Abstract: An ionization chamber of reduced size, with an end window of a diameter preferably smaller than 3 cm is mounted on the end of a hand probe containing an amplifier, the output of which goes through a cable to an indicating device. One of a set of aperture diaphragms may be slipped over the end window to reduce the area of the window transparent to beta rays. In addition to the end window, slots in the cylindrical wall of the chamber increase the solid angle through which radiation may enter the chamber. The end window and the slotted walls of the chamber are covered with tissue-equivalent material for measurement of radiation exposure dose by the electrodes of the chamber and their associated amplifier and indicator. An alternative design with a conical chamber and conical inner electrode and a smaller window provides reliable measurement, even in inhomogeneous radiation fields without the necessity of using diaphragms.