Activity Concentration of 238 U , 232 Th and 40 K Based on Soil Types in Perak State , Malaysia

The activity concentrations of U, Th and K in soil samples from different soil types has been collected in the state of Perak, Malaysia. The samples were determined using a high resolution co-axial HPGe gamma ray spectrometry system. Mean values for U, Th and K activity concentrations were found to be (127 ± 97) Bq kg, (304 ± 28) Bq kg, and (302 ± 29) Bq kg respectively. The highest concentration of U and Th was found for soil of the types Paleudults-Hapludults and K for the typic Aeric Tropaquepts. The external hazard values (Hex) are between 0.35 and 3.07. Soil type typic Paleudults and typic Hapludults have a hazard index, Hex value which is greater than 2, and the rest of the soil types (ten soil types) have Hex values between 1 to 2. The concentrations of U and Th in soil samples in the study area are higher than the reference value.


Introduction
The most common terrestrial radioelements that produce gamma-rays are uranium ( 238 U), thorium ( 232 Th) and potassium ( 40 K).There are several studies on naturally occurring radionuclide concentrations in normal background radiation areas, including Malanca et al. (1993), Quindos et al. (1994), Narayana et al. (2001), Tufail et al. (2006) and Ramli et al. (2009).The highest concentrations of radioactive minerals in soil are found in Brazil and India (Radhakrishna et al., 1993).The abnormally high terrestrial gamma radiation (HTGR) dose rates in Brazil are due to the presence of monazite sand along the Atlantic coast and volcanic intrusion in the state of Minas Gerais.Dose rate in this area ranged up to 2.1 Gy h -1 , which is more than 30 times the world average value (Malanca et al., 1993;Roser & Cullen, 1964).
The concentration value of 238 U, 232 Th and 40 K were varied depends on soil type, geological features and geographical conditions (Florou & Kritidis, 1992;Ramli, 1997).The high natural radiation levels commonly are associated with igneous rocks, such as granite, and lower levels commonly with sedimentary rocks (Tzortzis et al., 2003).The concentrations of uranium and thorium in high background radiation dose rate areas are associated with soil developed from igneous rocks (Kogan et al., 1969;Larsen & Gottfried, 1960).Probably majority of uranium is associated with the phosphatic sands and clays formations (Roessler et al., 1979).The concentrations of the naturally occurring radionuclide of 238 U, 232 Th and 40 K in soil samples were determined in Perak States of Malaysia

Soil Type in the Area
The area studied is located between the latitudes of 3 o 30′ to 5 o 21′ North, and the longitudes 100 o 00′ to 101 o 45′ East, within an area of 21,006 km 2 (Director of National Mapping Malaysia, 1996).The climate is tropical with temperature between 28 o C to 32 o C. The highest point in the area is 2,000 m above sea level.Perak state is overlaid by seven groups of soil series as classified according to the United States Department of Agriculture (USDA) taxonomy name (Director General of Agriculture Peninsular Malaysia, 1973).The groups of soils in Perak state (with local name in brackets) are: (a) Histosols, are also called organosols, comprising primarily of organic materials.There are two types of histosols in Perak, namely typic Fibric Tropohemists and typic Troposaprits and are known as peat and muck respectively.They are deposited mostly in swampy areas and not far from the coast of the southern parts of Perak state.
(d) Oxisols.These have a high concentration of iron, aluminium oxides, hydroxides and sesquioxides.In addition they also contain quartz and kaolin, small amounts of other clay minerals and organic matter.Soil series of this group are typic Hapludoxes (Munchong, Prang).
(f) Miscellaneous soils.Found in steep lands, these soil types are mostly located in the eastern part of Perak state.Most of the steep lands are in forested areas.
(g) Disturbed land (mined land and urban land).
Soil types according to the USDA soil classification in Perak state, as shown in Table 1.

Soil Sample Preparation
The soil samples were taken from several different locations and different terrestrial gamma radiation dose rate in Perak State, Malaysia.Soil samples preparation and materials were described elsewhere (Ramli et al., 2009). 238U, 232 Th and 40 K concentration in soil samples were determined by a coaxial high purity germanium (HPGe) gamma ray spectrometer.The IAEA reference nuclides 133 Ba, 22 Na, 137 Cs, 60 Co and 152 Eu were used for calibration.The IAEA standard samples, SL-14 and SL-2 were used as reference materials and were mixed with SiO 2 in Marinelli beakers.The activity concentration in soil samples (in Bq kg -1 ) due to radionuclide i and for a peak at energy E, was calculated by using Equation (1), where N Ei is the net peak area at energy Ε of radionuclide i, ε Ei is the detection efficiency, t is the counting time,  is the number of gammas per nuclear transformation, and M is the mass in kg of the measured sample.

