Radiation from an @[email protected] @[email protected] ionizes certain species in the @[email protected] A potential difference is applied and the resulting @[email protected] current is detected. The detector is only useful for substances with @[email protected] potentials below about \(11\ \text{eV}\). This makes it quite useful for detecting one component of a combined @[email protected] when the other component, for instance nitrogen, has a high @[email protected] The detector has a small linear dynamic range and is capable of detecting substances below \(1\ \text{ppmv}\).