resolution gamma spectrometry have allowed the determination of specific activity of the radionuclides of the two natural series of the Uranium (234Th, 234mPa, 214Pb and 214Bi) and of the Thorium (228Ac and 212Pb) from which we have calculated the specifi c activity of the 238U, in the fi rst case, and of the 232Th, in the second case, at the different depth in the three sampling stations; also we have verifi ed the secular equilibrium, demonstration that the soil profi le is undisturbed; specific activity of the 40K, an other primordial radionuclide; specific activity of the 137Cs, the only artifi cial radionuclide, introduced in the environment from the accident in the nuclear power reactor at Chernobyl, in the 1986. The secular equilibrium for the Uranium series has been verifi ed through the calculation of some parameters (relationship of mean activities of 214Bi and 234Th, correlation between 234Th – 214Pb and 234Th – 214Bi, presence of the peak at 1001 keV of the 234mPa). The mean activity for the gamma decay products (214Pb and 214Bi) is about 4.0÷6.0 Bq. In the third station, located in South West part of the forest (station C), we have found the inferior value, only 3.0 Bq, at 45 cm depth (layer S). The mean specifi c activity of the 238U are comprised between about 20 and 60 Bq kg-1 with more signifi cant variation in the south pedological profi le of the forest; moreover we have found that the distribution of the activity of 238U, in the forest soil’s, depend from the texture (in particular, from the percentage of clay and silt) and the cation exchange capacity values. Also the secular equilibrium for the Thorium series have been verifi ed using the comparison of mean activities of the daughters, 228Ac and 212Pb (relationship and correlation coeffi cient); the decay products activity is comprised between 4,0÷6,0 Bq, with minimum value about 3.0 Bq, in station C, at the depth of the S layer. The mean specifi c activity of 232Th (calculated from the activity of the 228Ac) is comprised between 30 and 50 Bq/kg for stations A, B and between 20 and 60 for the station C. Also the specific activity of this radioisotope depend of the chemical and physical characteristics of soil, in particular we have determined that depends from the distribution of clay and silt and from the cation exchange capacity values. Percentage of water in soil solution (humidity %) is other important characteristic of soils that infl uences the specific activity of the 232Th. The results of the 40K have been obtained using a characteristic peak of this radionuclide for gamma spectrometry (at 1460 keV energy). The mean specifi c activity, of this radionuclide, is about 683 Bq kg-1 in the station A, 656 Bq kg-1 for the station B and 720 Bq/kg in the station C. Also, the migration and distribution of this radioisotope is infl uenced from the presence of river’s water and for this reason the specifi c activity in the station located in the North East part of the forest (A) and in the South West part of the forest (C) is the highest. In the pedological profi le of the three stations the lowest concentration measured is 600 Bq kg-1 (at 70 cm depth, in the layer S, for the station B) and the highest is 900 Bq kg-1 (at 90 cm depth in station C). We have found that also the distribution of the 40K, in soil layer, is infl uenced from the humidity and cation exchange capacity. For this reason we assert that the distribution and migration of natural radionuclide (238U and his decay products, 232Th and his products and 40K) varies in function of two soils characteristics: the water content (humidity) and cation exchange capacity value, in the soil profi le. The measurements of the activity for the 137Cs have been carried out using the characteristic gamma peak of this radionuclide, at 661,7 keV. The maximum of the activity for this radionuclide is localized in upper layer between 10 and 15 cm depth and the specifi c activity in this layer is 22 Bq kg-1 for station A and about 17 Bq kg-1 for station B and C while the other depths the specifi c activity, in each stations, is less than 1 Bq kg-1. It depends on the fact that in the superfi cial soil layers of the forest there’s the highest contents of organic matter (3%) and acid pH. Caesium adsorption on soil and soil constituents is an ion exchange phenomenon that it can be modifi ed if the soil’s organic matter is associated with clay minerals. The acid reaction of soils is an important parameter to be observed: in fact, acid pH favours the development of fungi, accumulators of 137Cs. Also, some species of fungi not only accumulate radiocaesium effi ciently but also have slow turnover rate for this element, thus forming the major rediocaesium pool in the soil.

