Interactions within the Detector

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. 

Self-absorption Absorption of gamma photons within the sample before they can interact with the detector. 

Radiation detection equipment operates on the principle that the radiation interacts with gases or crystal within the detector to produce ionization that the instrument electronically converts to a reading. Instruments normally detect only certain types and certain levels of radiation. Most of these devices are not precision instruments, and the error rate can he up to 20%. For this reason, conservative estimates of exposure and dose should be used to ensure that emergency limits are not exceeded. 

The reader may recall that the gamma-ray interactions produce fast electrons which scatter within the detector. As they decelerate, a proportion of their energy will be emitted as bremsstrahlung rather than used to create electron-hole pairs. We need not worry about this as it is already taken into account when the various absorption and attenuation coefficients are calculated.)... 

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... 

---