Coincidence method
In particle physics, the coincidence method (or coincidence technique) is an experimental design through which particle detectors register two or more simultaneous measurements of a particular event through different interaction channels.
[1] The higher the rate of interactions or reaction products that can be measured in coincidence, the harder it is to justify such an event occurred from background flux and the higher the experiment's efficiency.
As an example, the Cowan and Reines’ neutrino experiment (1956) used a design that featured a four-fold coincidence technique.
[2] Particle detectors that rely on measurements of coincidence are often referred to as q-fold, where q is the number of channel measurements which must be triggered to affirm the desired interaction took place.
[3] Anti-coincidence counters or "vetos" are often used to filter common backgrounds, such as cosmic rays, from interacting with the primary detection medium.
For instance, such a veto is used in the gamma ray observatory COS-B.
[4] Coincidence designs are an essential technique for increasing confidence in signals and reducing random background within a wide range of particle detectors.
Common backgrounds include radioactive decay products (beta, alpha, and gamma radiation) and cosmic rays (protons, air showers).
compared to the rate at which all suspected signal triggers are measured
can also be defined by the product of all q channels of coincidence times the raw count of particles available for detection
:[7] Therefore, the ability of a detector to successfully confirm signals in coincidence is directly proportional to its efficiency.
A wide variety of operational particle detectors today contain some identifiable form of coincidence or anti-coincidence design.
In 1924, physicists Walther Bothe and Hans Geiger used the coincidence method to probe the Compton scattering of gamma rays and x-rays, a phenomenon whose quantum mechanical nature (see particle-wave duality) with regard to energy conservation was ambiguous at the time.
This setup included a coincidence circuit which measured the process to
= 1 ms and with an accuracy of 0.1 ms.[8] In 1954, Bothe won the Nobel Prize in Physics for this work.
In an attempt to build on the theoretical concept of a neutrino by providing empirical evidence for its existence, physicists Clyde L. Cowan and Frederik Reines constructed an experiment outside of a nuclear reactor expected to emit neutrinos.
Cowan and Reines decided to construct a four-fold coincidence experiment, for while the proximity to a nuclear reactor provided ample flux of neutrinos, it also created intense backgrounds (neutrons, gamma rays, etc.).
represents an antineutrino) released positions which interacted with one of two adjacent tanks of liquid scintillator.
The resulting photons could then be measured by photomultiplier tubes installed on the scintillator tanks.
While this interaction occurs, the neutron product from the original reaction follows a random walk through the cadmium-doped water until it is absorbed in a cadmium atom.
This process then produces more gamma rays, which are subsequently detected.
The overall system therefore includes two pairs of simultaneously recorded events, the correlation of which in time provides strong evidence for an interaction involving a neutrino.
[9] The invention of the coincidence method enlightened new techniques for measuring high-energy cosmic rays.
On such experiment, COS-B, launched in 1975 and featured an anti-coincidence veto for charged particles, as well as three scintillation detectors to measure electron cascades caused by incoming gamma radiation.
Therefore, gamma ray interactions could be measured with three-fold coincidence, after having passed a charged particle veto (see Anti-Coincidence).
[14] For instance, anti-coincidence counters can be used to shield charged particles when an experiment is explicitly searching for neutral particles,[15] as in the SuperKamiokande neutrino experiment.
is the rate of all detected, but uncorrelated events across multiple channels.
