[{"from": "human", "value": "Design a molecule that meets the criteria outlined in the description. The molecule is an unstable radioisotope of cesium (Cs) with radiocytotoxic application. Cs-131 is a gamma photon-emitting radionuclide with high energy and a relatively short half-life of 9.7 days. When used in prostate brachytherapy, Cs-131 demonstrated advantages over other commonly used isotopes."}, {"from": "gpt", "value": "[131Cs]"}]