- Email: [email protected]
Determination of Natural Radioactivity in Fertilizers by Gamma Ray Spectroscopy
PII:
Radiat. Phys. Chem. Vol. 51, No. 4±6, p. 621, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00222-3 0969-806X/98 $19.00 + 0.00
DETERMINATION OF NATURAL RADIOACTIVITY IN FERTILIZERS BY GAMMA RAY SPECTROSCOPY N. IBRAHIM Centre for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Most inorganic fertilizers contains natural potassium, with 0.01% radioactive 40K, while those fertilizers derived from phosphate rocks contain naturally occurring radioactive nuclides which are members of the thorium and uranium decay series. These radioactive nuclides will ®nd their way into the crop through the uptake of these fertilizers and might pose radiological hazards to human health through the consumption of agricultural produce. This study was carried out to measure the amount of radioactivity emitted by the naturally occurring nuclides of 238U, 232Th and 40K present in ®ve commonly used fertilizers available in Malaysia. Each type of fertilizer was thoroughly ground, homogenized and air-dried. They were placed in plastic containers of 200 ml capacity, then sealed and left for at least 3 weeks to allow radium and its short-lived progeny to attain radioactive equilibrium. The gamma spectra of the samples were collected using a high resolution HPGe gamma spectroscopy system with 20% eciency. The detector is maintained in a vertical position in a lead cylindrical shield of 12 cm thickness and 56 cm height. Samples were placed 10 cm from the surface of the detector and the counting time was 10 hours. For each material, each of the three sub samples were counted twice before an average was taken. The 226Ra (or 238U and 232Th radioactivities were estimated from the 609.3 keV and 583.1 keV gamma lines of 214Bi and 208Tl, respectively. 40K radioactivities were determined using the 1460 keV gamma line3. Radium equivalent activity (Raeq) was calculated based on the estimation that 370 Bq kgÿ1 of Ra 259 Bq kgÿ1 of Th and 4810 Bq kgÿ1 of K produce the same gamma ray dosage. Zaman et al., 1993 Table 1 shows the average speci®c activities in Bq kgÿ1 of dry weight obtained for ®ve types of fertilizer. As shown, the radioactivity contributed by uranium and thorium is higher for fertilizer # 4
compared to the other types. This is expected since this sample is of the phosphate type. For each fertilizer type, the activity contributed by 40K is higher than the contribution from U and Th. The organic fertilizer #5 which is derived from sheep droppings, does not show any activity from uranium. When we compare the speci®c activities of our fertilizers with the world average for soil (U: 25 Bq kgÿ1, Th: 28 Bq kgÿ1 and K: 370 Bq kgÿ1), we observe that the activity of uranium in fertilizer #4 is greater than the average activity recorded for soil Beretka and Mathew, 1985. The speci®c activity of K for fertilizers #2, 3 and 4 exceeded the 370 Bq kgÿ1 average value. In the case of thorium activity, all fertilizers under study have speci®c activities less than 28 Bq kgÿ1, the average value for soil. If we were to limit the radiation dose to 1.5 mSv per year than the maximum radium equivalent would be 370 Bq kgÿ1. Only fertilizer #4 possesses a greater Raeq. It should be emphasised that the actual amount of radioactivity taken up and retained in plants cannot be easily quanti®ed since the uptake follows a complicated pathway from soil to various parts of the plant. Our analysis indicates that fertilizers used to upgrade agricultural yields in Malaysia contain relatively low levels of radioactivity. However, one of the samples used in this study was found to have high radium equivalent. Human exposure as a result of these activities depends on many other factors such as air-¯ow patterns, time, air changes and others. In order to ascertain the potential radiation exposure to human health, follow-up studies have to be undertaken. REFERENCES
Beretka, J. and Mathew, P. J. (1985) Health Phys. 48, 87. Zaman, M. B., Hayumbu, P. and Munsanje, S. S. (1993) Int. J. BioChemiPhysics 2, 143.
Table 1. Speci®c activities of ®ve types of fertilizers, expressed as Bq kgÿ1 dry weight Samples Fertilizer Fertilizer Fertilizer Fertilizer Fertilizer
238
#1 #2 #3 #4 #5
U(609 keV) 0.38 20.01 1.26 20.05 5.33 20.11 112 24 N.D.
232
Th(583 keV) 0.81 20.07 2.52 20.21 1.49 20.14 48 23 1.10 20.05
N.D. Not detected.
