An Analysis for Distribution of Natural Radionuclides in Soil , Sand and Sediment of Potenga Sea Beach Area of Chittagong , Bangladesh

A HPGe (High Purity Germanium) detector based, low background gamma-ray counting system was used for activity measurement in soil, sand and sediment samples collected from Potenga sea beach area of Chittagong, Bangladesh. The specific radioactivities of Radium (226Ra), Thorium (232Th) and Potassium (40K) were measured in the above samples. The investigation reflects the lower deposition of minerals exist in the collected samples. From the measured specific radioactivities of the above three natural radionuclides, the radium equivalent activity (Raeq), the external hazard index (Hex), the external gamma absorbed dose rate and the annual effective dose were calculated. The obtained mean values of gamma absorbed dose rate in soil and sand samples and annual effective dose in soil, sand and sediment samples were found higher than the worldwide average as reported by United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000). The Raeq values were in the range of 47.86 to 293.76 Bq∙kg−1 and the Hex varied from 0.13 0.81, which indicated that the study area was radiologically safe for human being.


Introduction
Radioactive elements that are found in nature can be placed in three general categories, namely cosmogenic, primordial and anthropogenic.The high energy primary cosmic radiation induces various complex interactions with the earth's atmosphere, and results to cosmogenic radionuclides and hence produces secondary radiation [1].However, only a part of these radiations reach the earth's surface because the earth atmosphere acts as a filter and absorbs much of the energy of the cosmic rays.Primordial radionuclides which are left on earth since its creation, dominate in the wider environment such as in all types of soil, rocks, water, air, building materials, food and the human body [2].On the point of contribution to radiation dose received by mankind, the most important primordial radionuclides are uranium series, thorium series and 40 K [3].Of our daily life concern, the concentration of NORM (Naturally Occurring Radioactive Materials) which is characterised by cosmogenic and primordial can be increased above the natural background levels through specific industrial activities such as waste products of sludge etc. [4], and higher utilization of computed tomography (CT) and nuclear medicine [5].Therefore, without these artificial sources our annual background doses do not change significantly.
However, radioactive materials are subjected to ionizing radiation, which has sufficient energy to break some chemical bonds.Any living tissue in the human body can be damaged by ionizing radiation in a unique manner.The body attempts to repair the damage, but sometimes it is too widespread that the damage cannot be repaired and causes cancerous cells.
Potenga sea beach of Chittagong district has unique natural characteristics and is an ideal place to visit.Year after year, many kinds of organic and inorganic materials from the food and containers that are left behind by the tourist are accumulated and distributed to this area.Moreover, the activity levels vary greatly depending on soil type, mineral make-up and density.Higher levels of activity are usually associated with igneous and metamorphic rocks.Beach sand or soil is mineral deposits formed through the weathering and erosion of these kinds of rocks.Some constituent minerals of different concentration within the sand contain natural radionuclides [6].The nuclear weapon testing of some neighborhood countries may also elevate the level of natural radioactivity in environment.Therefore, the measurement of natural radioactivity and artificial radioactivity due to gamma rays from this place should be regularly examined.In the present study the knowledge of specific activities and distributions of 238 U ( 226 Ra), 232 Th, 40 K and 137 Cs in soil, sand and sediment samples collected from Potenga sea beach area were of interest since it provided useful information for estimation of the radiation exposures of human being and in monitoring of environmental radioactivity at that area.longitudes of 91˚47.245'-91˚47.710'and the latitudes of 22˚14.440'-22˚13.950'.At every location the sample was collected from 5 -10 cm depth.All investigated locations were along a line from north to south and formed three adjacent rows.Each sample was about 101 m apart from the nearest one and kept in a plastic bag and labeled according to the location.

Sample Preparation for γ-Ray Spectrometry
All investigated samples were dried under the direct sunlight for several days to evaporate the water.After removing unwanted objects each sample was also oven dried at about 105˚C to obtain constant weight.The investigated samples finally collected after passing through a 500 μm sieve to ensure the homogeneity of the sample.For the measurement of natural radioactivity, each dried sample was weighed and then taken into individual cylindrical plastic container of 8 cm diameter and 2.8 cm height.The sample in container was simply shaken by hand to allow the powder to settle down homogeneously.The volume of the soil sample amounting to about 131 cm 3 was kept equal to the standard γ -ray source material.The containers were sealed tightly and wrapped with thick vinyl tapes around their screw necks to keep air tight.These samples were stored for minimum four weeks before the measurement of radioactivity to allow for the attainment of secular equilibrium between 226 Ra and 232 Th and their progeny [7] [8].

