How does a Geiger counter work?

• A. measures primary ionization directly
• B. measures secondary ionization by using energy gathered to form secondary electrons
• C. uses electromagnets to deflect particles
• D. records photons as pulses of light

Answer: (B)

Geiger counters detect ionizing radiation by using a gas-filled tube kept at a high electric field. When radiation enters, it ionizes gas molecules, creating electron–ion pairs. The strong field then accelerates the freed electrons, causing them to collide with other gas molecules and create additional ionizations—a cascade called a Townsend avalanche. This rapid, self-amplifying process produces a measurable current pulse, which the electronics count as one radiation event. The device doesn’t measure how much energy was deposited by the radiation; it simply registers each ionization event as a pulse.

That’s why this option is correct: it describes counting the secondary ionization (the avalanche) that results from the initial interaction, using the energy to drive the chain reaction in the gas to form detectable secondary electrons. The other statements don’t fit how a Geiger counter works: it doesn’t directly measure primary ionization as a quantified value, it doesn’t rely on electromagnets to deflect particles, and it doesn’t record photons as light pulses.