Nickel 63 decays to stable copper 63 under beta radiation with 66.9 keV energy. Some centimetres are already enough to stop that, with aluminum a millimeter sho

Nickel 63 decays to stable copper 63 under beta radiation with 66.9 keV energy. Some centimetres are already enough to stop that, with aluminum a millimeter should probably already be enough. The real safety issues is if people pry it open and ingest or inhale the radionuclides.