ionized in the same way. Comparison of the ion currents in the two chambers allows compensation for changes due to air pressure, temperature, or the ageing of the source. Medical radiation measurement In medical physics and radiotherapy, ionization chambers are used to ensure that the dose delivered from a therapy unit or radiopharmaceutical is what is intended. The devices used for radiotherapy are called "reference dosimeters", while those used for radiopharmaceuticals are called radioisotope dose calibrators - an inexact name for radionuclide radioactivity calibrators, which are used for measurement of radioactivity but not absorbed dose. A chamber will have a calibration factor established by a national standards laboratory such as ARPANSA in Australia or the NPL in the UK, or will have a factor determined by comparison against a transfer standard chamber traceable to national standards at the user's site. Guidance on application use In the United Kingdom the HSE has issued a user guide on selecting the correct radiation measurement instrument for the particular application concerned. This covers all radiation instrument technologies, and is a useful comparative guide to the use of ion chamber instruments. See also Absorbed dose Bragg–Gray cavity theory Dosimetry Gaseous ionization detector Sievert chamber Stopping power (particle radiation) Notes References Ionising radiation detectors Particle detectors Radiation protection