Determination of Absorbed and Annual Effective Doses around Birnin Gwari Artisanal Goldmine , Kaduna State , Nigeria

This study presents result of outdoor absorbed dose rate and estimated annual effective dose rate from the naturally occurring radionuclides U, Th and K around BirninGwari artisanal goldmine using Instrumental Neutron Activation Analysis (INAA). The results showed that the mean absorbed dose rate is 96.52 nGy/h with a standard deviation of 33.3 nGy/h and the standard error of 9.62 nGy/h the range falls between 26.67-123.85 nGy/h. The mean effective dose rate was estimated to be 0.675 mSv/year with a standard deviation of 0.234 mSv and standard error of 0.068 mSv and in range between 0.183-0.867 mSv. The mean annual effective dose rate from this work is lower than the 1mSv/year recommended by ICRP for public radiation exposure control.


INTRODUCTION
Mining is a global industry known for its economic benefits of wealth creation and employment.In Africa, commercial scale mining provides important benefits in term of exports/foreign exchange earnings and tax receipt to nineteen African countries (Hayumbu and Mulenga, 2004).
The above mentioned socio-economic benefits of the mining industry not withstanding in developing countries, the industry is likely to be associated with three potential negative effects.The first one is the socio-economic dislocation an ill-prepared mining community goes through at mine closure, which arise from exploitation of a non-regenerative resources (Hayumbu and Mulenga, 2004).The second and third undesirable aspects arise when non-optimal management of mining operations results in environmental degradation and/or negative health impacts on miners and mining communities.Principal health problems among miners and mining communities from various countries that have been cited by the literature include respiratory disease, neoplasm/cancer, chronic hypertension, mental health and genetic impact (WHO, 1999) The major cause of these diseases can be attributed to the heavy metal contamination and naturally occurring radioactive materials (ICRP, 1994).
The International Basic Safety Standards (BSS) for protection against ionizing radiation and the safety of radiation sources (IAEA, 1996) specify the basic requirement for the protection of health and the environment from ionizing radiation.These are based on the latest recommendations of the International Commission on Radiological Protection on the regulation of Practices and interventions (ICRP, 2007).The BSS is applied to both natural and artificial sources of radiation in the environment and the consequences on living and non-living species.
Irradiation of the human body from external sources is mainly by gamma radiation from radionuclides of the 235 U and 232 Th decay series and from 40 K.These radionuclides may be present in the body and irradiate various organs with alpha and beta particles as well as gamma rays (Cember, 1996;UNSCEAR, 2000;IAEA, 2005).
Mineral ores in the naturally undisturbed environments and the radionuclides in the decay series are more or less in radiological equilibrium.However, this equilibrium becomes disturbed through human activities such as mining and has been identified as one of the potential sources of exposure to NORM (UNSCEAR, 2000).
In many developing countries including Nigeria, mining activities have not been subjected to radiological regulatory control.Data on radionuclide concentrations in raw materials, residues and waste streams and data on public exposure are scanty (Darko et al., 2005;Darko and Faanu, 2007).Consequently, there is general lack of awareness and knowledge of the radiological hazards and exposure levels by legislators, regulators and operators.
The aim of this study is to determine the absorbed dose rate and annual effective dose level of Naturally Occurring Radioactive Materials (NORM) at the mining locations in Birnin Gwari Local Government Area of Kaduna State, Nigeria.

MATERIALS AND METHODS
Twelve samples were collected from 12 gold pits at different depth of 2.5 m to 43.0 m in the study area which comprised of the following artisanal gold mining sites: Kakini Farinruwa and Tsohowar Gwari.Global Position System (GPS) was used to determine the location of each pit and a tape rule was used to determine the depth of each pit.Table 1 shows the location, depth and elevation of each pit where samples were collected.