Measurement Setup
The radioactivities of the prepared samples and International Atomic Energy Agency (IAEA) reference samples of the same geometry were measured by using a closed-end coaxial high-purity germanium (HPGe) γ -ray de- tector with a relative efficiency of 20%, resolution 1.8 keV (FWHM) at 1332 keV of 60 Co.The HPGe γ -ray detector (GC2018, CANBERRA, USA) was coupled with digital spectrum analyzer, DSA-1000 at the Radioactivity Testing and Monitoring Laboratory, Atomic Energy Center, Chittagong, Bangladesh.The system architecture shown in Figure 2 has been frequently used to measure radioactivity of bulky samples.The detector was well shielded to minimize the γ -ray background to be able to measure low radioactivity.The shielding consists of a cylindrical 5.08 cm thick lead with a fix bottom and removable cover.Due to low activity, each sample was measured at position of surface of the detector within the shielding arrangement [9].The γ -ray spectra were taken and analyzed by using the program GENIE 2000.Counting time interval was 10,000 seconds.To determine the radioactivity of the natural radioisotopes, sample counts with specific energies as well as background counts were taken accordingly.Background count was determined by a blank plastic container of the same geometry of the sample.All the background counts were subtracted from original sample counts to obtain the net peak areas.The net peak area in a γ -ray spectrum represents the number of counts collected for only that γ -ray energy of a radioisotope present in the sample.These peak areas are used to determine the radioactivity concentration of radionuclides present in the sample.

HPGe γ-Ray Detector Efficiency
The detector efficiency is an important factor for the measurement of bulk material because it depends on volume as well as on distance of the sample.In the present work a reliable efficiency vs energy curve for the HPGe γ -ray detector was determined by measuring the known activity of the volume uranium standard reference ma- terial (RGU-1), thorium standard reference material (RGTh-1) and potassium standard reference material (RGK-1) provided by the IAEA, Vienna [10].The standard radioactive sources have geometry similar to the investigated samples.Both the IAEA-RGU-1 and IAEA-RGTh-1 radioactive powder reference materials prepared by dilution of a uranium ore and thorium ore with floated silica powder of similar grain size distribution, respectively, were provided by the IAEA. 226Ra and 210 Pb are confirmed to be in radioactive equilibrium and no evidence for inhomogeneity was detected after mixing and bottling.By the sample-detector distance and the solid angle subtended by the sample, the random coincidence loss became negligible, because sample geometry is similar to standard radioactive sources used for efficiency calibration.These three standards γ -ray sources of same volume were used to generate efficiency versus energy curve for the HPGe γ -ray detector.The obtained efficiency versus energy curve is shown in Figure 3.

Data Analysis
The peaks of γ -ray spectrum found each of the investigation sample are attributed to the decay products of 238 U and 232 Th series and to 40 K.The 226 Ra ( 238 U) activity was determined individually from the net area of peak at energies of 351.9 keV (due to 214 Pb) and 609.31 keV (due to 214 Bi).Similarly, the counts at peak energies of 238.6 keV ( 212 Pb) and 911 keV ( 228 Ac) were used to determine the 232 Th activity.Finally, we took average of the values obtained from different peak energies.The activity of 40 K was determined from its 1460.83keV γ -line.
It should be mentioned that no peak appeared at energy of 661 keV in the spectrum due to decay of 137 Cs and it confirms that the artificial radioactivity in the investigated samples is below the detection limit.
From the measured gamma-ray count rate (CPS), the activity concentration of the 226 Ra, 232 Th and 40 K radionuclides were calculated by the following formula ( ) ( ) where, A represents the specific activity (Bq•kg −1 ), ε is the efficiency of the detector for the corresponding peak, Iγ is the gamma-ray emission probability and M is the mass of the sample in gramme.
The standard deviation for the error measurement has been expressed as

Radiation Hazard Index and Dose Rate Calculation
The most widely used radiation hazard index is called the radium equivalent activity.The exposure due to the gamma radiation in terms of radium equivalent activity eq Ra has been calculated using the following equation [11]: where Ra C , Th C and K C are the specific activities (Bq•kg −1 ) of 226 Ra, 232 Th and 40 K, respectively.Here, it is assumed that 370 Bq•kg −1 of 226 Ra or 259 Bq•kg −1 of 232 Th or 4810 Bq•kg −1 of 40 K produce the same gamma dose rate.The value of 370 Bq•kg −1 is allowed as a maximum value for public dose considerations [11].
In order to measure the radiation hazards, the external hazard index, ex H , is defined as [11]: The value of this index must be less than unity in order to keep the radiation hazard insignificant.The maximum value of ex H equal to unity corresponds to the upper limit of radium equivalent activity (370 Bq•kg −1 ).
The external gamma absorbed dose rate due to terrestrial γ -rays in the air at 1 m above ground level was calculated from the measured activities of 226 Ra, 232 Th and 40 K in the soil, sand and sediment samples.The calculations were performed according to the following equation [12]: where, D is the dose rate in nGy•h −1 .About 98% of the external gamma dose rate from the 238 U series is deli- vered by the 226 Ra subseries.So disequilibrium between 226 Ra and 238 U will not affect the dose estimation from the measurement of 226 Ra.In the above equation, it is assumed that all decay products of 226 Ra and 232 Th are in radioactive equilibrium with their precursors.
To estimate the annual effective dose, the conversion factor of 0.7 Sv/Gy from absorbed dose rate in air to effective dose received by adults and the indoor occupancy factor must be taken into account.Considering that people in the sea beach area of Potenga, on the average, spent 40% of their time outdoors.The annual effective doses were calculated as follows [12]:

