Apparatus for calibrating radiation detection instruments retains a beta radiation source, typically strontium 90, providing a known intensity of beta radiation in a safety shielded position within an aluminum mass to meter out standard doses in a plurality of ranges for calibrating corresponding instrument ranges. Safety features assure shielding of the radiation to prevent any exposure of personnel calibrating instruments to radiation. The calibrating apparatus is portable, but is supplied also in a wall mount housing structure with a horizontally extending shelf for receiving an instrument into calibration position. A pivotable treadle must be operated when placing the instrument into calibration position in order to release calibrating radiation from the source, which is retained in a safe storage position when the calibrating apparatus is not in use.