Gamma ray spectrometry measures the gamma rays emitted from natural or man-made radioactive elements that are present in solid and liquid samples. The natural decay of U and Th produce a series of radionuclides with different half–lives that can be used to determine the absolute ages and accumulation rates of sediments, including marine deposits, ferromanganese nodules and marine phosphorites. The U–series and Th–series elements that emit gamma rays include 234Th, 226Ra, 214Pb, 210Pb and 228Ac. Emission rates from these radionuclides can be quantitatively measured utilizing high–resolution semiconductor detectors. In addition, gamma ray spectrometry can identify the presence of man–made radionuclides including 137Cs, 60Co, 144Ce and 125Sb.