Electronic anticoincidence

Method of eliminating background events in particle physics

Electronic anticoincidence is a method (and its associated hardware) widely used to suppress unwanted, "background" events in high energy physics, experimental particle physics, gamma ray spectroscopy, gamma ray astronomy, experimental nuclear physics, and related fields. In the typical case, a desired high energy interaction or event occurs and is detected by some kind of detector, creating a fast electronic pulse in the associated nuclear electronics. But the desired events are mixed up with a significant number of other events, produced by other particles or processes, which create indistinguishable events in the detector. Very often it is possible to arrange other physical photon or particle detectors to intercept the unwanted background events, producing essentially simultaneous pulses that can be used with fast electronics to reject the unwanted background.

Early experimenters in X ray and gamma ray astronomy found that their detectors, flown on balloons or sounding rockets, were corrupted by the large fluxes of high energy photon and cosmic ray charged particle events. Gamma rays, in particular, could be collimated by surrounding the detectors with heavy shielding materials made of lead or other such elements, but it was quickly discovered that the high fluxes of very penetrating high energy radiation present in the near space environment created showers of secondary particles that could not be stopped by reasonable shielding masses. To solve this problem, detectors operating above 10 or 100 keV were often surrounded by an active anticoincidence shield made of some other detector, which could be used to reject the unwanted background events. An early example of such a system, first proposed by Kenneth John Frost in 1962, is shown in the figure. It has an active CsI(Tl) scintillation shield around the X ray/gamma ray detector, also of CsI(Tl), with the two connected in electronic anticoincidence to reject unwanted charged particle events and to provide the required angular collimation. Plastic scintillators are often used to reject charged particles, while thicker CsI, bismuth germanate ("BGO"), or other active shielding materials are used to detect and veto gamma ray events of non cosmic origin. A typical configuration might have a NaI scintillator almost completely surrounded by a thick CsI anticoincidence shield, with a hole or holes to allow the desired gamma rays to enter from the cosmic source under study. A plastic scintillator may be used across the front which is reasonably transparent to gamma rays, but efficiently rejects the high fluxes of cosmic ray protons present in space.

In gamma ray spectroscopy, Compton suppression is a technique that improves the signal by removing data that have been corrupted by the incident gamma ray getting Compton scattered out of the target before depositing all of its energy. The goal is to minimize the Compton edge feature in the data. The high resolution solid state germanium (HpGe) detectors used in gamma ray spectroscopy are very small, typically only a few centimeters in diameter and with thickness ranging from a few centimeters to a few millimeters. Since the detectors are so small, it is likely that the gamma ray will Compton scatter out of the detector before it deposits all of its energy. In this case, the energy reading by the data acquisition system will come up short: the detector records an energy which is only a fraction of the energy of the incident gamma ray. The desired events are typically accompanied by unwanted background processes that must be suppressed by enormous factors, ranging from thousands to many billions, to permit the desired signals to be detected and studied. Extreme examples of these kinds of experiments may be found at the Large Hadron Collider, where the enormous Atlas and CMS detectors must reject huge numbers of background events at very high rates, to isolate the very rare events being sought.