Differentiate between a Geiger-Müller counter and an ionization chamber used in radiation dosimetry.

• A. GM counter detects ionization current; Ionization chamber counts discrete events.
• B. GM counter detects individual ionization events; Ionization chamber measures an ionization current.
• C. GM counter measures dose rate via current; Ionization chamber measures energy.
• D. GM counter and ionization chamber both measure dose rate by current.

Answer: (B)

The key idea here is how each detector responds to radiation. A Geiger-Müller counter detects individual ionization events. Its gas-filled tube is biased so that each ionizing interaction starts a self-sustaining discharge, producing a single, easily counted electrical pulse. You read the instrument as a count of pulses, not as a continuous signal. Because it records discrete events and has a recovery time after each pulse (dead time), its response becomes nonlinear at higher radiation levels and it’s not ideal for precise dose-rate measurements.

An ionization chamber, on the other hand, measures the ionization current. Radiation creates ion pairs in the gas, and the applied electric field collects those charges, producing a current that's proportional to the number of ionizations per unit time. This current is integrated, giving a reading that is directly related to dose rate over a wide range. It’s generally energy-independent for photons in medical dosimetry and is well-suited for accurate dosimetry.

So, the Geiger-Müller counter detects individual events as pulses; the ionization chamber measures a continuous current proportional to dose rate.