Radiation, Protection of the Public and the Environment (Poster Session 1) Origin and Migration of Cs-137 in Jordanian Soils
Total Page：16
File Type：pdf, Size：1020Kb
Major scientific thematic areas: TA6 – Radiation, Protection of the Public and the Environment (Poster session 1) Origin and Migration of Cs-137 in Jordanian Soils Ahmed Qwasmeh, Helmut W. Fischer IUP- Institute for Environmental Physics, Bremen University, Germany Abstract Whilst some research and publication has been done and published about natural radioactivity in Jordan, only one paper has been published about artificial radioactivity in Jordanian soils (Al Hamarneh 2003). It reveals high concentrations of 137Cs and 90Sr in some regions in the northwest section of Jordan. The origin of this contamination was not determined. Two sets of soil samples were collected and brought from northwest section of Jordan for two reasons, namely; the comparable high concentration of 137Cs in this region according to the above-mentioned paper and because most of the population concentrates in this region. The first set of samples was collected in April 2004 from eleven different sites of this region of Jordan. The second set of samples has been brought in July 2005 from six of the previous sites where we had found higher 137Cs contamination. The second set was collected as thinner sliced soil samples for further studying and to apply a suitable model for 137Cs migration in soil. Activity of 137Cs was measured using a HpGe detector of 50% relative efficiency and having resolution of 2keV at 1.33MeV. Activity of 90Sr was measured for the samples of four sites of the first set of samples, using a gas-filled proportional detector with efficiency of 21.3% cps/Bq. The total inventory of 137Cs in Bq/m2 has been calculated and the correlation between 137Cs inventory and annual rainfall and site Altitude has been studied. In order to determine the origin of 137Cs in the Jordanian soil we have used two methods; 137Cs - 90Sr ratio and the diffusion convection model which describes the migration of 137Cs in Soil. 1. Introduction After the Chernobyl accident at 1986 many investigations, mainly in Europe, have been done on 137Cs in soil. From the radiological point of view, 131I and 137Cs are the most important radionuclides to consider, because they are responsible for most of the radiation exposure received by the general population. The releases of 131I and 137Cs are estimated to have been 1,760 and 85 PBq, respectively (UNSCEAR 2000 Report). The gamma emitting radionuclide 137Cs has a significant radiological importance because of its long half-life, which is about 30.17 years. This importance arises from the migration of 137Cs 1 downwards in soil and its partially absorption by plant roots, leading in turn to uptake into the vegetation and the human food chain (UNSCEAR 2000 Report). 137Cs has a low mobility in soil. Rosen et al. (1999) found that 137Cs activity concentrates mostly in the top 2 cm when the measurements were carried out a few years after the Chernobyl accident and in the top 7 cm in more recent measurements. The 137Cs concentration in surface soil decreases under the influence of various processes like decay, mechanical removing with rainwater, vertical migration and diffusion into deeper layers of soil (D. Krstic 2004). For the purpose of describing the vertical migration of 137Cs in soils several models have been developed. Recent models were developed by Szerbin (Szerbin1999) and Likar (Likar 2001), which based on the solution of the diffusion-convection equation. Szerbin et al. (1999) used the homogeneous Green function as a solution of the partial differential equation for infinite medium. Likar et al. (2001) used a Green function, which satisfies boundary conditions at the soil–air interface and claimed that his solution fits experimental results better than that used by Szerbin et al. (1999). The main aim of this paper is to study the migration of 137Cs in Jordanian soils and to determine its origin. For this purpose we will use two methods, namely, the 137Cs- 90Sr ratio and the solution of Szerbin (1999) where we found it fits our data better than the solution of Likar (2001). 