Analysis of Natural Radioactivity of Soil Samples using NaI (Tl) Detector from Fault Regions of Kolasib, Mizoram, India

Analysis of Natural Radioactivity of Soil Samples using NaI (Tl) Detector from Fault Regions of Kolasib, Mizoram, India

Laldingngheta Ralte1,Ramesh Chandra Tiwari1* , Vanramlawma1, Hmingchungnunga1

Department of Physics, Mizoram UniversityTanhril, Aizawl Mizoram, India.

Rosangliana2,B.Zoliana2

Department of Physics,

Govt. Zirtiri Residential Science College Ramthar, Aizawl, Mizoram, India.

Abstract Measurement of natural radioactivity in soil samples collected from Fault regions of Kolasib District, Mizoram has been carried out using 5x 4 NaI (Tl) Detector. The purpose of this study is to examine the safety levels of activity concentrations in soil.The average activity concentrations collected from the soil samples for 238U, 232Th and 40K were found to be 37 Bq/kg, 102 Bq/kg and 678 Bq/kg which are higher than worldwide values of 35 Bq/kg, 30 Bq/kg and 400 Bq/kg respectively. The average radium equivalent activity concentrations levels obtained from the soil samples were found to be of 237 Bq/kg and were found to be within the safe limit of 370 Bq/kg recommended by UNSCEAR. The average values of the external and internal hazard indices were

1. and 0.74 and are less than unity and are therefore considered safe and can be used for construction materials.

   KeywordsNaI (Tl) detector,Activity concentration, Radium equivalent activty, Hazard indices

   1. INTRODUCTION

      The study of radiation distribution and radiation levels is significant for assessing the exposure to human beings. Human beings have always been exposed to natural radiations from terrestrial and extraterrestrial radiations. Due to different geological features, absorbed dose rate from cosmic radiation varies with them. Natural radionuclides can be categorised as Cosmogenic and Primordial sources. The main difference between them is that cosmogenic comes from cosmic ray particles from the Sun and stars interacting with the earths atmosphere and undergoing nuclear reactions while Primodial are those which are in existence since the origin of Earth. The higher levels of terrestrial background radiation are mainly associated with igneous rocks [1-2]. The main source of gamma radiations on earth are usually radionuclides of 238U, 232Th and 40K and their corresponding daughter elements [3]. Rocks, soil present on earth, in water and in building materials used for construction purposes may be the sources of primordial long-lived radionuclides such as 238U, 226Ra, 232Th and 40K. Radionuclides and their radioactive isotopes present in air, water and food, from which they are mainly exposed by outdoor natural terrestrial radiations are inhaled by human beings [4].

      The main aim of this study is to examine the activity concentrations in soil. Analysis of radioactivity of soil samples is significant for acquiring data for dose estimation. The Gamma radiation resulting from the Primordial radionuclides of 238U, 232Th and 40K and their daughter elements are the main source of irridation of the human body. Data regarding natural radioactivity of soil samples around fault regions of Kolasib District has not been acquired by

      other researchers and this is the first time for this particular study in the areas. The objective of this is to get a base line data for the regions and to determine whether the average activity concentrations are well within the recommended maximum activity concentration as recommended by United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).

      In this study soil samples collected from the various fault regions of Kolasib District are studied using thallium (Tl) activated 5x4Sodium Iodide (NaI) detector. The activity concentrations of radionuclides of 238U, 232Th and 40K as well as Radium equivalent (Raeq) activity are measured. External and internal hazard indices were also measured so as to ensure that the soils from the areas of study are safe for construction purposes. Radium equivalent activity is most widely used index to assess radiation hazards and is weighted sum of activities or radiological effect of materials that contain different natural radionuclides 238U, 232Th and 40K [5].

      Radon and Thoron concentration measurements have been carried out in other areas of Mizoram using other devices by other scholars in the past. These studies have been done in areas of Mat Fault [6-7].Apart from this continuous measurements of radon concentration have also been carried out in Chite Fault [8-9]. The present study has been carried out for the first time in different fault regions of Kolasib.

   2. STUDY AREA

      Mizoram lies in the seismic zone V of seismic zonation map of India located between 22º19N and 24º19N latitude and 92º 16E and 93º 26 E longitude. The state of Mizoram is a hilly area with an average elevation of about 1000 meters to 1300 meters from sea level. It is a tropical region with moderate climate and the temperature varies from 11ºC to 24

      ºC during the winter season and 18 ºC to 29 ºC during summer. The study area of Kolasib lies in the northern parts of Mizoram sharing its borders with Assam and the geographical map of the studied region is shown in figure 1. The study area extends 23º5329.57to 24º2816.1 latitude and 92º3933.97 to 92º4840.71 longitude.

