A radioactive source was used to find the self-efficiency (4) of the Geiger counter. The initial radioactivity of the radioactive source is A=40 and its half-life is 30 days. The experiment was conducted after 9 months. The experiment was conducted so that only 0.025 percent of the particles emitted from the source fell on the Geiger counter. The average count rate recorded by the counter (R) is equal to cps