Scintillation semiconductor detectors

For the parallel recording of EEL spectra in STEM, linear arrays of semiconductor detectors are used. Such detectors convert the incident electrons mto photons, using additional fluorescent coatings or scintillators in the very same way as the TEM detectors described above. 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

A gamma-ray spectmm is produced nondispersively by pulse-height (multichannel) analysis using scintillation or semiconductor detectors. Resolving power, typically - 100 at 100 keV and - 700 at 2 MeV, is quite modest compared with that achievable in other spectral regions, but is sufficient to identify nucHdes unambiguously. 

Excitation of sample by bombardment with electrons, radioactive particles or white X-rays. Dispersive crystal analysers dispersing radiation at angles dependent upon energy (wavelength), detection of radiation with gas ionization or scintillation counters. Non-dispersive semiconductor detectors used in conjunction with multichannel pulse height analysers. Electron beam excitation together with scanning electron microscopes. 

Gas ionization, solid scintillation, liquid scintillation and semiconductor detectors, autoradiography. Single and multichannel pulse height analysers. Coincidence and anticoincidence circuits. 

The excellent, high-resolution y- and X-ray spectra which can be obtained from semiconductor detectors make the detectors very important in modern instruments. A typical spectrum is shown in Figure 10.11(b) which may be compared with the much broader peaks from a scintillation detector (Figure 10.11(a)). The spectra are not immune from the problem of Compton scattering (p. 461) but a good quality modem detector will have a photopeak to Compton peak ratio of 50 1 or better. Computer-aided spectrum analysis also serves to reduce the interference from the Compton effect. 

As discussed above, the measurement of characteristic y rays is very similar to the methods used in EDXRF. Early studies used a scintillation counter, typically a crystal of sodium iodide containing a small amount of thallium (Tite 1972). y ray absorption by these counters produces visible light, which is converted into an electrical pulse using a photosensitive detector. More recently semiconductor detectors have been used, either a lithium drifted germanium crystal, or, more typically, a pure ( intrinsic )... 

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. 

Successful detection of S3P-labeled molecules separated by capillary electrophoresis using the above detection schemes, in which a sensor was positioned external to the separation channel, was made possible by several factors. These included (1) the large energy associated with 0 decay of S3P (1.7 MeV), (2) the high sensitivity and small size of commercially available semiconductor detectors, (3) the availability of efficient solid scintillator materials and sensitive photomultiplier tubes, (4) the short lengths of fused silica (capillary wall thickness) and aqueous electrolyte through which the radiation must pass before striking the detector, and (5) the relatively short half-life of S3P (14.3 days). 

C-Carbon is now utilized more often in accordance with the development of new labeling techniques which involve so-call i natural labeling, i.e. replaces by isotopic exchange in molecules of the proteins to be labeled. The application of is more convenient than tritium labeling. Radiocarbon may be measure by various gas flow counters and semiconductor detectors, but liquid scintillation counting is still preferred. 

Kind of radiation Ionization chambers Proportional counters Geiger-Muller counters Scintillation detectors Semiconductor detectors... 

Figure 7.16. y-ray spectra of Co taken with a Nal (Tl) scintillation detector and a Ge (Li) semiconductor detector. 

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. 

Semiconductor detectors, made from single crystals of very pure germanium or silicon, are the highest performance detector type. The superior resolution of these detectors has revolutionized data-gathering for X-ray and gamma-ray measurements. The comparison of the pulse resolving ability of the three types of X-ray detectors scintillator, gas proportional and Si(Li) is shown in Fig. 5.18. 

Like scintillation detectors, semiconductor detectors are usually used in gamma spectrometer set-ups to identify radionuclides and determine their activities in a sample. A semiconductor detector is much more expensive and somewhat more troublesome to operate than a scintillation detector, but it can distinguish much better between different radiation energies and is better for nuclide identification. 

Array detectors scintillators coupled to very large arrays of semiconductor detectors that have largely replaced film in medical diagnostics... 

The instrumentation used to measure y-rays is generally a semiconductor detector (Nal(Tl) crystal) associated to a multi-channel analyser to cover energies over the range from about 60 keV to 3.0 MeV. The functioning is similar to that of liquid scintillators. 

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