Coincidence circuit

Comcidence experiments have been connnon in nuclear physics since the 1930s.The widely used coincidence circuit of Rossi [9] allowed experimenters to detennine, within tire resolution time of the electronics of the day, whether two events were coincident in time. The early circuits were capable of submicrosecond resolution, but lacked the flexibility of today s equipment. The most important distinction between modem comcidence methods and those of the earlier days is the availability of semiconductor memories that allow one to now record precisely the time relations between all particles detected in an experiment. We shall see the importance of tliis in the evaluation of the statistical uncertainty of the results. 

A second way to help resolve the thermal noise problem is to use two photomultiplier tubes for detection of scintillations. Each flash of light that is detected by the photomultiplier tubes is fed into a coincidence circuit A coincidence circuit counts only the flashes that arrive simultaneously at the two photodetectors. Electrical pulses that are the result of simultaneous random emission (thermal noise) in the individual tubes are very unlikely. A schematic diagram of a typical scintillation counter with coincidence circuitry is shown in Figure 6.2. 

Absolute activities of radionuclides may also be determined by coincidence measurements, provided that the decay scheme is relatively simple, e.g. only one f transition followed immediately by emission of one or more y-ray photons. The principles of the use of coincidence circuits are discussed in the following section. 

Many nuclear processes occui one after the other within a very short time of the order of picoseconds or less - for instance a. or fi decay followed by y-ray emission or emission of a cascade of y rays. The events are practically coincident, and for many purposes it is of interest to know whether two particles or photons are emitted practically at the same time or not. For detection and measurement of coincident events two detectors and a coincidence circuit are used. The detectors are chosen according to the coincidences to be measured, e.g. ot-y, fi y, y-y, X-y, y5-e or other types of coincidences, and the coincidence circuit records only events occurring within a given short time interval. Scintillation counters and semiconductor detectors are commonly used for these measurements. 

Coincidence circuits not only provide fidelity, but also provide a mechanism for signaling cross talk. Agonists that provide one of the input signals required by the... 

In PET scanners with block detectors, each detector pair is not connected by coincidence circuitry for practical difficulty, so the detectors are grouped together into banks or sets, which are then connected in coincidence opposite to each other. Essentially each detector in a set is connected by a coincidence circuit with a time window to a set of opposite detectors (both in plane and... 

A common method of correction for random events is to employ two coincidence circuits - one with the standard time window (e.g., 6 ns for LSO) and another with a delayed time window (say, 50-56 ns) of the same energy window. The counts in the standard time window include both the randoms plus trues, whereas the delayed time window contains only the randoms. For a given source, the random events in both time windows are the same within statistical variations. Delayed window counts are subtracted from the standard window counts to obtain the true coincidence counts, which are essentially free of any systematic errors associated with the PET scanner because they cancel by subtraction. 

Liquid scintillation counters use two photomultiplier tubes with a coincidence circuit that prevents counting of events seen by only one of the tubes. In this way, false counts due to chemiluminescence and noise in the phototube are greatly reduced. Quenching is a problem in all liquid scintillation counters. Quenching is any process which reduces the efficiency of the scintillation counting process, where efficiency is defined as... 

Positive mesons can be identified by observing their delayed decay products (again within the limit of the range being shorter than the mean free path for nuclear interactions). The delay can be measured directly by photographing a fast oscilloscope trace, or one can do it electronically by a series of delayed coincidence circuits. 

Qhe design of the seismoscopes has a non-fail-safe characteristic which makes the two-out-of-two coincidence circuit very questionable from a reactor safety standpoint Power Failure Detection... 

The conditions for the emission of a chemiluminescence photon during any small time interval A and the Independence of the reaction probability k of prior events, i.e. dependence only on the nature of M and the constant reaction conditions, are precisely the conditions for a Poisson distribution in time. Therefore, at any time during the course of a chemiluminescent reaction, the probability of finding two or more photons emitted within the resolving time A of a coincidence circuit will be given by... 

This probability is independent of the disintegration rate of the source and of the resolving time of the coincidence circuit, provided the latter is long compared with the fluorescence lifetime of the fluor. Using the same value for a as the previous section, for N = 10, - 0.45, while for N = 30, P... 

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