      Area/Sec= Net peak area per second (background subtracted)

      dps = Source strength

      Ab% = Gamma ray abundance factor

      Background subtracted net photo peak data for 232Th, 238U and 40K are shown in table 1. The Multichannel Analyzer was then used for determination of radioactivity content of the samples for a period of 50,000 seconds. Since radioactive equilibrium was established, the 238U and 232Th concentrations were determined from the concentrations of their daughter elements, namely 214Pb and 228Ac respectively. The activity concentrations were expressed in terms of Becquerel per kilogram (Bq/kg). The formula for activity concentration (A) was for each sample is given by:

      = × 100 × 100 × 1

      (2)

      %

      %

      Fig 1: Geographical Map of the studied region of Kolasib

   3. METHODOLOGY

      Thallium (Tl) activated 5x 4Sodium Iodide (NaI) detector was used for measurement of activity concentration of 238U, 232Th and 40K in the soil samples collected which was connected to a PC based multi-channel analyzer also known as GSPEC-SA (Version 2.5 X) as shown in figure 2. To overcome background radiation, the detector is enclosed in a cylindrical lead and iron shield. The concentrations of 238U were determined using a photo peak of photo peak of 295 keV (19%) from 214Pb, 232Th concentration were determined using a photo peak of 270 keV (4%) from 228Ac. 40K

      Where,

      N/T = Background subtracted net photo peak counts in time T.

      = abundance of gamma ray under consideration.

      = absolute detection efficiency obtained from the energy efficiency calibration.

      Wt = weight of the sample

      The radium equivalent activity provides a guideline for regulation of safety standards against radiation protection for the general public and it is calculated by using following equation

      Raeq (Bq / kg) = CU + 1.43 CTh + 0.077 CK (3)

      Where, CU, CTh and CK represents activity concentrations in Bq/kg of 238U, 232Th and 40K respectively.The formula for External hazard index is given by [12]:

      concentration were determined from its 1460 keV (11%) photo peak.

      The soil samples were collected from 12 selected

      =

      +

      370

      +

      259

      4810

      (4)

      fault regions of Kolasib District as sown in figure 1. The samples were dried and heated using a heater at a temperature

      by [13]:

      And the formula for Internal Hazard Index is given

      of about 110oC. The collected samples were then grinded into

      = + +

      (5)

      powder size and were sieved using a 500 m mesh.The grinded samples were then sealed inside an air-tight plastic container of 250ml and were kept undisturbed for a minimum period of 30 days to attain radioactive equilibrium. The weights of the samples were also measured and recorded.

      Before measuring the natural radioactivity in the sample, a three-point energy calibration is carried out using sources containing a mixture of several radionuclides [10]. In this present study, IAEA standard source of 60Co and 137Cs were used for energy calibration. This calibration allows the establishment of the relationship between the channel numbers of the analyzer and the known energy of the photons [11].

      185

      259

      4810

      For efficiency calibration, IAEA standard source of 238U, 232Th and 40K were analyzed using the multichannel analyzer for a period of 10800 sec. For the gamma energy peak obtained, the efficiency was calculated using the formula:

      (%) =

      × 100 × 100 (1)

      Fig 2: NaI Detector

      Where,

      %

      (%) = Percent Efficiency

   4. RESULT AND DISCUSSION

      The calculation for Efficiency Calibration of NaI Detector was done based on equation 2 using 232Th, 238U and 40K standard sources is given in the table 1. The efficiency measured was found to be 13% for 232Th, 7% for 238U and 2% for 40K.

      Table 1: Efficiency Calibration of NaI Detector using 232Th, 238U and

      40K standard source.

      The measurement of Activity Concentrations of 238U, 232Th and 40K radionuclide in soil samples collected from 12 different fault regions of Kolasib District are given in table 2. These values show different variations when compared with each other as shown in figure 3. The activity concentrations collected from the soil samples were found to be in the range of 12 Bq/kg to 58 Bq/kg for 238U, 34 Bq/kg to 161 Bq/kg for 232Th and 175 Bq/kg to 1190 Bq/kg for 40K with an average of 37 Bq/kg, 102 Bq/kg and 678 Bq/kg for 238U, 232Th and 40K respectively.

