An extrapolation chamber dosimeter has been developed for clinical dosimetry of electron beams and X-rays from medical linear accelerators. It consists of a new type extrapolation chamber

a water phantom and an intelligent portable instrument. With a thin entrance window and aφ20 mm collecting electrode made of polystyrene

the electrode spacing can be varied from 0.2 to 6mm. The dosimeter can accomplish dose measurement automatically

and has functions of error self-diagnosis and dose self-recording. The energy range applicable is 0.5-20MeV

and the dose-rate range 0.02-40Gy/min. The total uncertainty is 2.1%.