Abstract: The present invention provides a zero displacement ionization chamber for use in calibrating radiation units. The ionization chamber includes a thimble chamber having first and second electrodes with predetermined thickness and length separated from one another by a pre-defined air gap. The material density and air gap density are balanced by weighting the normal path lengths of radiation rays through the thimble chamber, thereby affecting the thimble chamber having an overall density substantially the same as water thus yielding a zero displacement when placed in water. The first and second electrodes are further electrically connected to a power source for creating an ionization effect when receiving radiation rays in the air gap so that a signal indicative of the radiation exposure may be detected by an electrical connection to one of the electrodes.