Half-life is the time for the number of undecayed nuclei, or a source’s activity, to halve. Repeated half-lives leave fractions ½, ¼, ⅛ and so on; subtract background from detector readings first. A longer half-life makes a material persist longer, while a short half-life can give greater initial activity for equal numbers of nuclei. Hazard also depends on radiation type, amount and exposure. Rearrange the relationship before substituting if a different quantity is required. Justify your answer.