Sample preparation:
The samples collected were taken to the Laboratory of Mineral Resources Engineering Department of Kaduna Polytechnic where they were crushed and sieved to tiny bits of 38 µm (Kogo et al., 2009).The crushed samples were dried at about 100 °C to a constant weight.The samples were then taken to Centre for Energy Research and Training Ahmadu Bello University Zaria, Nigeria for Neutron Activation Analysis.Between 0.150g -0.180g of the powdered samples were wrapped in a polyethylene then placed in 7 cm 3 .Rabbit capsules.The polyethylene and rabbit capsules containing the samples were cleaned by soaking in 1:1 HNO 3 (Nitric acid) and then washed with de-ionised water in order to eliminate every contamination prior to sample irradiation (El-Taher et al., 2003).

Sample analysis:
The concentration of elements of interest from the collected and prepared samples were investigated using Neutron Activation Analysis technique (NAA) with the Nigeria Research Reactor 1 (NRR1) No NRR1/DS/JC/09/16 at the Centre for Energy Research and Training, Ahmadu Bello University Zaria, Nigeria.

RESULTS
Absorbed dose rate: The gamma dose rates from the samples were calculated from the activity concentrations of the relevant radionuclides from Table 2 using Eq.( 1) below: where, A k , A u and A Th are the activity concentrations of 40 K, 238 U and 232 Th respectively and Table 3 shows the dose conversion factors of 40 K, 238 U and 232 th (UNSCEAR, 2000) The result from Table 3 shows that the mean absorbed dose rate in the study area is 96.52 nGy/h witha standard deviation of 33.31 nGy/h and the standard error of the mean is 9.62 nGy/h, meaning that

Sample code Depth
Location ---------------------------------------------------------  the maximum sampling error in the results cannot exceed 9.62 nGy/h.The 95% Confidence interval for the mean dose rate in the study area lies between 26.67-123.85nGy/h.In other words, there is 95% assurance that the mean dose rate in the study area lies between the limits (26.67-123.85)nGy/h as indicated in Table 3.The percentage contributions to absorbed dose rate due to 238 U, 232 Th and 40 K are shown in Fig. 1 Annual effective dose rate: The annual effective dose rate measured in mS v was calculated from the absorbed dose rate and is presented in Table 4.
The result from Table 4 showed that the mean effective dose rate is 0.685 mS v with a standard deviation of 0.234 mS v and the standard error of the mean is 0.068 mS v meaning that the maximum sampling error in the result from the study area cannot exceed 0.068 msv.The 95% confidence interval for the mean annual effective dose rate in the study area falls between 0.1832-0.867mS v .This shows that there is 95% assurance that the mean annual effective dose in the study area lies between the limits (0.1832-0.867) mSv.The mean absorbed dose rate calculated from soil activity concentration was 96.52±9.62 nGy/h in a range of 26.67-123.85nGy/h which is about 1 times the world wide average value of 60 nGy/h (UNSCEAR 2000), about 3 times the value of 29.9 obtained in Ghana (Faanu et al., 2010) and about twice the value of 46.2 nGy/h obtained in Nigeria (Zaria) (Muhammad et al., 2010).The difference could be attributed to the Fact that the samples collected in this study were from a mean depth of 17.35 m in the range between 2.5-43 m below the surface and also the geology and geochemical composition of the sampling site.
The mean percentage contribution of 238 U , 232 Th and 40 Kto the mean absorbed dose rate are 17.63% for 238 U, 40.71% for 232 Th and 41.66% for 40 K.The values show that 40 K contribution to the absorbed dose rate is more followed by 232 Th while the contribution of 238 U is lowest.Though the study area is K-Feldspar (MMDS, 2010) the % contributions of 232 Th compared favourably with that of 40 K.
The corresponding mean annual effective dose rate in this study is 0.675±0.068mSv in the range of 0.183-0.867mSv.In the determination of these values a dose conversion factor of 0.7 Sv/Gy an outdoor and indoor occupancy factor of 0.2 and 0.8 respectively were applied (UNSCEAR, 2000).The results from this study show that the annual effective dose rate is about 1

Fig. 1 :
Fig. 1: Plot of % contribution due to 238 U, 232 Th and 40 K by location

Table 1 :
Sample locations

Table 3 :
Percentage contribution to absorbed dose rate due to 238 U, 232 Th and 40 K

Table 4
Annual effective dose rate in (mS v )