621
40
K(1460 keV) 13.4 20.53 1688 251 2088 223 6003 2184 279 29
226 Raeq 2.6 2 0.4 135 220 168 221 643 282 23 22
Radiat. Phys. Chem. Vol. 51, No. 4±6, p. 621, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00222-3 0969-806X/98 $19.00 + 0.00
DETERMINATION OF NATURAL RADIOACTIVITY IN FERTILIZERS BY GAMMA RAY SPECTROSCOPY N. IBRAHIM Centre for Foundation Studies in Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Most inorganic fertilizers contains natural potassium, with 0.01% radioactive 40K, while those fertilizers derived from phosphate rocks contain naturally occurring radioactive nuclides which are members of the thorium and uranium decay series. These radioactive nuclides will ®nd their way into the crop through the uptake of these fertilizers and might pose radiological hazards to human health through the consumption of agricultural produce. This study was carried out to measure the amount of radioactivity emitted by the naturally occurring nuclides of 238U, 232Th and 40K present in ®ve commonly used fertilizers available in Malaysia. Each type of fertilizer was thoroughly ground, homogenized and air-dried. They were placed in plastic containers of 200 ml capacity, then sealed and left for at least 3 weeks to allow radium and its short-lived progeny to attain radioactive equilibrium. The gamma spectra of the samples were collected using a high resolution HPGe gamma spectroscopy system with 20% eciency. The detector is maintained in a vertical position in a lead cylindrical shield of 12 cm thickness and 56 cm height. Samples were placed 10 cm from the surface of the detector and the counting time was 10 hours. For each material, each of the three sub samples were counted twice before an average was taken. The 226Ra (or 238U and 232Th radioactivities were estimated from the 609.3 keV and 583.1 keV gamma lines of 214Bi and 208Tl, respectively. 40K radioactivities were determined using the 1460 keV gamma line3. Radium equivalent activity (Raeq) was calculated based on the estimation that 370 Bq kgÿ1 of Ra 259 Bq kgÿ1 of Th and 4810 Bq kgÿ1 of K produce the same gamma ray dosage. Zaman et al., 1993 Table 1 shows the average speci®c activities in Bq kgÿ1 of dry weight obtained for ®ve types of fertilizer. As shown, the radioactivity contributed by uranium and thorium is higher for fertilizer # 4
compared to the other types. This is expected since this sample is of the phosphate type. For each fertilizer type, the activity contributed by 40K is higher than the contribution from U and Th. The organic fertilizer #5 which is derived from sheep droppings, does not show any activity from uranium. When we compare the speci®c activities of our fertilizers with the world average for soil (U: 25 Bq kgÿ1, Th: 28 Bq kgÿ1 and K: 370 Bq kgÿ1), we observe that the activity of uranium in fertilizer #4 is greater than the average activity recorded for soil Beretka and Mathew, 1985. The speci®c activity of K for fertilizers #2, 3 and 4 exceeded the 370 Bq kgÿ1 average value. In the case of thorium activity, all fertilizers under study have speci®c activities less than 28 Bq kgÿ1, the average value for soil. If we were to limit the radiation dose to 1.5 mSv per year than the maximum radium equivalent would be 370 Bq kgÿ1. Only fertilizer #4 possesses a greater Raeq. It should be emphasised that the actual amount of radioactivity taken up and retained in plants cannot be easily quanti®ed since the uptake follows a complicated pathway from soil to various parts of the plant. Our analysis indicates that fertilizers used to upgrade agricultural yields in Malaysia contain relatively low levels of radioactivity. However, one of the samples used in this study was found to have high radium equivalent. Human exposure as a result of these activities depends on many other factors such as air-¯ow patterns, time, air changes and others. In order to ascertain the potential radiation exposure to human health, follow-up studies have to be undertaken. REFERENCES
Beretka, J. and Mathew, P. J. (1985) Health Phys. 48, 87. Zaman, M. B., Hayumbu, P. and Munsanje, S. S. (1993) Int. J. BioChemiPhysics 2, 143.
Table 1. Speci®c activities of ®ve types of fertilizers, expressed as Bq kgÿ1 dry weight Samples Fertilizer Fertilizer Fertilizer Fertilizer Fertilizer
238
#1 #2 #3 #4 #5
U(609 keV) 0.38 20.01 1.26 20.05 5.33 20.11 112 24 N.D.
232
Th(583 keV) 0.81 20.07 2.52 20.21 1.49 20.14 48 23 1.10 20.05
N.D. Not detected.
621
40
K(1460 keV) 13.4 20.53 1688 251 2088 223 6003 2184 279 29
226 Raeq 2.6 2 0.4 135 220 168 221 643 282 23 22