Coincidence-Anticoincidence Measurements

There are times in radiation measurements when it is desirable or necessary to discard the pulses due to certain types of radiation and accept only the pulses from a single type of particle or from a particle or particles coming from a specific direction. Here are two examples of such measurements. 

Detection of pair-production events. When pair production occurs, two 

511-MeV gammas are emitted back-to-back. To insure that only annihilation photon are counted, two detectors are placed 180° apart, and only events that register simultaneously (coincident events) in both detectors are recorded. 

Detection of internal conversion electrons. Radioisotopes emitting internal conversion (IC) electrons also emit gammas and X-rays. The use of a single detector to count electrons will record not only IC electrons but also Compton electrons produced in the detector by the gammas. To eliminate the Compton electrons, one can utilize the X-rays that are emitted simultaneously with the IC electrons. Thus, a second detector is added for X-rays and the counting system 

In theory, a true coincidence is the result of the arrival of two pulses at exactly the same time. In practice, this exact coincidence seldom occurs, and for this reason a coincidence unit is designed to register as a coincident event those pulses arriving within a finite but short time interval r. The interval t. 

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Notice that the rate at which the pulse rises (risetime) is determined by the decay time T. In certain measurements, e.g., coincidence-anticoincidence measurements (Chap. 10), the timing characteristics of the pulse are extremely important. 

For a successful coincidence or anticoincidence measurement, the detector signals should not be delayed by any factors other than the time of arrival of the particles at the detector. If it is known that it takes longer to generate the signal... 

For certain measurements, like coincidence-anticoincidence counting or experiments involving accelerators, the time resolution of the signal is also important, in addition to energy resolution. For timing purposes, it is essential to have pulses with constant risetime. 

An application of anticoincidence circuits is the anti-Compton spectrometer. The Compton continuum in y spectra can be reduced relative to the photopeaks by placing the Ge detector inside a second detector, usually a scintillation detector, connected in anticoincidence, so that only pulses in the central detector that are not coincident with those in the outer detector are recorded. Anti-Compton spectrometers are very useful for measurement of y rays of very high energy. 

If samples of very low activity are to be measured, the contribution of the background to the counting rate and hence the error of the measurement are relatively high. The influence of the background can be reduced by intensiflcation of the detector shielding and by coincidence or anticoincidence circuits. 

Some other scintillation materials, such as cesium iodide and bismuth ger-manate, have characteristics that are less favorable than Nal(Tl) for general use, but recommend them for some special measurements. For example, Csl and Nal(Tl) can be combined for coincidence or anticoincidence counting by distinguishing between output from the two detectors by their pulse shapes. 

From 3 to 12 Mev, measurements of the total cross sections in a large number of elements have been made by Nereson and Darden using the neutrons emitted by a fast neutron reactor as a source, and a special energy-selective detector. In that instrument, protons projected in the forward direction from a polythene foil were measured by an ionisation chamber connected in coincidence with a proportional counter located between it and the foil, and in anticoincidence with another counter beyond it. The energy of the protons was measured by the size of the pulses they produced in the chamber the energy resolution was about 10%. 

Figure 6. Left Panel The spectrum of the self-activity of the LaBrs Ce detector (from ref [24]). Right Panel Self-activity spectra of LaBrs. Ce measured in coincidence (grey Une) and anticoincidence (black line) with an anti-Compton shield. The spectra are normalized on the acquisition time (from ref [24])...

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