Activity Concentration of 222 Rn Gas, 226 Ra, 232 Th and 40 K in Crops and Soil Taken from Safwan Granges Using Active, Passive and Gamma Spectroscopy Techniques

It is widely expected that fertilizer has great effect on the soil crops, especially when it contains a radioactive elements. The radioactive content present in crops will increase the number of deferent type of cancer cases in consumers and will explain the importance of this study. In this work we used the passive method (can technique) to measure radon concentration and gamma ray spectrometer for gamma ray concentrations measurement type NaI(Tl) 3”x3”. The samples show an average radon concentration of 48.58 Bq/m 3 , area radon exhalation rate of 0.091 Bq.m -2 .h -1 and mass exhalation rate of 0.007 Bq.kg -1 .h -1 . The average specific gamma activity concentration for 226 Ra, 232 Th, 238 U and 40 K were found to be 37.94, 27.88, 13.2 and 388.0 Bq/kg respectively. The gamma hazard indices R eq , H ex , H in , D, E out were found to be 105.58 Bq/kg, 0.285, 0.383, 49.35 nGy/h, 0.06 mSv/y respectively. The concentrations of radionuclides found in this study are nominal and do not pose any health hazard.


INTRODUCTION
The transfer of metals and radioactive element through the planets to humans is a serious health hazard. It mostly causes lung, stomach, kidney, bladder cancers as well as leukemia [1]. To increase the crop production and to improve soil nutrient properties, farmers used different categories of fertilizers which are essential in this time. Most of the materials contain 238 U and 232 Th are radon gas emitters, which is a progeny of Radium, which in turn is derived from Uranium decay. The natural radioactivity arises mainly from 40 K and nuclide from the 238 U and 232 Th series and their decay product [2]. A combination of soil and fertilizer is essential for understanding changes in the background radiation [3]. The radionuclide (Uranium, Thorium, Radium and Potassium) can be transferred from soil mixed with fertilizer to roots and accumulate in various part of plant like leaves grain and stem [4,5]. These nuclei have a half-life comparable to the edge of the earth and enhanced 87% of the total radiation dose received, the rest are manmade [6,7]. The fertilized soil is considered as the main source of continuous radiation exposure to human and acts as a medium of migration of radionuclide to the biological system.
The aim of the present work is to investigate the activity concentration of NORM in fertilized soil and crops in Safwan Granges, using radon gas detection and gamma ray spectroscopy.

STUDY AREA
The study area falls in Safwan region near Kuwait border as shown in Fig. 1. Safwan is lying just south-western part of Basrah Governorate, southern Al-Zubiar district, is enclosed by salt water on eastern side by Khor Al-Zubiar channel between longitudes 47° 52' and 48° 30' E, and latitude circles 30° 10' and 29° 50' N, with an area of 24 km 2 south-west, in which sediments are made up alluvial fan and sandy Dibddibba deposits. Dibddibba aquifer has acquired its name from Dibddibba formation which is composed of unconsolidated deposits, i.e. alluvial, often represented by the most common exploited aquifer. Soil of the area comprises essentially of ill-sorted gravely sand, with gypcrete top cover. The size of the gravels varies from coarse gravels (5 -20 cm), around the apex of the fan to fine gravels and pebbles (2 -5 cm), in the peripheral parts. The thickness of the gypcrete varies from (0.5 -1.5) m. Geographically; the surficial fine-grained soil occupies about (50%) of Basrah area. Most cities of Basrah Governorate are located on this type of soil. These soils have a range of depth (7-15) m of depth in the area. The area contains many granges used fertilizers for crops growth [8][9][10].

