- 10 An instrument for measuring nuclear radiations designed on the principle of ion-pair formation consists of a gas-filled chamber with two electrodes, an electrical power supply to the electrodes and a meter for measuring electrical current. When an ionizing radiation enters the gas chamber at a time when a potential is applied to the electrodes, the ion-pairs that are formed will provide positive ions that will collect on the cathode and negative ions which collect on the anode and thereby create a pulse of current which will be indicated on the current meter. The amplitude of the pulse depends upon the number of ionpairs produced and the applied voltage. Three common types of ionization instruments that are basically similar but differ in the applied voltage are the ionization chamber, the proportional counter and the Geiger-MUller counter. Another type of instrument for measuring nuclear radiation is the scintillation counter. Certain materials -- phosphors -- when exposed to nuclear radiations will emit small flashes of light -- scintillations. The scintillations are received from the phosphor by the photocathode which responds by emitting electrons that are greatly increased by stages in the photomultiplier tube until a measurable