2. Materials and methods 2.1. Sampling The first set of soil samples was collected from eleven different locations of the northwestern section of Jordan in April 2004 (Figure 1, Table 1). We chose a suitable place to take the samples according to the following conditions: the surface was undisturbed (i.e. not cultivated recently) and with a low slope of the area. The samoles were taken using an auger, in a simple random way from an 10m x 10m area. Fifteen cores have been taken from every site. The soil profiles in Ah1, Ah2, Ah3, Ah4, Ah5 and Ah6 were taken to 25cm depth and divided into 5cm thick layers. The soil profiles in Ah8, Ah9, Ah10 and Ah11 were taken to 30cm depth and divided into 6cm thick layers. The soil profile Ah7 was taken to 51cm depth and divided into 7cm thick in the first three layers and into 10cm thick in the last three layers. The GPS coordinates have been determined for every site using a handhold GPS receiver. The second set of samples was taken in July 2005 from six of the previous sites, namely; Ah3, Ah4, Ah5, Ah6, Ah9 and Ah10, where we had previously found higher 137Cs contamination in the first set of samples. These samples were coded Ah3new, Ah4new, Ah5new, Ah6new, Ah9new and Ah10new. The samples were taken using two stainless steel plates of 10cm x 10cm and 10cm x 20cm dimensions. The slice thickness reduced and can be obtained from the graphs (Figure 9−Figure 13). 2 2.2. Samples Preparation for Gamma Analysis The first set of samples was dried in an oven at 105oC until a constant weight was reached. After removal of all stones and vegetation, the samples were milled and homogenized using a mixer of about 5rps. An amount of 105g of every sample was mixed with 11g wax and compressed to produce a circular pallet of 7cm diameter, 2cm thickness and density of 1.5g/cm3. The last step was done to obtain a standard geometry, for which an efficiency calibration for the detector existed already, which is in our case 7cm diameter and 2cm thick discs of density of 1.0g/cm3 and 1.5g/cm3. This step has been omitted for the new set of samples because we found that most of the dry sample densities were around 1gm/cm3 and the efficiency difference between the two densities for 137Cs does not change significantly (the difference was within the statistical error of the 137Cs data). Every sample was then sealed and labeled with necessary information about the sample. Sample Site GPS coordinates Code N E Alt.(m) Ah1 Kufr Sum 32o 40´ 35o 49´ 506 Ah2 Foua’ra 32o 36´ 35o 45´ 373 Ah3 Baliela 32o 25´ 35o 55´ 740 Ah4 Qafqafa 32o 20´ 35o 58´ 927 Ah5 Dair Allyyat 32o 17´ 35o 52´ 882 Ah6 Suf (Abien) 32o 21´ 35o 46´ 1036 Ah7 Ain El Basha 32o 04´ 35o 49´ 647 Ah8 Wadi El Naqah 32o 04´ 35o 45´ 985 Ah9 Bala´ama 32o 16´ 36o 05 708 Ah10 El Ramtha 32o 35´ 35o 58´ 542 Ah11 As Subeihi 32o 08´ 35o 42 537 Table 1: samples identification. 3 Figure 1: Jordan's map with samples locations. The samples in the new set were sieved using a 2mm sieve, then they were dried at an oven at temperature of 105oC until a constant weight was reached. The dried samples were then filled in 20mm high, 70mm diameter plastic petri-dishes of 76cm3 volume. Thus we obtained the same geometry as for the first samples set. 2.3. Samples Preparation for Beta Analysis The sample preparation has been done for four profiles of the first set of samples namely; Ah4, Ah5, Ah6, where we found the higher depositions of 137Cs and Ah9, where we found the lowest deposition of 137Cs. The analysis for the rest of the profiles of the first set is going on at the moment. To determine 90Sr concentration the 90Sr has to be extracted from the soil. For this purpose the well-known Nitric Acid Method has been used. The extraction has been done according to ``Meßanleitungen für die Überwachung der Radioaktivität in Boden``. 3. Analysis 3.1. Gamma Analysis The samples were submitted for analysis of gamma emitting radionuclides using a HPGe detector of 50% relative efficiency and having resolution of 2.0kev at 1.33MeV. The system was set up to cover about 2MeV–photon energy ranges over 4k channels. Measurement time was always more than 70000s. The analyses for the first set of samples were carried out using the software of GAMMA+ version 1.02.1 (Silena), while Genie-2000 spectroscopy software v2.1 (Canberra) has been used to analyze the new set of samples. 3.2. Beta Analysis The extracted 90Sr samples have been submitted for beta analysis using a gas-filled proportional detector of type LB 750 L for 50mm 4 dishes (Berthhold) wit an efficiency of 21.3% cps/Bq. Measurements times were 500min or longer. 3.3. Results and Discussion of Gamma Analysis The vertical distribution of 137Cs for the first set of samples in the eleven sites could be categorized into three groups (Figure 2 - Figure 4) according to the 137Cs inventory, the surface concentration of 137Cs and profile shape (Figure 5). The highest surface concentration of 137Cs was found in Ah4 (Figure 3), and the lowest was found in Ah11 (Figure 4). The profiles Ah2- Ah6, Ah9 and Ah10 show an approximate exponential decay for 137Cs with depth. The flat shape for the two profiles A8 and Ah11 (Figure 3 and Figure 4) indicates that these two sites might have been cultivated.