Natural radioactivity levels (K, Th and Ra) in some areas of Punjab, India

Radioactivity, natural and man-made, is omnipresent in the earth’s crust in different amounts. Natural radioactive materials under certain conditions can reach hazardous radiological levels. So, it becomes necessary to study the natural radioactivity levels in soil to assess the dose for the population in order to know the health risks and to have a baseline for future changes in the environmental radioactivity due to human activities. 226 Ra, 232 Th and 40 K analysis has been carried out in soil samples collected from some areas of Punjab, India using gamma-ray spectrometry. The technique of gamma ray spectrometry was applied using high purity germanium gamma-ray detector and a PC based MCA. Radium equivalent activities are calculated for the analyzed samples to assess radiation hazards arising due to the use of these soil samples in construction of dwellings. The measured activity in the soil ranges from 23.17 to 57.87 Bq kg −1 , 59.03 to 160.40 Bq kg −1 and 228.06 to 501.03 Bq kg −1 for 226 Ra, 232 Th and 40 K with mean values of 37.93, 84.47 and 351.17 Bq kg −1 respectively. It has been observed that on the average the outdoor terrestrial gamma air absorbed dose rate is about 84.65 nGy h −1 .


Introduction
The world is naturally radioactive, and around 90% of human radiation exposure arises from natural sources such as cosmic radiations, exposure to radon gas, and terrestrial radiations.Natural radionuclides in rocks and soil generate a significant component of the background radiation exposure to human beings, which depend on the composition of the soils and rocks in which they are contained.A significant contribution to total dose from natural sources comes from terrestrial radionuclides such as 238 U, 232 Th and 40 K present in soil (Khan et al. 1998;Menon et al. 1982; Turhan et al. 2008).Since these radionuclides are not uniformly distributed, the knowledge of their distribution in soil and rocks plays an important role in radiation protection and measurement (Khan et al. 1994).The radiation dose comes from gamma-rays, which are emitted from rocks, soil and some building materials composed of the earth's crust.The estimation of exposures of humans to the various sources of radiation is important.Some of the exposures are fairly constant and uniform for all individuals everywhere for example, the dose from ingestion of 40 K in food.As radiation of natural origin is responsible for most of the total radiation exposure, knowledge of the dose received from natural sources is very important for not only of its effects on health but also of the incidence of other radiation from man-made sources (UNSCEAR 2000).The nuclides having half-life comparable with the age of earth or their corresponding decay products, existing in terrestrial materials, such as 40 K, 238 U and 232 Th radionuclides are of great interest.
Most of the developed and developing countries in the world are carrying out nation-wide surveys to assess the amount of radioactivity in order to establish possible radiological hazards and to take safety measures if necessary (Beretka and Mathew, 1985;Mollah et al., 1986;Paredes et al., 1987;Nazaro and Nero, 1988).Naturally occurring radionuclides of terrestrial origin (primordial nuclides) are present in various degrees in all media in the environment, including the human body.There have been many surveys to determine the background levels of the radionuclides in soils like igneous, granite and sedimentary.All of these measurements indicate that gamma-emitting radionuclides in 226 Ra, 232 Th series and 40 K, made approximately equal contributions to the externally incident gamma radiation dose to individuals in typical situations both indoor and outdoor.The present study was undertaken to measure the specific activity and the gamma-ray absorbed dose of naturally occurring radionuclides ( 226 Ra, 232 Th and 40 K) in soil samples of some areas of Punjab, India using gamma-ray spectrometry as it provides a fast and simple method of measuring the concentration

Geology of the area
Punjab state extends from the latitudes 29.30 • to 32.32 • North and longitudes 73.55 • to 76.50 • east.It is bounded on the west by Pakistan, on the north by Jammu and Kashmir, on the northeast by Himachal Pradesh and on the south by Haryana and Rajasthan.Due to the presence of a large number of rivers, most of the Punjab is a fertile plain.The southeast region of the state is semi-arid and gradually presents a desert landscape.A belt of undulating hills extends along the northeastern part of the state at the foot of the Himalayas.The soil characteristics are influenced to a very limited extent by the topography, vegetation and parent rock.The variation in soil profile characteristics is much more pronounced because of the regional climatic differences.On the basis of soil types Punjab can be divided into three distinct regions; southwestern, central, and eastern.Southwestern Punjab, India, has been taken as region of interest in the present work.Our studies area consists of three districts of Punjab viz.Faridkot, Ferozepur and Muktsar (fig.1).
Muktsar, with an area of 2615 km The Ferozpur District lies between latitude 29 • 55 and 31 • 09 and between longitude 73 • 53 and 75 • 24 .On the northeast, the river Satluj generally separates it from the Jalandhar and Kapurthala districts.The united stream of the Satluj and the Beas generally separates it from the Amritsar District in the northwest, and farther down from the Pakistan, with the exception of some areas on each side of the river.