Passive technique
Twenty soil and crop samples were collected from different locations in granges of tomatoes in Safwan region near the border of Kuwait. The samples name and number are listed in Table 1. Soil-2 4 Option Atrosy 5 Soil-3 6 Eggplant 7 Soil-4 8 Tomato-2 9 Soil-5 10 Tomato-3 11 Soil- 6 12 Tomato-4 13 Soil-7 14 Okra 15 Soil- 8 16 Qrnabit Broccoil 17 Soil-9 18 Tomato- 5 19 Soil-10 20 Tomato-6 The can technique was adopted for estimation the radon concentration and exhalation rates for crops and soil samples [11]. The samples were dried in a hot air oven at a temperature 110°C for 24 h and then crushed and sieved for uniform emanation of radon. Small masses (0.22 kg) were put inside hard plastic containers of 7.5 cm diameter and 30 cm height. In this study we used CR-39 track detectors to measure the track density. The detector 1.5 cm x 1.5 cm areas was fixed on the top inside of the containers using double side adhesive tape. A semi-permeable membrane was fitted in between the detector and sample, which allow only radon to diffuse through. The exposing time was 90 days, and then the detectors were removed and etched with 6.25 N NaOH at 70°C during 7h. After etching, the detectors were washed in distilled water and then dried in air. The track density, due to alpha particles emitted by samples, was determined by using optical microscope (400X). The radon concentration was calculated using the following expression [12,13]; Where T is the exposure time in day, ρ is track density in T/cm 2 and K is the calibration factor, which used as K= 0.2857±0.0143 Tr/cm 2 .d per Bq/m 3 measured from previous calibration experiment [11].
At the equilibrium state, the radon flux (exhalation) from each sample inside the can is written as [14,15]; where E x is exhalation rate in unit Bq.m -2 .d -1 , A Rn is radon concentration measured by the detector in unit Bq.m -3 , ߣ is radon decay constant, T is the exposure time, V is the volume of the can and S is the surface area of sample in the can.
The radon exhalation rate in terms of mass is calculated from the relation; where E M expressed in Bq.kg The effective radium content calculated from the relation [16]; Where ߩ is track density recorder, h distance between the detector and sample, S surface area of sample, T is exposure time and K is the calibration factor.

Active technique
RAD7 (Durridge Company) is a highly versatile electronic instrument used for radon detection that can measure radon concentrations in real time. For the present purposes the instrument was used to measure radon concentration emanated from soil. The soil sample was loaded into 30 cm x 7.5 cm can used as an emanation cylinder. The cylinder connected online with RAD7 system as shown in Fig. 2. The height of the active volume of the container is 25 cm, to insure 222 Rn detection only. The alpha RAD7 detector was operated in grab mode for one day protocol, with cycle 0.5 h and recycle 48. The removable lid was equipped with two gas-tight tubes, one used to pump air contains radon gas in RAD7 chamber (ZnS) and the other used to pump fresh air to the container. The system is a closed loop in which the air circulates continuously. The concentration of radon emanated from each sample inside the emanation chamber was allowed to build up with time and it was measured in 0.5 h cycle for an average time of 24 h. The output data are produced by computer software (Capture 1.2.0) was supplied by DURRIDGE Company.

Gamma Ray Measurements
Samples of crops and soil were heated in the oven at 110°C for 24 h to remove moisture, grinded to fine powder, put inside Marinalli beakers and then stored for 30 days to allow the equilibrium between 226 Ra and 222 Rn. The activity concentration of 226 Ra, 228 Ra, 238 U, 232 Th and 40 K was estimated from the gamma spectrum using Na(Tl) detector3x3 inch with a 1024 channel computer analyzer USX supplied by Spectrum Technique Company. The detector was employed with lead shielding, 4 cm thickness, which reduced the background. The detector was calibrated using standard sources of 57 Co (peak 122 keV), 137 Cs (peak 662 keV) and 60 Co (peaks 1173 keV, 1333 keV). The detector resolution is about 8% at 662 keV of 137 Cs. The efficiency calibration was achieved using eight standard sources include the calibration sources. The system was running freely, for 12 h live time, to evaluate the background spectrum. The Marinalli beaker contains sample was placed over the detector for counting.
After measuring the net count (area under the peak) for each peak, the activity concentration for each environmental isotope calculated from [17]; where ε is absolute gamma peak efficiency of the detector at this particular gamma-ray energy, ‫ܫ‬ ఊ decay intensity for the specific energy peak (including the decay branching ratio information), M the mass of the sample in kg and t is the counting time of the measurement in second.
For calculation of the specific activity concentration for each Normal Origin Radioactive Material (NORM), or Technical Enhanced Normal Origin Radioactive Materials (TENORM), one has to recognize the belongcity of each peak according to gamma decay of each isotope. The activity concentration of 226 Ra, is calculated from the weighted average concentration of gamma ray lines; 295 keV(19.2%), 352 keV (37.1%), 609 keV ( 46.1%), 1120 keV (15%) and 1760 keV (15.4%). The peak of 186 keV assumed to be from 235 U since it has slight effect on the total concentration after subtracting the background, 42.8% for Ra and the rest for 235 U. The determination of existence of 232 Th is achieved by the average of lines; 338 keV (12%), 969 keV (17%). The case of 238 U is recognized by 1001 keV (83%), 766 keV (29%) and 2204 keV (5%). For 40 K, this directly determined using 1460 keV (10%) peak [18]. Sample of environmental gamma ray spectrum in 512 is shown in Fig. 3.

