Ionizing radiation detection and dosimetry

Every radiation detector works by turning the energy an ionizing particle deposits into some measurable electrical signal, but different detector types tap into that energy at different stages. A Geiger-Müller tube uses the ionization itself: an incoming particle strips electrons from the fill gas, and a strong electric field triggers a runaway avalanche that produces one large, uniform pulse per event — cheap and robust, but it can only say "something happened," not what it was or how much energy it carried. A scintillator instead converts deposited energy into flashes of light, which a photomultiplier or similar sensor turns into a pulse whose height tracks the energy of the event, making particle identification and spectroscopy possible. A semiconductor detector skips the light step entirely and collects the electron-hole pairs an event creates directly as charge, giving the best energy resolution of the three at the cost of more demanding electronics and often cooling.

None of these detectors is a perfect counter. Every detection method needs a finite amount of time to process one event — its dead time — during which a second particle arriving too soon simply isn't registered. At low count rates this is invisible, but as the true event rate climbs, the fraction of missed events grows, and the detector's reported rate silently falls further and further behind the real one unless that dead time is measured and corrected for. Different techniques also specialise: alpha spectrometry needs very short particle ranges and careful sample preparation to resolve individual decay energies, liquid scintillation counting suspends a sample directly in a scintillating cocktail to catch even the weakest beta emitters, and gamma spectrometry exploits how cleanly gamma rays deposit characteristic energies to identify which isotopes are present, not just that radiation exists.

A raw count rate, however it's produced, still isn't a dose — and a dose is what actually matters for radiation safety. Dosimetry closes that gap by converting detected events into the absorbed dose: the energy actually deposited per unit mass of tissue, in grays. Because the same count rate can correspond to very different absorbed energies depending on the radiation type and its energy, a dosimeter has to be calibrated against a known source before its readings mean anything, and that calibration is what turns a device that merely clicks into one that can state, in physically meaningful units, how much radiation exposure actually occurred.