Measurement of natural radioactivity
In order to measure the natural radioactivity in soil, surface soil samples were collected from different locations from three shown districts of Punjab, India namely Faridkot, Ferozepur and Muktsar.One sample from each location was collected by digging a hole at a depth of 0.5 m in the ground surface.After collection, samples were crushed to fine powder by using mortar and pestle.Fine quality of the sample was obtained using a scientific sieve of 100 micron-mesh size.Before measurement, samples were dried, packed, sealed in an airtight PVC container and kept for about 4 weeks to allow radioactive equilibrium between radon ( 222 Ra) and its short-lived decay products.An average 0.25 kg of soil was used per sample.Using high purity germanium (HPGe) detectors based on a high-resolution gamma spectrometry system, the activity of soil samples was determined.The details of the technique are the same as reported elsewhere (Mehra et al., 2007).The efficiency calibration for the system was carried out using secondary standards of uranium ore in the same geometry as for the sample counting.The secondary standard was calibrated with the primary standard (RGU-1) obtained from the International Atomic Energy Agency.Gamma transitions of 186 keV and 609 keV were used for 226 Ra, 1461 keV for 40 K, 338, 463, 911, 968 keV for 228 Ac, 727 keV of 212 Bi, and 238 keV of 212 Pb for 232 Th for the laboratory measurement of the respective activity concentration.The samples were counted for a period of 72,000 s each.A typical spectrum is shown in fig. 2. Based on the applied conditions, the achieved detection limits were 2 Bq kg −1 , 3 Bq kg −1 and 2 Bq kg −1 for 226 Ra, 232 Th and 40 K, respectively.The concentration of radionuclides was calculated using the following equation where, CPS = Net count rate per second B.I. = Branching ratio Intensity, and Eff = full peak efficiency of the detector.

Radium equivalent activity
The radium equivalent activity was calculated through the following relation

Calculation of annual effective dose
Annual estimated average effective dose equivalent received by the population was calculated using a conversion factor of 0.7 Sv Gy −1 , which was  used to convert the absorbed rate to human effective dose equivalent with an outdoor occupancy of 20% and 80% for indoors (UNSCEAR, 1993).The annual effective doses were determined as follows Outdoor(nSv) = (Absorbed dose)nGyh −1 × 8760 h × 0.2 × 0.7 SvGy −1 , (4)

External hazard index (H ex )
The external hazard index H ex can be calculated using the following equation (Beretka and Mathew, 1985) where C Ra , C Th and C K is the activity concentration of 226 Ra, 232 Th and 40 K in Bq kg −1 , respectively.If the value of this index is less than unity, the radiation hazard is insignificant.The maximum value of H ex equal to unity corresponds to the upper limit of Ra eq (370 Bq kg −1 ).

