RADIOACTIVITY DETECTOR

PURPOSE:To improve the accuracy of detection by ejecting laminar shielding flow along the radioactivity detection window formed on the inside wall in the internal flow passage of a measuring device to form a shielding layer of the gaseous flow between the inside wall of a GM tube and the gas to be detected flowing in the internal flow passage thereof. CONSTITUTION:A cooling gas is ejected from an ejection hole 20 to form beforehand laminar shielding gaseous flow along the inside wall in the internal flow passage of a GM tube 10 before a high temp. gas is passed in the internal flow passage 14 of the tube 10. If the high temp. gas is passed in the passage 14 of the tube 10 in this state, radiations are made incident from the radioactive dust contained in the high temp. gas to the inside of the tube 10 through a detection window 10a. The laminar shielding gaseous flow formed of the cooling gas functions as a heat insulation material and cools and protects thoroughly the tube 10 against the high temp. gas.