Raccolta dati sulla presenza di radionuclidi naturali ed artificiali nell'area Bosco Negri - 2: Risultati delle indagini Radiometiche su campioni delle indagini, serie 238U e 232Th; 40K e 137Cs

ODDONE, MASSIMO
2007

Abstract

In the second part of this research we have studied the natural and artifi cial radioactivity in the soils of the Siro Negri forest. The analysis with high resolution gamma spectrometry have allowed the determination of specific activity of the radionuclides of the two natural series of the Uranium (234Th, 234mPa, 214Pb and 214Bi) and of the Thorium (228Ac and 212Pb) from which we have calculated the specifi c activity of the 238U, in the fi rst case, and of the 232Th, in the second case, at the different depth in the three sampling stations; also we have verifi ed the secular equilibrium, demonstration that the soil profi le is undisturbed; specific activity of the 40K, an other primordial radionuclide; specific activity of the 137Cs, the only artifi cial radionuclide, introduced in the environment from the accident in the nuclear power reactor at Chernobyl, in the 1986. The secular equilibrium for the Uranium series has been verifi ed through the calculation of some parameters (relationship of mean activities of 214Bi and 234Th, correlation between 234Th – 214Pb and 234Th – 214Bi, presence of the peak at 1001 keV of the 234mPa). The mean activity for the gamma decay products (214Pb and 214Bi) is about 4.0÷6.0 Bq. In the third station, located in South West part of the forest (station C), we have found the inferior value, only 3.0 Bq, at 45 cm depth (layer S). The mean specifi c activity of the 238U are comprised between about 20 and 60 Bq kg-1 with more signifi cant variation in the south pedological profi le of the forest; moreover we have found that the distribution of the activity of 238U, in the forest soil’s, depend from the texture (in particular, from the percentage of clay and silt) and the cation exchange capacity values. Also the secular equilibrium for the Thorium series have been verifi ed using the comparison of mean activities of the daughters, 228Ac and 212Pb (relationship and correlation coeffi cient); the decay products activity is comprised between 4,0÷6,0 Bq, with minimum value about 3.0 Bq, in station C, at the depth of the S layer. The mean specifi c activity of 232Th (calculated from the activity of the 228Ac) is comprised between 30 and 50 Bq/kg for stations A, B and between 20 and 60 for the station C. Also the specific activity of this radioisotope depend of the chemical and physical characteristics of soil, in particular we have determined that depends from the distribution of clay and silt and from the cation exchange capacity values. Percentage of water in soil solution (humidity %) is other important characteristic of soils that infl uences the specific activity of the 232Th. The results of the 40K have been obtained using a characteristic peak of this radionuclide for gamma spectrometry (at 1460 keV energy). The mean specifi c activity, of this radionuclide, is about 683 Bq kg-1 in the station A, 656 Bq kg-1 for the station B and 720 Bq/kg in the station C. Also, the migration and distribution of this radioisotope is infl uenced from the presence of river’s water and for this reason the specifi c activity in the station located in the North East part of the forest (A) and in the South West part of the forest (C) is the highest. In the pedological profi le of the three stations the lowest concentration measured is 600 Bq kg-1 (at 70 cm depth, in the layer S, for the station B) and the highest is 900 Bq kg-1 (at 90 cm depth in station C). We have found that also the distribution of the 40K, in soil layer, is infl uenced from the humidity and cation exchange capacity. For this reason we assert that the distribution and migration of natural radionuclide (238U and his decay products, 232Th and his products and 40K) varies in function of two soils characteristics: the water content (humidity) and cation exchange capacity value, in the soil profi le. The measurements of the activity for the 137Cs have been carried out using the characteristic gamma peak of this radionuclide, at 661,7 keV. The maximum of the activity for this radionuclide is localized in upper layer between 10 and 15 cm depth and the specifi c activity in this layer is 22 Bq kg-1 for station A and about 17 Bq kg-1 for station B and C while the other depths the specifi c activity, in each stations, is less than 1 Bq kg-1. It depends on the fact that in the superfi cial soil layers of the forest there’s the highest contents of organic matter (3%) and acid pH. Caesium adsorption on soil and soil constituents is an ion exchange phenomenon that it can be modifi ed if the soil’s organic matter is associated with clay minerals. The acid reaction of soils is an important parameter to be observed: in fact, acid pH favours the development of fungi, accumulators of 137Cs. Also, some species of fungi not only accumulate radiocaesium effi ciently but also have slow turnover rate for this element, thus forming the major rediocaesium pool in the soil.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11571/104065
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