First detection of fallout Cs-135 and potential applications of [sup 137]Cs/[sup 135]Cs ratios

All three cesium isotopes, natural Cs-133 (stable) plus fission Cs-135 (half-life 2.3 Ma) and Cs-137 (half-life 30 y), were measured in two coastal sediment samples using a thermal ionization mass spectrometer equipped with a retarding potential quadrupole lens filter. The [sup 135]Cs/[sup 133]Cs ratio was about 1[times]10[sup [minus]9] while [sup 137]Cs/[sup 135]Cs was about 0.5 owing to the decay during the past 30 years from their production ratio of about 1. This appears to be the first detection of fallout Cs-135 in nature. Being an isotope ratio, [sup 137]Cs/[sup 135]Cs is a far more powerful chronometer-tracer than Cs-137 alone. In order to explore its potential, the authors present briefly two simple models to illustrate how this ratio may be used to quantitatively estimate recent sedimentation and erosion rates.