G.M. tube environment radioactivity meter having a sensitivity degradation monitoring device with a natural radioisotope and means for automatically correcting the measured radioactivity

A G.M. tube environment radiameter including a sensitivity degradation monitoring device, comprising:

• a) a weak radioactive source arranged in a fixed position with respect to said G.M. tube;
• b) a shielding disc moved by a power operated support adapted to alternatively move the shielding disc between two stable positions, namely a first position, in which said shielding disc shields the G.M. tube from said weak radioactive source so pulse intensity generated by the weak radioactive source is negligible with respect to pulse intensity generated by background radioactivity, and a second position, in which said G.M. tube is directly exposed to said weak radioactive source thereby causing the G.M. tube to provide pulses of magnitude equal to pulses generated by the G.M. tube in response to background radioactivity;
• c) a cyclic counter circuit for contemporaneously counting pulses generated by the G.M. tube in response to both the weak radioactive source and the background radioactivity when the shielding disc is in the second position;
• and
• d) a circuit for measuring and storing pulse intensities both in the test stage when the shielding disc is in the second position and in subsequent times when the shielding disc in the first position as well as for performing a ratio of said pulse intensities.