Reply to the letter-to-the-editor submitted by Dr. M. U. Khandaker at JRNC referring to the article entitled as “Environmental impact of some cement manufacturing plants in Saudi Arabia” J Radioanal Nucl Chem (2014) 302:1103–1117 doi 10.1007/s10967-014-3383-8

We appreciate this opportunity to reply to the comments given in the letter of Dr. M.U. Khandaker about our recent published article in J Radioanal Nucl Chem (2014) 302:1103–1117. Our reply is presented in the next paragraphs according to the ordering of the letter’s comments. Secular equilibrium in the decay of radioactive series is normally assumed in environmental samples. The disequilibrium is occurred when one or more of the progeny are completely or partially removed or added to the system. The relatively short half-lives of the Th daughters mean that disequilibrium in this decay series is unlikely. On the other hand, because of the much longer half-lives of many U daughters, disequilibrium in this decay series is present, even for closed systems. In particularly, U series disequilibrium can result from precipitation/dissolution, alpha recoil, diffusion and the Szilard–Chalmers effect [1]. It is widespread in the exposed surficial environment and was even observed in many raw materials. The present work dealt with cement samples which were subjected to many chemical and physical processes that can easily lead to disequilibrium in U series assuming that all raw materials used in the production were in secular equilibrium which can hardly happen. Therefore, both the Th and Ra activity concentrations were measured in the present work. The quantification of Th activity (or U, assuming radioactive equilibrium) using the 63.3 and 92.6 keV (92.4 ? 92.8 keV doublet) c-ray photopeaks is highly preferable, especially in case of using n-type X-gamma HPGe detectors (as the used detector in this study had a carbon epoxy window) because they are typically at highest detection efficiency. The high detector efficiencies combined with the high emission probabilities make these c-ray photopeaks the optimal choice for routine c-ray analysis on most environmental samples even when regular p-type HPGe detectors are used [2–8]. The typical ratio of U/Ra activity in soil are reported with range from 0.55 to[2 [6, 9, 10]. Therefore, the observed disequilibrium in this study between the radioactivity of Th and Ra is normal in soil samples assuming radioactive equilibrium between the U and Th. If there are significant contributions of spectral interferences from other radionuclides to the 63.3 and 92.6 keV photopeaks of Th isotope as mentioned in Khandaker’s letter, then this will lead to a higher calculated activity of Th than that of Ra, which is not found in this study. The possible interference with 63.3 photopeak of Th from 63.8 keV of Th and 63.9 keV of from Th can be neglected unless the measured sample is highly enriched in U which is hardly the case in most of the environmental samples. Since the isotopic activity ratio of U/U is 21.6 [11–13], the contributions of any radionuclides in U decay series to the measured c-ray spectra of H. E. Hassan (&) T. Sharshar H. M. Badran Physics Department, Faculty of Science, Taif University, P.O. Box 888, Al-Hawiah, Taif, Saudi Arabia e-mail: hebrahim_hassan@yahoo.com