Gammasphere at the Argonne National Laboratory, Illinois, USA is the most sensitive gamma ray ‘microscope’ used in nuclear physics research. It is a cylinder measuring 3 m (10ft) high pierced with 110 gamma ray detectors. These detectors focus inwards to a central target area which is bombarded with ions from a particle accelerator. Gammasphere is so sensitive that it can detect gamma rays produced in these collisions that are 300,000 times weaker than the strongest produced in the collisions.