ABSTRACT
Time is a fundamental variable in almost all experimental measurements. For some experiments, time is measured directly, as in the determination of radioactive halflives and the lifetimes of excited states of atoms and molecules. Velocity measurements require the measurement of the time required for an object to travel a fixed distance. Time measurements are also used to identify events that bear some correlation to each other and as a means for reducing background noise in experiments that would otherwise be difficult or impossible to perform. Examples of time correlation measurements are so-called coincidence measurements where two or more separate events can be associated with a common originating event by virtue of their time correlation. Positron annihilation in which a positron and an electron combine to yield two gamma rays, is an example of this kind of coincidence measurement. Electron impact ionization of atoms in which an incident electron strikes an atom, ejects an electron and is simultaneously scattered is another example. In such an experiment the ejected and scattered electrons originate from the same event and their arrival at two separate detectors is correlated in time. In one of the very first coincidence experiments Bothe and Geiger [1] used the time correlation between the recoil electron and inelastically scattered x-ray photon, as recorded on a moving film strip, to identify Compton scattering of an incident x-ray and establish energy conservation on the microscopic level. An example of the use of time correlation to enhance signal-to-noise ratios can be found in experiments where there is a background signal that is uncorrelated with the signal of interest. The effect of penetrating high-energy charged particles from cosmic rays can be eliminated from a gamma ray detector by construction of an anti-coincidence shield. Because a signal from the shield will also be accompanied by a signal at the detector, these spurious signals can be eliminated.