. Gamma ray spectrum of soil sample measured by NaI
Radium equivalent activity (Ra eq ) is used to assess the hazards associated with materials that contain 226 Ra, 232 Th and 40 K in Bq kg -1 , which is, determined by assuming that 370 Bq kg -1 of 226 Ra or 260 Bq kg -1 of 232 Th or 4810 Bq kg -1 of 40 K produce the same γ dose rate. The Ra eq of a sample in (Bq kg -1 ) can be achieved using the following relation [19]; ܴܽ = ‫ܣ(‬ ோ ) + ‫ܣ(‬ ் × 1.43) + ‫ܣ(‬ × 0.077) (6) The published maximal permissible Ra eq is 370 Bq kg -1 .
The external and internal hazard indicesare an evaluation of the hazard of the natural gamma radiation. The prime objective of this index is to limit the radiation dose to the admissible permissible dose equivalent limit around 1mSvy −1 . In order to evaluate this index, one can use the following relations [20]; In order to estimate the annual effective dose rate in air, the conversion coefficient from absorbed dose in air to effective dose received by an adult must be considered. This value is published in UNSCEAR 2000 and UNSCEAR 1993, to be 0.7 SvGy -1 for environmental exposure to gamma rays of moderate energy. The outdoor occupancy factor is about 0.2 [18].
The annual effective dose equivalent is given by the following equation [19];  Table 2 shows the results of radon concentrations measured by passive (using equation 1) and active technique by RAD7. Fig. 4 shows the correlation coefficient between passive and active techniques this correlation equal (R 2 =0.95), which means that, the correlation is excellent.

Radon Results
The maximum value of radon concentration in    5 shows the correlation between total radium concentrations measured by gamma ray spectroscopy and radon measured by CR39 track detectors or exhalation rate. There looks to be strong and positive correlation between them (R=0.98). Fig. 6 shows the correlation between effective radium concentrations measured by CR-39 track detectors and total radium activity concentrations measured by gamma ray spectroscopy. The correlation looks positively strong, with correlation factor R=0.98.

Gamma Ray Results
A detailed analysis of the results in Table 4, indicates that there are no correlations between radioactive elements belong to different series. Figs. 7, 8 and 9 were introduced to check this correlation.
The radium equivalent activity, external and internal hazard index and annual effective dose calculated using equations (6) to (9) are presented in Table 5.
The results revealed that the measured activity levels from natural occurring radioactive in the investigated soil and crops samples are comparable to the corresponding worldwide level. The calculated result of radium equivalent activity ranged from 35.85±6.11 Bq/kg to 196.77±14.50 Bq/kg with a mean value of 105.58±13.60 Bq/kg, and is lower than 370 Bq/kg reported by UNCEAR2000. The average values of H ex =0.282 and H in =0.383 are less than 1 as recommended.
The observed gamma dose rate varies from 16.01 to 94.91 nGy/h with an average value of 49.35 nGy/h. This result is less than the reported value of UNSCEAR2000 [22] which is approximately 57nGy/h. The annual effective dose varies from 0.019 mSv/y to 0.115 mSv/y with the mean value of 0.06 mSv/y. The worldwide average effective dose, according to UNSCEAR2000is 0.07 mSv/y, this means that, our results were in agreement with the worldwide average value. In other word, the fertilized soil and the crops are safe to deal with.

CONCLUSIONS
The present measurements concentrate on data analysis for radon concentration and gamma ray emission from soil and crops samples collected from Safwan granges near the border of Kuwait. The samples show acceptable radon gas concentration in fertilized soil and crops. Area radon exhalation and mass exhalation have been calculated and show low values. The activity concentration of radium-226, thorium-232 and potassium-40 were measured also for the same samples. The values were below the recommended limit by the ICRP as the maximum annual dose to member of public. The radium equivalent activity and hazard indices are found to be lower than the safe limit. The values of the annual observed effective show that the radioactivity of the natural radionuclides found in the surveyed area is nominal and does not pose any potential health hazard to the farmers and occupiers.