Charged-particle detectors

The choice of detector for charged particles (electrons or positive/negative ions) depends on the design of a particular instrument and the type of experiment. Basically, the detector generates a signal from incident electrons or ions, either by directly inducing a current or by generating secondary electrons, which are further amplified. The three most common types are described below. 

A Faraday cup is a metal (conducting) cup that is placed in the path of a charged particle beam and recaptures secondary particles. Basically, when charged particles impinge on the metal surface, the metal will become charged (ions are neutralized in the process). The metal can then be discharged to measure a small current, equivalent to the number of deposited electrons/ions. 

A Faraday cup, or cylinder electrode detector, is very simple in construction. The basic principle is that 

Electron multiplier tubes are similar in design to photomultiplier tubes. They consist of a primary cathode and a series of biased dynodes that eject secondary electrons. Therefore, any incident charged particle induces a multiplied electron current. A channeltron is a hom-shaped continuous dynode stmcture that is coated on the inside with an electron-emissive material. Any charged particle, but also high-energy U Vor X-ray photons, striking the channeltron creates secondary electrons that have an avalanche effect to create the final current. 

A microchannel plate (MCP) is an array of 10 -lO miniature electron multipliers, oriented parallel to one another. Standard devices have channel diameters in the range 10-100 pm, and their length-to-diameter ratio is of the order 40-100. Typically, the channel axes are oriented at a small angle ( 8°) to the MCP input surface. 

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Figure 2. (a) Reflection TOF mass spectrometer, (b) Depicts the electrostatic potentials. With a judicious selection of potential, the daughter ions arising from metastable decay arrive at the detector prior to the parent ions which have higher kinetic energy. MCP denotes a microchannel plate charged particle detector, (a) Taken with permission from ref. 22 (b) Taken with permission from ref. 19. 

The major components of the system include a krypton gas injector assembly, a radiation detector assembly, detector electronics and a data processor. The heart of the system is the radiotracer detector assembly which uses semiconductor charged particle detectors to measure the relative concn of tracer gas in the ullage. Krypton-85 is considered the safest radioisotope for such use since there is virtually no bodily retention of this gas... 

The two layers of charged particle detectors each consist of sixteen doublesided silicon microstrip detector modules 162 mm long, 19 mm wide, and 380 Uva thick. These are constructed from two 81 mm long double sided detectors (n-bulk), produced by SINTEF, Norway. Each detector has 128 p+ readout strips with two floating intermediate strips (for the measurement of < ) and 128 n+ pads on the opposite side (for the measurement of z). The floating intermediate strips... 

A modem version of the charged particle detector is called PIPS, an acronym for Passivated Implanted Planar Silicon. This detector employs implanted rather than surface barrier contacts and is therefore more mgged and reliable than the Silicon Surface Barrier (SSB) detector it replaces. 

For a-spectrometry, silicon charged-particle detectors (surface-barrier detectors) have been used. They can be used over an extensive range of energies (20 kV-200 MeV). The inherent resolution of these surface-barrier detectors is surpassed only by that of magnetic spectrometers. 

Semiconductor charged particle detector Energy measurement -isotope ratios. Very convenient-capable of high resolution. Requires low geometry for high resolution. Detector may become irreversibly contaminated by volatile radioactivity. 

For the calibration of charged-particle detectors, emission characteristics of some convenient conversion electron and alpha decay sources are presented in O Tables 56.5 and O 56.6, respectively. 

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