Radionuclides of caesium are environmentally important since they are formed as significant high yield fission products ( 135 Cs and 137 Cs) and activation products ( 134 Cs and 136 Cs) 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 137 Cs, 134 Cs and 136 Cs are routinely measurable at high sensitivity by gamma spectrometry, routine detection of long-lived 135 Cs by radiometric methods is challenging. This measurement is, however, important given its significance in long-term nuclear waste storage and disposal.
M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPTseparation 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 135 Cs/ 137 Cs, with particular focus on ICP-MS detection and the methods applied to interference separation.
Sources and properties of caesium isotopesHigh yield fission products 135 Cs and 137 Cs (Table 1) are present in environmental samples as a result of releases from nuclear power plants and reprocessing sites, nuclear accidents, and fallout from atmospheric weapons testing [1]. Caesium-137 is established as an important radionuclide in radiation protection, environmental monitoring, sediment dating, land erosion and waste disposal [2]. By comparison, 135 Cs is a long-lived radioisotope with a comparatively low radiation risk; however it is a significant contributor to the long term radiological risk associated with deep geological disposal [3]. Furthermore, the
135Cs/ 137 Cs ratio varies with reactor, weapon and fuel type, and therefore can be used as a forensic tool to identify the source of radioactive contamination [1,[4][5][6]. Other isotopes of caesium include the activation products ( 134 Cs and 136 Cs, Table 1). These shorter-lived activation products (2.06 years and 13.2 days, respectively) provide information on fuel irradiation history and have importance in health physics terms but are not considered further here.( Table 1) Caesium-137 decays by beta emission to short-lived metastable isomer Cs is therefore achievable using beta counting or gamma spectrometry. Gamma spectrometry is generally favoured because it exploits the high gamma intensity of the 662 keV energy that is not susceptible to significant absorption. The ability to directly count most samples without the need for any chemical separation is also beneficial, and a high-purity germanium (HPGe)well detector can readily achieve a detection limit of ~2.5 mBq g -1 (for a 20 g sample counted for half a day), which is suitable for many environmental samples [3]. By comparison, 137 Cs measurement by beta counting in environmental samples requires chemical separation from other beta-emitting nuclides prior to measurement.Caesium-135 decays with a maximum beta particle energy of 2...