Radiological Hazard Index
To determine the relative hazard of materials that contain 238 U, 232 Th and 40 K were used by the radium-equivalent activity Ra eq as shown in Equation (2) (Krieger, 1981;Beretka & Mathew, 1985), where, H ex is the external hazard index which is calculated by using Equation ( 3), where C U , C Th , and C K are the activity concentrations of 238 U, 232 Th and 40 K in Bq kg -1 respectively.H ex can be considered to be low if its value is less than or equal to 1 (Krieger, 1981;Beretka & Mathew, 1985).The index value of 1 is equivalent to 370 Bq kg -1 of radium.This will cause a radiation dose of 1.5 mSv per year which has been considered as an acceptable radiation dose level due to Technologically Enhanced Naturally Occurring Radioactive Material (TENORM) for member of the public (ICRP, 1991).

Results and Discussion
The dose conversion factors obtained as shown in Table 2.The results were compared with various reported values.Conversation factors obtained in this study are in good agreement with those of earlier studies.Statistical testing was used to evaluate the results from this study with respect to published values.The results obtained are comparable with the published values if p > 0.05 or t-test value < t-table value by Student's t-test.The test results obtained indicated that no significant difference exist between the results obtained by present study and the earlier studies.Therefore the dose conversion factors obtained can be used to predict terrestrial gamma radiation dose rate from 238 U, 232 Th and 40 K concentration, or more practically to check the validity of measurements done when direct instrumental calibration with respect ultra low environmental doses is impractical (Ramli et al., 2005).
The mean values of 238 U, 232 Th and 40 K activity concentrations in different soil types in Perak State are presented in Table 2.The activity concentrations of the soil samples for 238 U ranged from 7 Bq kg -1 to 554 Bq kg -1 , with the mean value of (127 ± 97) Bq kg -1 ; for 232 Th ranged from 23 Bq kg -1 to 1 806 Bq kg -1 , with the mean value of (304 ± 28) Bq kg -1 ; and for 40 K ranged from 6 Bq kg -1 to 2,522 Bq kg -1 , with the mean value of (302 ± 29) Bq kg -1 .The average activity concentrations of 238 U and 232 Th are higher than the world.But, the activity concentration for 40 K is lower than world average value of 400 Bq kg -1 .The most abundant radionuclide is thorium ( 232 Th).It is about 42% of the total ( 232 U + 232 Th + 40 K), 238 U is 17% and 40 K is 41%.The activity concentrations of 232 U, 232 Th and 40 K in different of soil samples are given in Figure 1.Beck et al., 1972; 3 Clouvas et al., 2000; 4 Jibiri, 2001)   Figure 1.The activity concentration of 238 U, 232 Th, and 40 K in soil classifications The calculated external hazard values are between 0.35 and 3.07.They are given in Table 3. Fourteen of the soil types have values lower than unity.They cover 62% Perak state.They are not considered to pose significant radiological hazard.Conversely, soil type typic Paleudults and typic Hapludults have a hazard index, H ex value which is greater than 2. They are found in 11% of the Perak state.The rest of the soil types (ten soil types) are found in 27% of the area and have H ex values between 1 to 2. Thus according to ICRP 1991 recommendations, caution has to be taken if these soil types or its derivatives are to be incorporated as building materials, especially those from the typic Paleudults and typic Hapludults type.

Conclusion
The results indicate that the concentration of 238 U and 232 Th in soil samples are higher than the reference values by UNSCEAR 2000.The mean activity concentration was higher for soils of granitic origin (typic Paleudult-typic Hapludult locally known as Renggam-Bukit Temiam and miscellaneous soil), and lower for soils originating from shale and alluvium (Tropic Fluvaquent locally known as Serong and Aeric Tropic Fluvaquents locally known as Selangor).
The results obtained on hazard index, H ex for various soil types in Perak state indicate that caution needs to taken for most of the soil types if they or their derivatives are to be used as building material.

Table 1 .
Soil classification according USDA in Perak State

Table 2 .
Dose conversion factors for 238 U, 232 Th and 40 K were given in the literature and as derived from experimental data

Table 3 .
Specific concentration of 238 U, 232 Th and 40 K, Hazard Index, and Radium equivalent activity in soil samples