Results and discussion
The measured values for the activity concentration of the natural radionuclides 226 Ra, 232 Th and 40 K together with their average values in soil samples of 25 locations in Punjab (Faridkot, Ferozepur and Muktsar) are reported in table 1.The world average concentration is 35, 30 and 400 Bq kg −1 for 226 Ra, 232 Th and 40 K, respectively.In general, the average activity concentration of 226 Ra and 232 Th in soil of these areas is higher than the world figures reported in UNSCEAR (2000), whereas the activity for 40 K is under the safe limit.Comparatively high values of 226 Ra and 232 Th in soil samples from this area may be due to the Punjab sediments that derive from the Shiwalik Himalaya and occur in the form of Alluvium.Residential houses and dwellings in the investigated area were mostly built with bricks made of this soil.There exists information about cancers, average lifetime of people, typical health diseases, etc. in the area very close to the study area (Singh et al., 1995(Singh et al., , 2009)).To assess the radiological risk of soil used as a building material, it was useful to calculate the radium equivalent activity and external hazard index (Orgun et al., 2005;Beretka and Mathew, 1985).The values of radium equivalent activity (Ra eq ) around the investigated areas were less than the acceptable safe limit of 370 Bq kg −1 (OECD, 1979).The calculated total absorbed dose and annual effective dose rates of samples are also shown in table 1.The calculated total absorbed dose and annual effective dose rates of samples are also shown in table 1.The International Commission on Radiological Protection (ICRP) has recommended the annual effective dose equivalent limit of 1 mSv y −1 for the individual members of the public and 20 mSv y −1 for the radiation workers (ICRP, 1993).These dose limits have been established on the prudent approach assuming that there is no threshold dose below which there would be no effect.This means that any additional dose would cause a proportional increase in chance of a health effect.The world wide average annual effective dose is approximately 0.5 mSv and the results for individual countries being generally within the 0.3-0.6 mSv range (UN-SCEAR, 2000).Table 2 gives the levels of the natural radionuclides in the soil samples of various other countries.When the present data for Punjab are compared to the ones worldwide, it is seen that the overall values of 40 K are in the lower range and the value of 226 Ra and 232 Th matches with those of the other countries.The highest values of radionuclides recorded (fig.2) in the soil samples from the investigated area are originating from the Shiwalik Himalaya (composed of radioactive rich granites, sandstones and quartzites) and occur in the form of Alluvium.Plotting the activity of 232 Th as function of the 40 K activity of shows a poor correlation (R 2 = 0.03, N = 25) in the soil samples of the investigated area (fig.3).The calculated values of H ex for the soil samples range from 0.37 to 0.84 around the investigated area with an average value of 0.50 (table 1).Since all these values are lower than unity, according to the Radiation Protection report (European Commission, 1999), soil from these regions can be considered as safe and can be used as construction material without posing any significant radiological threat to the population (fig.4).

Conclusions
1.The average activity concentration of 226 Ra and 232 Th in soil of these areas are higher than the world average figures reported in UNSCEAR (2000) whereas the concentration of 40 K is rather low (table 3, fig.5).
2. High values of 226 Ra and 232 Th in soil samples from this area may be caused by Punjab sediments originating from the Shiwalik Himalaya.They occur in the form of Alluvium.

Figure 1 :
Figure 1: Map showing the area surveyed.

Figure 2 :
Figure 2: Gamma-ray spectra of samples showing various peaks of radionuclides.

Figure 3 :
Figure 3: Variation of 232 Th with 40 K activity concentrations in the soils of the investigated area.

Figure 4 :
Figure 4: Bar diagram showing the values of the external hazard index at different locations in Punjab.

Figure 5 :
Figure 5: Bar diagram showing the activity concentration of 226 Ra, 232 Th and 40 K at the different sampling sites.
2 , which constitutes 5.19% of the area of Punjab, is situated in the southwestern zone of Punjab.It lies between 30 • 69 and 29 • 87 latitude and 74 • 21 and 74 • 86 longitude.It is bounded by the States of Rajasthan and Haryana in the South, district Faridkot in North, Ferozepur in West and Bathinda in the East.Faridkot is situated between 29 • 54 to 30 • 54 latitude and 74 • 15 to 75 • 25 longitude.It lies in the southwest of the state and is surrounded by Ferozepur District in the northwest, Moga and Ludhiana Districts in the northeast and districts of Bathinda and Sangrur in the south.The District covers an area of 1469 km 2 , which is 2.92% of the total area of the State and accommodates a population of 5, 52,466 which is 2.27% of the total population of the state.
Th and C K is the respective activity concentration of 226 Ra, 232 Th and 40 K in Bq kg −1 .While defining the Ra eq activity, it has been assumed that 370 Bq kg−1 226Ra or 259 Bq kg −1 232 Th or 4810 Bq kg −1 40 K produce the same gamma dose rate.The radium equivalent activity (Ra eq ) in these soil samples ranges from 136 Bq kg −1 in Faridkot to 310 Bq kg −1 in Sitoguno with a mean value of 183 Bq kg −1 which is less than the safe limit (370 Bq kg −1 ) recommended by the Organization for Economic Cooperation and Development (OECD), (1979).

Table 1 :
Analytical results for the activity concentration of 226 Ra, 232 Th and of 40 K (Bq kg −1 ) determined for each of the measured samples together with their total uncertainties, radium equivalent, external hazard index, total absorbed dose and effective dose rate at various locations of Northern Punjab, India.

Table 3 :
Comparison of natural radioactivity levels in soil at different locations of Punjab (India) with those in other countries as given in UNSCEAR (2000).