Radioactivity measurement using Liquid Scintillation techniques

Techniques for measuring activity by liquid scintillation first appeared around seventy years ago, and have made their mark in the fields of life and earth sciences, environmental monitoring and primary radioactivity metrology.

These techniques involve mixing the radioactive solution to be measured with a scintillating liquid that transforms the ionizing radiation produced by radioactive decay into detectable, quantifiable light.

The main advantages of liquid scintillation are the ease with which radioactive sources can be prepared, the geometric detection efficiency of 4π, and the absence of a physical barrier between the radionuclide to be measured and the detector, enabling the detection of low-energy radiation. Liquid scintillation activity measurement is one of the only methods for measuring the activity of pure beta radionuclides, where radioactive decay is not accompanied by gamma radiation detectable by other techniques. It is also one of the only methods for measuring radionuclides that decay by electron capture, especially those that lead to the emission of low-energy ionizing radiation.

Liquid scintillation can also be used as an absolute method of measuring activity, i.e. without using a standard.

Modern liquid scintillation counters can have very low detection limits, allowing the measurement of micro-activities. One application is carbon-14 dating and geological tracing.

The main drawbacks of this technique lie in its overall energy efficiency, which is low and variable depending on the composition of the scintillating source. This makes it necessary to calculate the detection efficiency for each measurement condition.

Mastery of liquid scintillation activity measurement techniques begins with an understanding of the physico-chemical phenomena involved in the process of light emission, detection and pulse analysis. It is then based on the quality of scintillation sources, determination of their luminous efficiency, calibration of detectors and assessment of measurement uncertainty. Finally, it requires the use of reliable, verifiable measuring equipment.

The aim of this article is to provide an overview of these liquid scintillation techniques, so that users can better control the measurement process and optimize the quality of results.