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Geography Laboratories

Radionuclide Dating

Ortec Gamma Well Detectors

Ortec Gamma Well Detectors (Upright and J-shape)

These detectors are used to date soils and sediments from salt marshes and lakes using radioactive isotopes caesium - 137 (137Cs), lead - 210 (210Pb) and Lead - 214 (a mediator for Ra - 226). Caesium - 137 is a man-made radionuclide created by atmospheric testing of nuclear weapons and can be used to date sediments deposited since 1950. Lead - 210 is a naturally occurring radionuclide that is part of the 238U decay series and can date sediments up to 150 years old.

We have three upright well detectors which are suitable for samples with normal 210Pb concentrations. Our three J-shaped detectors are low background detectors which are ideal for samples with low 210Pb concentrations. One of these J-shaped detectors is equipped with a carbon fibre endcap, making it suitable for ultra-low background detection.

Prior to analysis sediment samples must be freeze-dried and ball-milled. Plus, dry bulk density must also be calculated for use in dating models. The samples must be placed in sealed cylindrical vials for 28 days to allow 226Ra and 214Pb to equilibrate prior to gamma analysis. The samples are inserted into a liquid nitrogen cooled germanium detector and gamma emissions from the sample are recorded using an analogue-to-digital convertor interfaced with a computer. Samples are typically counted for seven days, but can be counted for shorter time (3 - 4 days) if the activity is particularly high. One detector is usually assigned to each sediment core to avoid problems associated with cross calibrating detectors.