Detector interactions within

Improved optics sources, more robust fluorescent probes, molecular tags that are X-ray excitable, and more sensitive detectors will contribute much to the ability to image molecular interactions within cells at desired resolutions. 

When a 511-keV photon interacts within the detector and is absorbed in the crystal, light photons are produced, which strike the photocathode of the PM tube. A pulse is generated at the end of the PM tube and amplified by an amplifier the energy and the spatial position of the photon are determined and finally a count is recorded. When two such events are detected by two detectors in the time window, a coincidence event is recorded. 

Silver-activated zinc sulfide [ZnS(Ag)] has been used since radioactivity was first measured to detect alpha particles. It is relatively insensitive to electrons and gamma rays because it is not transparent to its own radiation. Radiation interactions within the detector are not recorded only its surface, where alpha particles interact, emits scintillations. The ZnS is doped with silver to shift its scintillations to a longer wavelength for better PMT response. 

The charge collection time is quite short— typically lying in the range of 25 to 100 ns, depending on detector bias voltage, detector thickness, and the position of the photon interaction within the detector. The detector is operated at 77 K to lower the lithium mobility in the crystal and to reduce the noise that would be caused by excessive diode reverse leakage current at higher temperatures. 

Figure 2.10 Additional possibilities for interaction within a real detector...

Atmospheric and albedo neutrons can interact within detectors at balloon altitudes and in low-Earth orbit. Target nuclei can emit secondary particles or gamma rays that contribute to the background counting rate of the... 

The muon spin relaxation technique uses the implantation and subsequent decay of muons, n+, in matter. The muon has a polarized spin of 1/2 [22]. When implanted, the muons interact with the local magnetic field and decay (lifetime = 2.2 ps) by emitting a positron preferentially in the direction of polarization. Adequately positioned detectors are then used to determine the asymmetry of this decay as a function of time, A t). This function is thus dependant on the distribution of internal magnetic fields within a... 

Module 3, Column and Mobile Phase Design (CMP). This is the core module for ECAT. It can currently specify i) analytical column and mobile phase constituents for reverse phase chromatography of common classes of organic molecules ii) reverse phase, ion exchange phase and hydrophobic interaction chromatography of proteins and peptides iii) a limited set of specialty classes of molecules best treated by straight phase chromatography (e.g., mono- and disaccharides). The rules for selection of the HPLC detector are under development within Module 3. Some of the rules for detector mobile phase compatibility are already encoded. A set of rules for detector selection is ready but not yet encoded. 

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