Determination of Radon and Uranium Concentrations in soil samples Iraqi cities

The maximum concentrations of radon was (261.54Bqm-3) in soil sample No. (18) : Basra city , while the minimum activity was (86.80 Bqm-3) in soil sample No. (6): Salahaddin city. The uranium concentrations in the same cities have been measured in (ppm) unit the maximum concentration was (0.0201) in soil sample No. (18):Basra city , while the minimum activity was (0.0067) in soil sample No. (6):Salahaddin city.


INTRODUCTION
The polymer Solid State Nuclear Track Detectors (SSNTDs) such as CR-39 and CN-85 are very sensitive to the energetic alpha-particles since they cause intense damage trail as it passing through them .So, CR-39 and CN-85 as well as LR-115(II) are used widely for detecting and measuring the α -particle emitter isotopes such as radon gas [1][2][3][4][5][6][7] and Uranium [8,9].The concentration of the radioactive materials is correlated with the number of etched tracks per unit area of the detector surface(tracks density),which can be counted with the aid of an optical microscope.This visual counting method is replaced by the measurement of the optical density [10,11],or the diminishing in the laser beam intensity as it transmit through the etched SSNTDs [12,13].
In the natural environment, radon element has three radioactive isotopes, namely: 222 Rn (3.82 d), 220 Rn (55 s) and 219 Rn ( 4 s).Normally, 220 Rn and 219 Rn and their progenies are neglected when studying "radon problem" [14,15].The immediate radon precursor is Radium (226Ra ), with a half-life of 1600 years, spread widely, particularly in materials which are made from mineral products.The forerunner of radium is Uranium ( 238 U); which has a half-life of 4.47×10 9 years [16].Radon is picked up by ground water passing through rocks and soil containing such radioactive substances; it enters water supplies when this water is pumped up a well [17].

Materials and Methods :
Collection of soil samples :180 samples of soil distributed in 18 cities in Iraqi Country were taken from location of study, from depth (5 cm) as 10 samples for each city as shown in fig.(1) .Then the samples were cleaned, dried in an oven at 70Co for 12 hours finally they were powdered and sifted by using special sieve (250 µm in diameter) [15].which are available in market were analyzed.For each sample about 20 gms of soil was kept in a small clean vial (10 cm height and 2.2 cm diameter) and marked with a proper number for identification.Solid State Nuclear Track Detectors (SSNTD) with sheet thickness 300µm were used in this study, which is usually known as CR-39 plastic detector [16,17 ].The detectors were small square pieces of size 1cm × 1cm covered with plastic properly and hung inside the sample of the vial [18 ].The open mouths of vial were closed properly and then preserved in suitable place free from disturbanc e.After completion of the exposure, these were etched in (6.25 N) NaOH solution at a constant temperature of 70±10Co for six hours in a constant water bath.After removal, the detectors were etched with a sharp pin simultaneously in a beaker.For immersing a detector in the beaker and to let it stand erect each detector was fixed with a paper clip properly at the bottom of the beaker [19 ].The central portion of the detector strips was scanned by using a microscope (kruss-mbl 20000) at a magnification of 400(40x objective and 10x eyepiece).The determinations of the concentrations of alpha particles from radon gas in samples were performed by using CR-39 from the intercast Europe srl company.The alpha tracks per cm2 in each detector were determined using an optical microscope.
The radon gas concentration in samples was obtained by using the sealed-cup technique as shown in Fig. 2.The test tubes were sealed and stored for 85 days (from 1-7-2013 to 23-9-2013).

Results and Discussion :
For the purpose of calculating 222 Rn concentration levels in soil samples, the radon activity density (C) in the air gap above the soil samples were determined by measuring the tracks density on the detector according to the following relation [20]: Where C is the 222 Rn concentration in the test tube above the sample measured in (Bq/L), ρ is the surface density of tracks on the exposed detectors(Tr/cm 2 ), T is the exposure time(85 day) and K is the 222 Rn gas diffusion constant [21].Such that Where r is the test tube radius (1.1cm), is the critical angle for CR-39 (35degree) and is the range of 222 Rn alpha particle in air (4.15 cm) [21].By substituting r, and in equation( 2) we get:- The radon activity density in the soils samples (CRn) in the test tube was calculated by using a model proposed by Somogyi [22].According to this model, the number of radon atoms exhaled from the sample surface is equal to the number of radon atoms in the can air above the soil samples multiplied by the probability of decay, which can be written in the following form [22]: where λ is the decay constant of ( 222 Rn) = 7.554×10 -3 (hr -1 ), h is the distance from the surface of soil in the sample cup to the detector (9.5 cm), T is the exposure time of the sample (2040 hr ) and Lis the depth of the samples (0.5cm).The activity of ( 222 Rn) in the soil samples will be calculated in (Bq) unit from the following relation: Where V is the Volume of the sample in the test tube The experimental results obtained in this work for Radon and Uranium Concentrations levels in different types of soil samples in Iraqi cities are presented in table 1.The variable values of radon and uranium concentrations may be mentioned that Iraq cities have variable geologies ,despite the limitation of existing data the observational and experimental data presented here is the first investigation and study provided a basis for the radon and uranium map in whole Iraq.

Conclusion :
In this investigation the concentration of radon in Iraqi country cities soil have been measured.We found that the uranium levels in whole soils samples in this field of study within the acceptable value (11.8 ppm) in addition to the radon concentrations in all soil samples was less than the safety limit .

Fig.( 2
Fig.(2): The test tubes were sealed and stored for 85 days

- 3 )
in soil sample No. (18): Basra city , while the minimum concentration was (86.80 Bqm -3 ) in soil sample No. (6):Salahaddin city.The uranium concentrations in the same cities have been measured in (ppm) unit the maximum concentration was (0.0201) have been observed in soil sample No. (18):Basra city , while the minimum activity was (0.0067) has been observed in soil sample No. (6):Salahaddin city .

Table 1 .
Radon and Uranium Concentrations in soil samples in Iraqi cities