Coincident events

The anticoincidence and coincidence spectra are recorded separately in order to realize the increased sensitivity for noncoincident events without loss of sesitivity for truly coincident events. 

Figure 12. Schematic of the electronic system for recording simultaneously anticoincidence and coincidence events...

Chapter 2 in Watson has a good discussion of fluid flow dynamics and of the avoidance of coincidence events. 

Core stream The core stream is the stream-within-a-stream that has been injected into the center of the sheath stream and is maintained there by the hydrodynamic considerations of laminar flow at increasing velocity. The core contains the sample particles that are to be analyzed in the flow cytometer. If the sample is injected too rapidly, the core stream widens and particles may be unequally illuminated. In addition, with a wide core stream, coincidence events are more likely. 

The residence time was determined for our neutron counter by measuring the time intervals between beta start signals and neutron stop signals. With a residence half-time of 11 ms and a coincidence resolving time of 40 ms. 92 of the true coincidence events were included. The fraction of true events not detected does not influence the present results because we normalize the Pn measurements to a known Pn value measured under identical conditions. The coincidence rate was measured by a simple overlap coincidence module where the beta pulse Input was stretched to 40 ms by a gate and delay generator. To measure the accidental coincidence rate, the same beta pulse was sent to a second coincidence module and overlapped with neutron pulses which had been delayed 45 ms. After correcting each coincidence rate for deadtime effects, the difference was the true coincidence rate. 

The y-ray spectroscopic information was obtained using an array of five Ge detectors with pentagonal Nal anti-Compton shields located at 63° to the beam and three additional Ge detectors at 24°. Two-fold or higher coincident events from these detectors were used to trigger the 72 Nal detectors of the Spin Spectrometer (SS) at ORNL. [JAA83] An average Compton suppression factor of 3.5 for the Co spectrum was obtained. The Ge detectors were placed at 20.8 cm from the target. 

In principle it is also possible to get rid of the complicated function in the coincidence case by defining an area Atrue of coincident events for each selected passi value the coincident events have to be measured for all pass2 values, and the individual results must be added. According to equ. (10.64) one gets... 

Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

The curved arm-like features are barely visible in the A >i2 - 012 map plotted on a linear scale (Fig. 1.8a). This is because the number of the corresponding events shows that this sequential process forms only 17% of all the three-body explosion events. The other coincidence events forming the dense distribution in the central part correspond to the concerted or nonsequen-... 

Fig. 1.8. The Api2-6 i2 momentum correlation map for the (1,1,1) pathway of CS2 at a field intensity of 0.2PW/cm2, obtained for a all of the coincidence events, b Ekin = 20 1.5eV, c 16 0.5eV, and d 12.5 1.5eV. Solid curves drawn along the vertical and horizontal axes in each map represent the projections of the 2D distribution onto the Api2 and 12 axes, respectively. The 12 distribution expected from the geometry of the neutral CS2 is shown with a dotted line in a...

The relative yields of the respective explosion pathways can be derived from the number of coincidence events. For CH3CN, the relative yields for the n = 1 and 2 pathways with respect to the n = 0 pathway are 1.3(1) and 1.0(1), respectively. The comparable yields for the three explosion pathways show that the migration of hydrogen atoms competes with the two-body Coulomb explosion of acetonitrile in the intense laser field (0.15 PW/cm2). 

Fig. 7. Proposed direct measurement of p-a sticking [37,38]. (a) The /it created in the emitter is stopped in the source, where fusion takes place producing a++ or (pa)+. The degrader separates the two species by their stopping powers (recall that dE/dx oc Z2). Collinear coincidence of the neutron with the charged events suppresses the background. (b) Simulated Si spectrum for the coincidence events, showing a clear separation between a and pa peaks...

A small dose of a soluble fast-decay positron-emitting artificial radioisotope (produced as needed not too far from the PET instrument 6C11, 8015, 9F18 or 37Rb82) is put into human tissue (e.g., blood) the positron typically travels about 1 mm, meets an electron from within the human body, and the pair decays into two y photons of energy 0.51 MeV each, within microseconds to nanoseconds. Two spin states are possible for the positron— electron ion pair before their annihilation singlet and triplet. The annihilation rate for the triplet state depends sensitively on the electron density of the body tissue. Two y counters are set in coincidence mode, and several hundred thousand coincidence events are used to provide valuable tissue information (in addition to a CT scan). 

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. 

A time window Ate is usually set to measure coincidence events and another Atb = RAtc is set to measure the uncorrelated or random background events. If the background timing spectrum is flat, the number of true coincidence counts Nt accumulated in time T is given by... 

Coincident—Events timed to have a constant time difference. 

In a full ring system, the data are collected simultaneously by all detector pairs, whereas in partial ring systems, the detector assembly is rotated around the patient in angular increments to collect the data. In acquiring the coincidence events, three steps are followed ... 

Figure 3.1. (a) True coincidence events (b) Random coincidence events detected by two detectors connected in coincidence along the dotted line. The two 511-keV photons originated from different positron annihilations, (c) Scattered coincidence events. Two scattered photons with little loss of energy originating from two annihilation events may fall within PHA window and also within coincidence time window to be detected as a coincidence event by two detectors. 

The location of the detector pair in the detector ring is determined for each coincident event. 

As stated in Chap. 2, in a PET scanner, block detectors are cut into small detectors and coupled with four PM tubes, which are arranged in arrays of rings. Each detector is connected in coincidence to as many as N/2 detectors, where N is the number of small detectors in the ring. So which two detectors detected a coincidence event within the time window must be determined. Pulses produced in PM tubes are used to determine the locations of the two detectors (Fig. 3.2). As in scintillation cameras, the position of each detector is estimated by a weighted centroid algorithm. This algorithm estimates... 

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