Determination of ¹³⁵Cs and ¹³⁷Cs in environmental samples: A review

Radionuclides of caesium are environmentally important since they are formed as significant high yield fission products (¹³⁵Cs and ¹³⁷Cs) and activation products (¹³⁴Cs and ¹³⁶Cs) during nuclear fission. They originate from a range of nuclear activities such as weapons testing, nuclear reprocessing and nuclear fuel cycle discharges and nuclear accidents. Whilst ¹³⁷Cs, ¹³⁴Cs and ¹³⁶Cs are routinely measurable at high sensitivity by gamma spectrometry, routine detection of long-lived ¹³⁵Cs by radiometric methods is challenging. This measurement is, however, important given its significance in long-term nuclear waste storage and disposal. Furthermore, the ¹³⁵Cs/¹³⁷Cs ratio varies with reactor, weapon and fuel type, and accurate measurement of this ratio can therefore be used as a forensic tool in identifying the source(s) of nuclear contamination. The shorter-lived activation products ¹³⁴Cs and ¹³⁶Cs have a limited application but provide useful early information on fuel irradiation history and have importance in health physics. Detection of ¹³⁵Cs (and ¹³⁷Cs) is achievable by mass spectrometric techniques; most commonly inductively coupled plasma mass spectrometry (ICP-MS), as well as thermal ionisation (TIMS), accelerator (AMS) and resonance ionisation (RIMS) techniques. The critical issues affecting the accuracy and detection limits achievable by this technique are effective removal of barium to eliminate isobaric interferences arising from ¹³⁵Ba and ¹³⁷Ba, and elimination of peak tailing of stable ¹³³Cs on ¹³⁵Cs. Isobaric interferences can be removed by chemical separation, most commonly ion exchange chromatography, and/or instrumental separation using an ICP-MS equipped with a reaction cell. The removal of the peak tailing interference is dependent on the instrument used for final measurement. This review summarizes and compares the analytical procedures developed for determination of ¹³⁵Cs/¹³⁷Cs, with particular focus on ICP-MS detection and the methods applied to interference separation. - Highlights: • ¹³⁵Cs and ¹³⁷Cs are present as a result of anthropogenic nuclear activities. • Measurement has implications for multiple aspects of the nuclear industry. • Advances in mass spectrometry enable low-level detection of ¹³⁵Cs and ¹³⁷Cs. • Accurate measurement of the ¹³⁵Cs/¹³⁷Cs ratio can identify the contamination source. • Detection limits are controlled by removal of interferences from stable Ba and Cs