Instrument used for detecting gamma rays and X-rays in which radiation triggers an electrical discharge resulting in a pulse of electric current whose strength is proportional to the energy of the radiation.

Similar in construction and operation to an ionization chamber. Usually the proportional counter is a metal chamber. filled with gas, with a central electrode maintained at a positive voltage with respect to the shell. The voltage level of the central electrode is related to the critical voltage value where gas amplification begins. The proportional counter is operated in that limited voltage range where the charge flow across the counter is proportional to the primary ionization. In well logging, it is designed for the detection of neutrons. A gas is used which is suitable for the production of ionizing particles upon reaction with incident neutrons. The gas commonly used is He3 but may be BF3. The BF3 gas, maintained at about 1 atmosphere, requires a voltage level of 2400-2500 volts and produces a larger pulse than that in He3; but, the He3 maintained at higher pressure and operating at about 1300 volts is more efficient in the detection of neutrons. Compare ionization chamber, Geiger-Mueller counter, and scintillation counter.

counter tube whose output pulse is proportional to number of ions produced