Cesium



The natural isotope 133 Cs is stable, but two radioactive isotopes of cesium are of interest. These are 135 Cs and 137 Cs. Both are radioactive with half-lives of 2.3 My and 30 y, respectively. They are produced with equal yields of about 6 to 7 % in binary fission of actinide nuclides. The nuclei are introduced into the environment through atmospheric nuclear explosions and release from the nuclear industries. 137 Cs is a well-known radionuclide (and radiotracer) and time marker while 135 Cs has been neglected because of its much longer half-life precludes detection by counting. In addition, in conventional mass spectrometers, the tail from the stable 133 Cs swamps its signal (since the abundance of 135 Cs is typically 10 -9 smaller). The first measurement of the relevant cesium isotopes was made using two coastal sediment samples and a thermal ionization mass spectrometer. The [ 135 Cs/ 133 Cs] ratio was about 1•10 -9 and the [ 137 Cs/ 135 Cs] ratio was about one to five due to the decay occurring through the past thirty years from their production ratio of about one. It was noted that this appeared to be the first detection of fallout 135 Cs in nature and it may be inferred that the isotope ratio [ 137 Cs/ 135 Cs] is much more powerful as a tracer than 137 Cs alone. Two models were given to illustrate how the ratio may be used in order to quantify estimates of recent sedimentation and rates of erosion (Lee et al. 1993).