Photomultiplier tube particle detector

This eqnipment is used when the intensity of the scattered light is high and an analog ontpnt from the photomultiplier tube (the detector) is available. The power spectrum, P(g,(o), of the output signal is extracted. For example, in the case of translational diffnsion of monodisperse spherical particles ... 

When the energy of the charged particle beam is too large to easily stop the beam in a Faraday cup, the beam intensity is frequently monitored by a secondary ionization chamber. These ion chambers have thin entrance and exit windows and measure the differential energy loss when the beam traverses them. They must be calibrated to give absolute beam intensities. If the charged particle beam intensity is very low (<106 particles/s), then individual particles can be counted in a plastic scintillator detector mounted on a photomultiplier tube. 

Radioisotope detection of P, 14C, and Tc was reported by Kaniansky et al. (7,8) for isotachophoresis. In their work, isotachophoretic separations were performed using fluorinated ethylene-propylene copolymer capillary tubing (300 pm internal diameter) and either a Geiger-Mueller tube or a plastic scintillator/photomultiplier tube combination to detect emitted fi particles. One of their reported detection schemes involved passing the radiolabeled sample components directly through a plastic scintillator. Detector efficiency for 14C-labeled molecules was reported to be 13-15%, and a minimum detection limit of 0.44 nCi was reported for a 212 nL cell volume. 

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. 

Many types of plastic scintillators are commercially available and find applications in fast timing, charged particle or neutron detection, as well as in cases where the rugged nature of the plastic (compared to Nal), or very large detector sizes, are appropriate. Sub-nanosecond rise times are achieved with plastic detectors coupled to fast photomultiplier tubes, and these assemblies are ideal for fast timing work. 

The phoswich detector is used for the detection of low-level radiation in the presence of considerable background. It consists of two different scintillators coupled together and mounted on a single photomultiplier tube. By utilizing the difference in the decay constants of the two phosphors, differentiation between events taking place in the two detectors is possible. The combination of crystals used depends on the types of particles present in the radiation field under investigation. ... 

Figure 7.4 Definition of time response characteristics of an electron multiplier detector. The initiating event is here assumed to be an effectively instantaneous ( delta function ) flash of light, but could equally well be a single charged particle. Reproduced from Photomultiplier Tubes Basics and Applications (3rd Edn), Hamamatsu Corporation, with permission.

The most widely used modern scintillation detector consists of a transparent crystal of sodium iodide that has been activated by the introduction of 0.2% thallium iodide. Often, the crystal is shaped as a cylinder that is 3 to 4 in. in each dimension one of the plane surfaces then faces the cathode of a photomultiplier tube. As the incoming radiation passes through the crystal, its energy is first lost to the scintillator, this energy is subsequently released in the form of photons of fluorescence radiation. Several thousand photons with a wavelength of about 400 nm are produced by each primary particle or photon over a period of about 0.25 ps, which is the dead time. The dead time of a scintillation counter is thus significantly smaller than the dead time of a gas-filled detector. 

Scintillation methods offer the possibility of high-efficiency detectors with a more rapid time response than the BF3 counter. As mentioned in the previous section, the basis of the scintillation detector is the conversion, in a suitable crystal, such as thallium-activated sodium iodide, Nal(Tl), of the kinetic energy of the charged particle to light, which can be amplified by a photomultiplier tube to provide an electrical pulse. Again, the neutron has to interact to produce either a charged particle or a 7 ray, the latter of which may in turn interact to produce ionizing particles. 

The total scattered intensity is die photomultiplier tube is the result of the superposition of all die individual scattered waves. The Brownian motion ofthe particles causes the relativephases of the light scattered from differentparticles to vary which in turn cause the intensity at the detector to fluctuate in time. [16]... 

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