      1400

      1200

      1000

      800

      600

      400

      200

      0

      Location Codes

      U-238

      Th-232

      K-40

      K1

      K3 K5

      K7

      K9 K11

      ACTIVITY CONCENTRATION

      (Bq/kg)

      Sl.no	Element	Counting Time (Sec)	Activity (Bq)	Net Area	Branching Intensity (%)	Counts per second	Efficiency () (%)
      1	232Th	10800	3854	229392	4	21	13
      2	238U	10800	1636	267803	19	24	7
      3	40K	10800	4557	195742	11	18	2

      Fig 3: Activity Concentrations of 238U, 232Th and 40K radionuclide in soil samples collected from fault regions of Kolasib District

      The Radium equivalent activity concentrations were also determined and shown in table 2. The measured values of Radium equivalent activity concentrations are within the range of 108 Bq/kg to 346 Bq/kg with an average of 237 Bq/kg. Figure 4 shows the various ranges of Radium equivalent activity concentrations of the different regions.

      Raeqv Activity (Bq/Kg)

      400

      300

      Table 2. Activity Concentrations, Radium equivalent activity, External and internal hazard indices of 238U, 232Th and 40K radionuclide in soil samples collected from fault regions of Kolasib District

      200

      100

      K1

      K2 K3 K4 K5 K6 K7 K8 K9 K10 K11 K12

      0

      Location code

      Hazard Indices (Bq/kg)

      SAMPL E CODE	ACTIVITY CONCENTRATIO N (Bq/kg)			Radium equivalen t Activity (Bq/kg)	Externa l Hazard Index (Hex)	Interna l Hazard Index (Hin)
      	238U	232Th	40K
      K-1	12	34	615	108	0.29	0.32
      K-2	32	89	774	220	0.59	0.68
      K-3	36	101	858	247	0.66	0.76
      K-4	51	141	1190	344	0.93	1.06
      K-5	43	119	885	282	0.76	0.88
      K-6	58	161	736	346	0.93	1.09
      K-7	25	68	765	182	0.49	0.56
      K-8	22	62	221	129	0.34	0.40
      K-9	53	147	175	277	0.74	0.89
      K-10	29	82	660	197	0.53	0.61
      K-11	50	138	642	297	0.80	0.93
      K-12	32	88	619	206	0.55	0.64

      Fig 4: Radium Equivalent Activity Concentrations of 238U, 232Th and 40K radionuclide in soil samples collected from fault regions of Kolasib District

      1.2

      1

      0.8

      0.6

      0.4

      0.2

      0

      Hex

      Hin

      1 2 3 4 5 6 7 8 9 10 11 12

      Location codes

      Fig 5: External and internal hazard indices of 238U, 232Th and 40K radionuclide in soil samples collected from fault regions of Kolasib District

      The external and internal hazard indices were also measured as given in table 2 and are within the range of 0.29

      to 0.93 with average value of 0.63 for Hex and 0.32 to 1.09 with average value of 0.74 for Hin. The various ranges of external and internal indices are shown in figure 5.

   5. CONCLUSIONS

The average natural activity concentrations of 238U, 232Th and 40K collected from the soil samples are 37 Bq/kg,

102 Bq/kg and 678 Bq/kg which are higher than the corresponding worldwide values of 35 Bq/kg, 30 Bq/kg and 400 Bq/kg respectively, but these values are lower than IAEA critical values of 10,000 Bq/kg for 40K and 1000 Bq/kg for all other radionuclides [14].

The Radium equivalent activity concentrations levels obtained from the soil samples are within the range of

108 Bq/kg to 346 Bq/kg with an average of 237 Bq/kg. Radium equivalent activity concentration levels for the different regions have been found to be within the safe limit of 370 Bq/kg recommended by UNSCEAR (2000) [15].

The values of the external hazard indices ranges from 0.29 to 0.93 with an average value of 0.63 and the values of internal hazard indices ranges from 0.32 to 1.09 with an average of 0.74. According to Radiation Protection 112, soil from the region is safe and is no thret to the population if the values of the corresponding external and internal indices are less than unity [16]. From the results we can see that the average values of both the external and internal hazard indices are less than unity but the values of K- 4 and K-6 have values of 1.06 and 1.09 which is more than 1(unity). So soil from these areas excluding K-4 and K-6 are considered safe and can be used for construction materials.

ACKNOWLEDGEMENT

The authors are thankful to the Board of Research in Nuclear Sciences, Department of Atomic Energy sanction number 36(4)/14/67/2014-BRNS/36016, Government of India, for providing financial assistance through research project.

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