Specific Activities
The measured specific activities of the natural radionuclides 226 Ra ( 238 U), 232 Th and 40 K in soil, sand and sediment samples are given in Table 1, together with the standard deviation from the several values and their average for a radionuclide.The activity ratios of 232 Th and 226 Ra are also given in    41.52 ± 7.27 -86.12 ± 6.72, 67.39 ± 7.45 -102.64 ± 5.68 and 787.51 ± 7.56 -1125.40 ± 6.34 Bq•kg −1 , respectively.In most of the soil samples the activity ratios of 232 Th and226 Ra are greater than unity, which indicate the higher activity concentration as well as gamma dose rate produced by 232 Th rather than that of 226 Ra.A variation of 24.72 ± 9.92 -89.12 ± 2.73, 38.43 ± 8.34 -98.59 ± 2.47 and 401.83 ± 6.82 -649.22 ± 3.67 Bq•kg −1 in the radioactivities of the above three radionuclides, respectively, were found in the sand samples.The 232 Th/ 226 Ra ratios of sand samples show the greater concentration of 232 Th than that of 226 Ra.The radioactivities of the above three radionuclides in sediment samples are varied in the range of 17.09 ± 7.44 -67.10 ± 3.41, 16.14 ± 4.77 -87.59 ± 1.99 and 542.38 ± 6.75 -654.94 ± 5.36 Bq•kg −1 , respectively.The activity ratios of 232 Th and 226 Ra showed that the activity concentration of 232 Th in most of the sediment samples was higher than that of 226 Ra.It should be mention that in most of the sediment samples the radioactivities due to 40 K were found below the background level and it may be happened due to the difference in geological properties.Most probably mud of soil contain large amount of 40 K, but sediments that are collected from sallow sea water were frequently disturbed by the wave of the sea water and makes clear from the mud and possibly results to lower concentration of 40 K.The comparison of the measured mean specific activities of natural radionuclides among the investigated samples (Figure 4) shows the high activity concentration of 226 Ra, 232 Th and 40 K in soil samples were found than that of sand and sediment samples.Moreover, in account of concentration of radionuclides and hence the gamma dose rate production, the three category soil samples decreasing in ordered as, soil > sand > sediment.The correlations between 226 Ra and 232 Th activities as shown in Figures 5-7 are approximately linear with the correlation coefficients of 0.49, 0.89 and 0.65 in soil, sand and sediment samples respectively.
The presently measured mean values of specific activities of the natural radionuclides 226 Ra, 232 Th and 40 K are compared (Table 2) with the corresponding reported data from different countries [13]- [23].For the above three radionuclides the worldwide mean values are rather low compared to present investigation.The mean specific activities due to 226 Ra obtained in this work are nearer to those for Pakistan and Thailand.The 226 Ra activities for Brazil collected in Table 2 are significantly higher and the reason is high mineral deposition in that beach sand of Brazil.The mean values of 232 Th activities in Bq•kg −1 found in this work are comparable to Potenga (75.93) [13], Pakistan (84), Thailand (63.8 and 45) and China (44.9) and a little bit lower than Kuakata [14] (90.56 and 93.72), but much lower than Cox's Bazar (4736.07 and 215.13) [15], India (2825) and Brazil (1735, 6422 and 55,537).The higher content of heavy minerals like Monazite, Kyanite, etc., in the soil/sand of Cox's Bazar, India and Brazil cause this difference.The mean 40 K activities of this work are close to Kuakata, Cox's Bazar, Pakistan and Egypt.The overall mean activities of soil in this investigation are found higher than that of other countries collected in Table 2 and the reasons may be the geological properties of soil and also may be the different kinds of organic and inorganic materials from the food and containers that are left behind by the tourist.
At a glance the 226 Ra and 232 Th activities found in this investigation indicate the lower deposition of minerals exist in soil, sand and sediment samples collected from the Potenga sea beach area.

Radium Equivalent Activity, External Hazard Index and Dose Rate
The radium equivalent activity, external hazard index, gamma dose rate and annual effective dose were calculated from the measured activities of 226 Ra, 232 Th and 40 K and the results are given in Table 3.The radium

Table 1 .
The corresponding charts are shown in Figures 4-7.The 226 Ra, 232 Th and 40 K radioactivities in soil samples are varied in the range of Figure 4. Comparison of the measured mean specific activities of natural radionuclides among the investigated samples.

Table 1 .
Measured specific radio activities due to 226 Ra,232Th and 40 K in the investigated samples collected form Potenga sea beach area of Chittagong.

Table 2 .
Comparison of the measured mean specific activities due to natural radionuclides in soil, sand and sediment samples with the values reported for soil and sand of various countries in literature.

Table 3 .
Radium equivalent activity (Ra eq ), external hazard index (H ex ), dose rate (D) and annual effective dose calculated from the measured specific radioactivities.