Calibrating radiotherapy equipment. Physicist setting up a primary standard ionisation chamber. In 2003, this spherical ionisation chamber was established as the world's first primary standard for measuring radioactive isotopes (atoms with an unstable nucleus) used for high dose rate brachytherapy (inserting radioactive samples near localised cancers to treat them). Here, it is being used to calibrate the dose of radiation emitted by the radioactive isotope (an atom with an unstable nucleus) iridium-192, a process called dosimetry. Photographed at the National Physical Laboratory in Teddington, UK.