ANALYTICAL APPLICATION OF CHARGED PARTICLE INDUCED NUCLEAR REACTIONS FOR LOW MASS ELEMENTAL ANALYSIS

Author
Dr. Engr. A. K. M. Fazlul Hoque
Professor
Natural Sciences
Daffodil Internation University

ABSTRACT

Proton Induced Gamma Emission (PIGE), a nuclear reaction based analytical technique, has been developed at the 3 MeV Van de Graaff Accelerator Laboratory of Atomic Energy Centre, Dhaka (AECD) for analyzing the low Z elements in the range of 3 ~ Z ~15. With a view toexploit the potential of PIGE technique as a tool for practical applications, attempts have been made to fmd the experimental evidences for its suitability and scope. Gamma ray yields from PIGE experiments for some low Z elements have been measured and analyzed in order to assess its capability and to identify its limits. The thick target PIGE yields of the light elements such as lithium, beryllium, boron, oxygen, fluorine, sodium, magnesium and silicon that are essential for the development of the methodology have been measured at the proton energy of 2.9 MeV.
The thick target excitation function for the elements of interest namely lithium, boron, fluorine and sodium have been measured in the proton energy range of 2.3-3.0 MeV and that facilitated the identification of these elements.

The low energy proton-induced reactions m the light elements are energetically possible for PIGE studies but in heavy elements, the Coulomb barrier inhibits the reaction rate. _ The penetrability or transmission coefficients have been calculated to estimate the dependence of the y-ray yields on the mass of the target element. The sensitivity of the low Z elements such as lithium, beryllium, boron, oxygen, fluorine, sodium, magnesium, silicon and phosphorus have been measured at 2.9
MeV proton energy, with both the internal and the external beam techniques. The sensitivity for the elements, lithium, boron, fluorine and sodium is found to be the highest at this proton energy and these can be measured at the mglkg level. The sensitivity functions for lithium, boron, fluorine and sodium have also been measured in the proton energy range of2.3-3.0 MeV, that helped to select the excitation energy for the present experimental setup. The PIGE method has been utilized for isotopic speciation of the different isotopes of magnesium present in the locally available vegetables to assess its suitability for the isotopic analysis of different elements present in the samples of interest.

To ensure the analytical quality of data, a stringent protocol following the American Chemical Society guidelines has been adopted and also a Quality Assurance (QA) assessment has been included in the study. In order to validate the method, IAEA standard soil-7 and US rock standard sample AGV -1 have been analyzed and compared. The results of this intercomparison of analysis of fluorine are found to agree within ±5%.

Fluorine concentration in 6 water samples has been measured using the Ion Selective Analysis (ISA) method which is considered to be the best electrochemical method and the results are compared with those of PIGE analysis. The agreement between the results of the two methods is found to lie within ± 11 %. The correspondence between the results of the two methods indicates the accuracy of the measurements of fluorine in water sample using PIGE. In the process of the development of PIGE, the characteristics of the High Purity Germanium (HPGe) detector in terms of its energy calibration, energy resolution and efficiency have also been 11 studied. The quoted energy efficiency of the 59 cm3 HPGe detector was 12.3% at 1332 keV.
Analytical techniques have been developed for the quantitative analysis of materials of different origin. A mathematical formalism has been developed and implemented to obtain the matrix correction factors necessary to derive pure element yields that resulted from the individual elements present in the compound/mixture targets, taking the proton energy degradation into accmmt. The effect of the variability of the matrix on the gamma ray yields has been theoretically studied and then compared with the experimental results. Attempt has also been made to fmd a universal standard for concentration calibration for use in the quantitative analysis of the data obtained using PIGE technique.

Analytical methods based on the PIGE technique have been applied for the study of the concentration of fluorine in water. In total 223 water samples collected from different sources, both rural and urban areas of Bangladesh have been analyzed. Of these analyte samples, 102 were from the shallow tube wells, 15 were from the deep tube wells and 82 were from the city water supplies; 13 samples collected from the ponds, lakes and fountains and 11 rain water samples have also been analyzed for fluorine concentration.

