Investigation of radioactivity level in heavy minerals of Cox's bazar beach sand by neutron activation analysis

Abstract

This thesis describes the study of radioactivity level in heavy mineral found in the Cox’s Bazar beach sand by Neutron Activation Analysis (NAA). Heavy mineral sand is a source of Zirconium, Titanium, Tungsten, rare earth elements, Diamonds, Sapphire, Garnet, etc, and they have various industrial applications. The Cox’s bazaar is an important tourist place in Bangladesh and it is the largest sea beach in the world. Many people from home and abroad come here around all times in the year. It is reported that the beach sand contains radioactive uranium, radium, thorium, and their daughter products and heavy minerals, which cause radiological hazards. There are seventeen deposits which contains 20.5 million tons of raw sand, and 4.4 million tons of heavy minerals. Only eight types of economically important heavy minerals, namely, Ilmenite, Magnetite, Zircon, Rutile, Garnet, Leucoxene, Kyanite and Monazite occur in this deposit. In the present study, we have investigated the elemental status in beach sand using Neutron Activation Analysis technique. At the same time, the radioactivity concentrations of 238U, 232Th and their daughters and 40K in heavy minerals have been analyzed. Knowledge of radioactivity levels of these elements is important to assess the possible radiological hazards for the human health. For the investigation of impurities concentrations and radioactivity in heavy minerals at the study area, a total of 30 samples were collected from 6 locations with 5 depth variations. The depths were varied from 0-1 ft, 1- 2 ft, 2- 3 ft, 3 - 4 ft and 4-5 ft, below this depth, the water level reached. After collection, all the samples were air dried at room temperature and relative humidity condition. Each sand sample was sieved to remove the aggregates and organic species, dried at about 110°C in an oven until having constant weight to remove the moisture and crushed into fine powder with an Agate mortar and pestle. The weight of each sample is kept around 100 mg. Weights are taken with the help of a micro balance. The bullet type irradiation vial containing sample are then sealed and preserved carefully for irradiation. To apply relative standardization approach, we used 3 standards: IAEA-Soil-7, IAEA-SL-1 and NIST 1633b Coal Fly Ash. Each of the standards was prepared as the same way as the sample prepared. In case of short irradiation, each sample vial is taken into a larger polyethylene irradiation tube (Rabbit) and is irradiated in the 3 MW TRIGA reactor using the pneumatic transfer system with the thermal neutron flux of 2.75 1012 n/cm2/sec at the reactor power 250 kW for one minute. The individual irradiation was performed for each sample within the polyethylene container irradiated with the thermal neutron flux of 1 1013 n/cm2/sec at the reactor power 500 kW for 40 minutes and 1 hour for determination of long lived radionuclides. After irradiation, samples and standard were turned into radioactive. Gamma ray counting was done by High Purity Germanium (HPGe) detector. By the short irradiation at 1st counting we determine Al, Ti, V, Mg etc. and from 2nd counting we determined Eu, Dy, Mn, Na etc. After 1st counting of long irradiation As, K, La, Na, Sm, Yb, U etc. and from 2nd counting determined Ce, Co, Cr, Cs, Eu, Fe, Gd, Hf, Sc, Ta, Tb, Th, Zn, etc, were determined using the radioactive standardization approach of NAA. The measured U, Th and K concentrations (ppm) in beach sand were converted to the activity concentrations (Bq/kg) and compared with the world reference value. The present study revealed that the Cox’s Bazar beach sand contains high level of radioactivity coming from U, Th and K compared to the world references. This study could be useful as baseline data for radiation exposure to sand and their impact on human health.