Radioactivity in Fish - Dictionary Becquerel (Bq): The International System of units (SI) for radioactivity, used to measure the rate of radioactive decay. One Becquerel is defined as one disintegration per second”?. Cesium (134Cs): Cesium (134Cs) is a fission product with a half-life of 2.06 years. 134Cs decay is accompanied by beta particle and gamma ray emissions. 134Cs can be present in the environment as a nuclear fission product. Cesium (137Cs): Cesium (137Cs) is a fission product with a half-life of 30.17 years. 137Cs decay is accompanied by a mid-energy gamma ray emission. 137Cs can be present in the environment as a nuclear fission product. Potassium (40K): Potassium (40K) is a natural radioisotope with a half-life of 1.25×109 years. 40K undergoes 3 types of radioactive decay one of which results in a mid-energy gamma ray emission. Counting Time (h): The duration the sample was measured, in units of hours. Iodine (131I): Iodine (131I) has a half-life of eight days and can be present in the environment as a nuclear fission product. Bq/kg: Becquerel per kilogram. The units used to report radioactivity concentration in fish. MDC: Minimum Detectable Concentration (MDC) is the smallest concentration of radioactivity that can be reliably detected in a sample, for a given time of measure and sample size. Uncertainty: The amount the calculated value may vary from the true value.