14 Int. J. Low Radiation, Vol. 10, No. 1, 2015 Copyright © 2015 Inderscience Enterprises Ltd. Natural radioactivity and hazardous index of major South Indian river sediments S. Murugesan* Department of Physics, Sathyabama University, Chennai 600119, Tamil Nadu, India Email: binunair.phd@gmail.com *Corresponding author S. Mullainathan Department of Physics, A.V.C. College of Engineering, Mayiladuthurai 609305, Tamil Nadu, India Email: s_mullai@rediffmail.com V. Ramasamy Department of Physics, Annamalai University, Annamalai Nagar 608002, Tamil Nadu, India Email: srsaranram@rediffmail.com Abstract: The activity concentrations of 238 U, 232 Th and 40 K have been determined by gamma ray spectrometry with an NaI(Tl) detector in sediments of Palar River, Tamil Nadu, India. The absorbed dose rate, radium equivalent concentration, external (H ex ) and internal (H in ) hazardous indices are calculated from criteria formula and compared with the international recommended limits. The Radioactive Heat Production (RHP) rate and Excess Lifetime Cancer Risk (ELCR) are also calculated. The observed dose rate from ERDM (Environmental Radiation Dosimeter) at 1 m above the ground level at each site of the river is measured and correlated with calculated absorbed dose rate. The distribution of quartz, feldspar, magnetic susceptibility and weight of the magnetic minerals is correlated with radioactivity results. From the observations, the weight of the magnetic minerals is an index to identify the sediments of low or high radiological risk. Keywords: sediments; Palar River; RHP; radioactive heat production rate; ELCR; excess lifetime cancer risk; quartz; magnetic susceptibility. Reference to this paper should be made as follows: Murugesan, S., Mullainathan, S. and Ramasamy, V. (2015) ‘Natural radioactivity and hazardous index of major South Indian river sediments’, Int. J. Low Radiation, Vol. 10, No. 1, pp.14–33. Biographical notes: S. Murugesan is serving as an Associate Professor at Department of Physics, Sathyabama University, Chennai, India. He has published more than 15 research articles in various national and international journals. His field of interest is spectroscopy and nuclear physics.