We present a new method to determine ²²⁶Ra in aqueous environmental samples, based on the rate of ingrowth of ²²²Rn from ²²⁶Ra, using the radium delayed coincidence counter (RaDeCC). We use the same instrument setup that is used for the determination of ²²³Ra and ²²⁴Ra. In contrast to methods published earlier, the approach does not require a modification of the counting equipment, counting separately for ²²⁶Ra, or waiting for radioactive equilibrium.

Acrylic fibers impregnated with MnO2 (Mn-fiber) have become a valuable tool for concentrating dissolved radium for oceanographic applications. With four naturally-occurring radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra, and ²²⁸Ra) of vastly different half-lives (3.6 days to 1600 years), radium can be a powerful tool for tracing terrestrial water discharges into the ocean and studying coastal mixing processes.

Four radium isotopes, decay products of the ²³⁸U–²³²Th–²³⁵U series radionuclides, occur in nature: ²²⁴Ra (t_1/2=3.66 days), ²²³Ra (t_1/2=11.4 days), ²²⁸Ra (t_1/2=5.75 years), and ²²⁶Ra (t_1/2=1600 years). Beginning with the GEOSECS program of the late 1960s – early 1970s, these radium isotopes have been widely applied to the study of a variety of oceanographic processes (Moore, 1972; Trier et al., 1972).