In the fission of U-235, two cesium isotopes form with a high cross section, Cs-134 and Cs-137. Their ratio of the two isotopes as the result of fission is well deter­mined; however, the decay rate of Cs-134 is higher. Therefore, the ratio of the two fission cesium isotopes gives information on the time of the environmental pollu­tion by radioactive cesium. Cs-134 can be produced in the 133Cs(n, Y)134Cs nuclear reaction; its half-life is 2 years, and it has (3_ and gamma radiation. The half-life of Cs-137 is 30 years, and it emits (3_ and gamma radiation. Since cesium ion is strongly sorbed on soils, its migration is rather slow. For these reasons, Cs-137 can be applied to study soil formation and erosion. In these studies, the activity of Cs-137 already present in the soil from the nuclear pollutions is measured. For more information see Section 8.8.

8.6.18 Renium-186

186Re is produced by the 185Re(n, Y)186Re nuclear reaction. Its half-life is 90.6 h, and it has (3_-radiation. It has medical applications.