Daly detectors are high-performance ion counters with a very high dynamic range that are used to analyze gases. They allow to measure small samples and isotopes of low occurrence and are therefore often used in molecular beam experiments in combination with time-of-flight mass spectrometry. The construction of a Daly detector enables the photomultiplier used to be separated from the high vacuum of the mass spectrometer, which prevents possible contamination and increases the life span of the PMT. The Daly detector also allows for a higher acceleration after the field-free region of a time-of-flight mass spectrometer flight tube, which can improve the sensitivity for high mass ions.
For this purpose, an aluminized cathode (a so-called “Daly-knob” or a conversion dynode) is kept at a very large negative potential in such a detector, so that ions are so strongly attracted to it that secondary electrons are emitted when they hit it. In turn, a high voltage is applied between the cathode and a scintillator, which accelerates the electrons in the direction of the scintillator, so that they trigger photons there, which are passed through a glass window into the photomultiplier tube. This detects and ultimately counts the photons that pass through the glass window. The glass window only lets through the photons and thus prevents contamination, which increases the service life of the photomultiplier to a service life of up to 5 years.