Detector electric field variations

The pulse risetime is essentially equal to the collection time. For the detector shown in Fig. 12.39a, the risetime will vary between 60 and 120 ns. For other detector geometries the variation in risetime is greater because the electrons and holes, following the electric field lines, may travel distances larger than the thickness of the detector core (Fig. 12.396). The variation in risetime for the detector of Fig. 12.396 will be between 60 and 200 ns. The distribution of pulse risetimes for commercial detectors is a bell-type curve, not exactly Gaussian, with a FWHM of less than 5 ns. 

A variation on HPLC-CDS is based on magneto-optical rotation. In a longitudinal electric field, almost all molecules show optical activity [134]. The electrical field orients the molecules through their permanent dipole moments and/ or through any anisotropic electrical polarizability. Kawazumi et al. [135] used this principle to build a detector for HPLC. Although the device was complicated, and prone to respond to minute changes in the refractive index or temperature of the mobile phase, it showed promise as a universal detector. This universality would only apply to detectability. Compounds with strong dipole moments and those that are easily polarizable would have more intense responses than compounds that do not. 

The basic principles of thermal ionization mass spectrometry (TIMS) operation were described in Chapter 1 a drop of the liquid sample is deposited on a filament, a low electric current heats the filament, and the solution is evaporated to dryness. The filament current (temperature) is then raised and atoms of the sample are emitted and ionized (either by the same filament or by a second electron emitting filament). The ions are accelerated by an electric field, pass through an electrostatic analyzer (ESA) that focuses the ion beam before it enters a magnetic field that deflects the ions into a curved pathway (in some devices, the ions enter the magnetic field before the ESA—referred to as reverse geometry). Heavy and light ions are deflected by the field at different curvatures that depend on their mass-to-charge ratio. A detector at the end of the ion path measures the ion current (or counts the ion pulses). There are many variations of ion sources, ion separation devices, and detectors that are used in TIMS instruments and specifically adapted for ultratrace or particle analysis. 

Semiconductor materials are commonly employed for optical detection mainly because the semiconductor detectors are usually small, require a low bias voltage, and can be easily assembled with the rest of the receiver. In semiconductors, photons interact with electrons and are absorbed in processes whereby the electrons make a transition from a lower to a higher energy state. These electrons are detected in an external circuit. The absorption is dependent on the photon energy and each material has its characteristic absorption spectrum. The absorption spectrum can be sensitive to electric field and temperature variations. 

The variation in electric-field strength across a detector depends upon the shape of the detector and the type of intrinsic region. For example, in a p-type high purity planar detector at a point a distance x from the p-h (negative) contact, the electric field, E x), is given by the following relationship ... 

PGT do not agree that the weak field regions of a non-bulletized detector are a problem. In Section 1.15.1 of their Nuclear Product Catalog, they state that The variation in the electric field in the crystal, which such a shape supposedly avoids, is minor They go on to explain that, what they call, the straight-across design of their detectors makes for more secure clamping of the detector inside the end cap, which minimizes microphonics resulting in lower resolution. 

Figure 6.69 gives an example for an optical current sensor. The light path is wound around a current-carrying conductor equidirectionally with the azimuthal magnetic field of the current. The rotation of the plane of the electric vector is not detectable on its own and is converted to light intensity variations by a polarizer/analyser combination. A photo diode is used as a light intensity detector. The optical sensor itself is installed in the - e - compartment, the electronics shall be protected in an adequate type of protection, e.g. in a small flameproof - d - enclosure or in encapsulation - m -. In the special case of an energy distribution system with combined - e - and - d - compartments, the optical fibres may enter the d-compartment to the electronics inside via bushings complying with d -standards EN 50018 or IEC 60079-1 respectively (Fig. 6.70). The evacuation of the sensors into the e-compart-ment results in additional available space in the more expensive d-compart-ment, compared with increased safety - e -. ...

Hygroscopic materials which, when moist, become electrically conductive may cause false alarms due to variations in moisture content (and therefore in conductivity) on passing the metal detector. The most reliable protection against tramp metal is provided by the combination metal detector-magnetic drum— metal detector (Fig. 25). In this arrangement the first metal detector operates the switch-on/switch-off of the drum separator, whose magnetic field therefore is activated only when metal is detected on the conveyor. Any non-magnetic metal that passes the drum will produce a response from the second metal detector. 

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