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Electron-ion recombination combines a free electron with a positive ion, reducing the number of free charge carriers. The released binding energy must be carried away, for example by a photon or another particle.

In an ionization chamber, recombination before charge collection reduces the measured signal. A simplified local model has loss rate $\alpha n_e{n}_i$, with coefficient $\alpha$ depending on the species and conditions. Increasing the collection field can reduce the time available for recombination, but complete collection must be established experimentally rather than assumed from the applied voltage alone.

An ionization chamber measures ionizing radiation by collecting the electric charge that it creates in a gas. Two electrodes establish an electric field; liberated electrons and positive ions move toward opposite electrodes. In the chamber's intended operating regime the field collects the initial charge with negligible electron avalanche multiplication, distinguishing it from a proportional counter.

Varying the applied electric potential difference while holding the incident radiation fixed produces a collection curve. Too weak a field allows electron-ion recombination before collection; an approximately flat region indicates near-complete collection under those conditions. Increasing the field far enough can instead cause gas multiplication. A 2013 synchrotron radiation experiment compared free-air chambers with different electrode gaps and chopped the X-ray beam, showing that collection depends on both geometry and the temporal delivery of radiation.^[ref]

For a simple counting model, let each interacting particle create on average $n$ electron-ion pairs and let $f$ be their collected fraction. An interaction rate $r$ then gives mean electric current $I=efnr$, where $e$ is the elementary charge. This accounting explains why loss of charge can mimic a reduced incident particle flux; converting a current to absorbed dose requires additional information about energy deposition and the irradiated mass.