Detection-coincidence counting

For the usual interpretation, once the photon is detected at D2, nothing more from 2 remains in the interferometer because of the collapse of the wave-function. Therefore at detector Dj only the wave 2 from the usual source arrives. Since the path of this wave does not cross the phase shifting device and, even more, is only one wave, the coincidence count does not depend on the phase. 

Many experiments (see Section I.B) require the energy analysis and detection of two or more particles with time correlation in other words, coincidence counting techniques must be used. Coincidence methods have long been used in nuclear physics because of the convenient fast detectors that have long been available. The more recent availability of fast, high-gain electron multipliers has created the possibility of coincidence measurements in electron spectroscopy. Various aspects of coincidence measurements have been discussed elsewhere.100 102... 

We report here the design and characterization of three simple, on-line radioisotope detectors for capillary electrophoresis. The first detector utilizes a commercially available semiconductor device responding directly to 7 rays or particles that pass through the walls of the fused silica separation channel. A similar semiconductor detector for 7-emitting radiopharmaceuticals separated by HPLC was reported by Needham and Delaney (XI). The second detector utilizes a commercially available plastic scintillator material that completely surrounds (360 ) the detection region of the separation channel. Light emitted by the plastic scintillator is collected and focused onto the photocathode of a cooled photomultiplier tube. Alternatively, a third detection scheme utilizes a disk fashioned from commercially available plastic scintillator material positioned between two-room temperature photomultiplier tubes operated in the coincidence counting mode. 

Figure 3. Exploded diagram showing the design of the coincidence radioisotope detection scheme. The fused-silica capillary is exposed to a 2-mm length of plastic scintillator material located between two photomultiplier tubes operated in the coincidence counting mode.

The 4jt P-y coincidence counting is an improved version of the P y coincidence counting method. Here, a 4ji p gas flow type proportional counter is usually used as the P-detector, and a Nal(Tl) scintillation counter located near the 4tc P counter wall is employed to detect the y-rays. 

A disadvantage of the coincidence scintillation cameras is that they have low sensitivity due to low detection efficiency of Nal(Tl) crystal for 511-keV photons, which results in a longer acquisition time. To improve the sensitivity, thicker detectors of sizes 1.6-2.5 cm have been used in some cameras, but even then, coincidence photopeak efficiency is only 3-4%. This increase in crystal thickness, however, compromises the spatial resolution of the system in SPECT mode. Fast electronics and pulse shaping are implemented in modern systems to improve the sensitivity. Also, there is a significant camera dead time and pulse pileups due to relatively increased single count rates in the absence of a collimator in PET mode. Low coincidence count rates due to low... 

The very high detection efficiency of this apparatus, which normally exceeds 50% as directly determined from the coincidence counting statistics, invites attempts to detect other particles in coincidence in order to learn more... 

The major positron group and the gamma ray can be measured by coincidence counting (see above for °Co). In addition to the noted gamma ray, 511-keV annihilation radiation is detected by the gamma-ray detector it is generated at the rate of two per positron, i.e., 1.81 per disintegration. 

In the method of delayed coincidences, the coincidence counting rate between detectors responding selectively to genetically related particles, e.g., a jS- or a-par-ticle and a y-ray or an electron or between y-rays, is plotted as a function of a time delay inserted in the electronic circuits used to detect one of them. The development of this method and of improved electronic techniques has led to the use of resolution times of between 10" and 10 sec. We shall not discuss these methods here, for they have been very adequately reported by Bell [i9], 2ff]. Further analysis of the experimental data has made it possible to measure lifetimes of the order of 10 sec. The limit to the method seems to be determined by the scintillating properties of the phosphor and its physical size. 

The He neutron counters detect all neutrons arising from both (a, n) and fission reactions. Sophisticated pulse processing electronics, called neutron coincidence counting circuits (shift register), measure the number of neutrons that are detected within a predefined time interval (gate width), and differentiate between time correlated (coincidence) neutrons emitted from the fission events and single neutrons created as a result of a-particle interactions. The measured coincident neutrons (doubles) are proportional to the mass of the even-even Pu isotopes ( PUefFecdve = 2.43 Pu -I- Pu -I- 1.69 Pu). 

Normal coincidence counting techniques rely on the detection of two coincident neutrons (doubles) and making an assumption based either on the multiplication or on the (a, n) neutron rate. These two analysis methods - passive calibration and known-alpha - require... 

The surface system mentioned above in the context of optimization also incorporated a veto detector. Hurtado et al. (2006) examined four different gating scenarios to allow the plastic scintillator veto detector to prevent background counts from being recorded. Two of those, in effect, involved detecting coincidences between veto counts and HPGe counts while the other two used a... 

The photomultipliers feed the coincidence-counting electronics, that includes a time-to-amplitude converter and a multichannel analyzer, yielding the time-delay spectrum of the two-photon detections (Fig. 11). This spectrum involves a flat background due to accidental coincidences (i.e. between photons emitted by different atoms). True coincidences yield a peak around the null-delay, with an exponential decrease (time constant t ). 

A method claimed to be applicable for a minimum detectable activity of less than 0.05 pCI/i of sample at the time of counting has been reported (86). Carrier was mixed with the milk sample and the mixture was heated to about 75°. Iodide was collected on an anion exchange resin (resin not specified) and then eluted as lodate by means of hypochlorite. lodate was converted to free Iodine by treatment with hydroxylamlne hydrochloride and sodium nitrite. An l2"I cycle was performed and the Iodine extracted Into toluene in which medium It was determined photometrically for chemical yield. 2-Methyl butene was added to the toluene solution and the liquid sample was mixed with a toluene-based liquid scintillation counting solution and 8-gated y coincidence counting was carried out. 

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