What does a reading on a Geiger–Müller counter represent?

• A. The total number of particles emitted per unit time
• B. The count rate is the number of disintegrations detected per unit time, corrected for background
• C. The energy of each detected event
• D. The background radiation level

Answer: (B)

A Geiger–Müller counter outputs a pulse for every ionizing event that occurs in the tube, so the reading you see is the number of detected events per unit time—the count rate. Because there’s always some ambient background radiation, the raw count rate includes events not from your sample. Subtracting this background gives the net count rate, which reflects the disintegrations detected from the source. The instrument does not measure the energy of each event, and it doesn’t directly tell you the total number of disintegrations occurring in the source per unit time—only the portion that the detector actually records, which depends on efficiency and geometry.