The fluorine concentration in these samples was found to be in the range of 0.03 – 2.32 mg!L and the mean is 0.45 ± 0.43 mg!L. It has been found that the fluorine concentration in the deep tube well (>50 m) water is the 111 highest among the water samples analyzed from different sources and the mean of which is 0.90 ± 0.62 mg/L. The surface water contains the lowest amount of fluorine and the mean for the 13 surface water samples mentioned earlier is 0.14 ± 0.12 mg/L. The mean fluorine content in the rain water is 0.26±0.14 mg/L which is higher than that of the surface water. These samples were collected during the period of September-October, 1999.

The study reveals that the fluorine concentration in the water of city supplies of Bangladesh is much lower than the maximum permissible limit. The maximum permissible limit of fluorine in drinking water in Bangladesh is set at 1 mg/L. The recommended lower level of fluorine in drinking water is 0.5 mg/L, the fluorine content lower than which is considered harmful for health. The concentration of fluorine in the water of city supplies ranges from 0.03 – 1.10 mg!L with a mean of 0.32 ± 0.21 mg/L. It is observed that the majority of the water samples studied
contained fluorine less than 0.5 mg!L, indicating a deficiency of fluorine in the city water supply.
The mean of fluorine concentration in the shallow tube well water is 0.55 ± 0.49 mg/L, within the range of 0.04 – 2.32 mg/L. This indicates that the fluorine concentration in the groundwater of Bangladesh is, in general, low. However, the fluorine concentration is found to be higher in the tube well water of the south-western region of Bangladesh. The mean value of the fluorine concentration in tube well water in this region is 1.10 ± 0.52 mg/L within the range of0.37- 2.32 mg/L.

PIGE, developed in this study has also been applied for the analysis of fluorine in the dental enamel of Bangladeshi population to determine both prophylactic and toxicological effects of fluorine on human health. For adult teeth sampling, the age group of 15 years and above has been selected and 150 adult teeth have been analyzed for the fluorine contents in their enamel. 263 milk teeth (incisor) were collected for the measurement of fluorine in the dental enamel. The mean fluorine concentration in the adult dental enamel is found to be 2007±1505 mglkg within the range of 233 to 9528 mglkg. The concentration of fluorine in the milk teeth enamel ranges from 111 to 6423 mglkg and the mean is 1527±1028 mglkg. The results indicate that the fluorine concentration in the teeth of 40% of adult population and 24% of the children lies well above the normal level of 500 – 1000 mglkg. It is also observed that the milk teeth enamel contain higher amount of fluorine compared to that of the adult population.

Attempt has also been made to apply PIGE for the measurement of boron in soil. The boron content in the soil samples analyzed lies in the range of 19.9 – 153 mglkg with a mean of 52.7±32.1 mglkg. The boron concentrations in 5 soil samples, measured using PI GE, were also analyzed using Nuclear Reaction Analysis (NRA) method for comparison. The good correspondence observed between the results obtained by NRA and PIGE (±16%) for boron in soil samples, suggests that the PIGE technique is a reliable method for the analysis of boron in soil matrix.

PIGE technique has also been used for the measurements of fluorine, sodium, magnesium and phosphorus in the locally available vegetables. The results suggest that PIGE can be effectively used for the low Z element analysis present in trace quantities in different biological samples such as vegetables, fruits, crops, etc.
The present study demonstrates the characteristics of the nuclear reaction based PIGE methodology for analytical applications. The results of the present study show the strength and capabilities of the PI GE for the low Z elemental analysis, especially when present in trace quantities in the samples of different origin. The data presented on the concentration of fluorine in drinking water may be used to evaluate the quality of the drinking water in terms of the fluorine content. It can also be used to satisfy the requirements of the baseline data as well as to detect any fluorine hazard that might be occurring in any region of rural Bangladesh. The data presented on the concentration of fluorine in dental enamel may be used to detect any possible fluorine toxicity in the different age groups of the Bangladeshi population.

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