Avalanche effect

An interesting variant of a CEMS counter is the parallel-plate avalanche counter (PPAC) [18, 19], which carries the sample between parallel electrodes made of Perspex coated with graphite (Fig. 3.8, left). A counter gas is used to amplify the low conversion-electron current emitted by the sample, with an avalanche effect taking place between the plates. Compared with the CEMS proportional counters, PPAC gives a larger signal-to-background ratio, faster time response, simpler construction, and better performance at low temperatures. 

These electrons strike other gas molecules releasing more electrons. The total number of electrons generated in this manner is typically several million times more than were emitted from the cathode. This current of electron flow is known as the avalanche effect. 

It s like an avalanche effect. You get all these assaults and you get sick, but you don t believe this is happening. Then there s one... 

Nanocrystals and nanowires are utilized in a new generation of solar collectors (a nanometer is one billionth of a meter). In conventional solar cells, at the P-N junction one photon splits one electron from its "hole companion" as it travels to the electron-capturing electrode. If solar collectors are made of semiconducting nanocrystals that disperse the light, according to TU Delft s professor Laurens Siebbeles, an avalanche effect results and one photon can release two or three electrons, because this effect maximizes photon absorption while minimizing electron-hole recombination. This effect of the photon-scattering nanoparticles substantially increases cell efficiency. 

When the oxygen partial pressure is raised to a critical value, the process becomes unstable An avalanche effect occurs since the rise in oxygen partial pressure decreases the pumping speed for the reactive gas. The target surface becomes fully oxidized. 

When the oxygen partial pressure is decreased to a critical limit, the cathode surface becomes partially metallic. The sputter yield is increased and the 2nd avalanche effect occurs for the transition from the oxide to the metallic mode. 

The sample is a fine cylinder usually comprised of a capillary filled with the powder we wish to study. It can also be made of a thin wire, particularly when the material we wish to study is a metal. In any case, this sample is placed in the center of a curved, position sensitive gas detector used to simultaneously detect all of the diffracted beams. We saw before that when these gas detectors are irradiated with X photons, the gas is ionized and a local avalanche effect takes place which leads to the ionization of the neighboring atoms. The size of this ionized zone depends on... 

A helium gas tube is shown schematically in Fig. 3.10a. It consists of an earthed steel tube filled with He the pressure is usually around 10 bar. A high voltage, 1800 V, anode runs down the length of the tube. The charged ionisation products caused by the proton and triton ( H", Eq. (3.5), are accelerated towards the anode, causing further ionisation and an avalanche effect. This results in a gain of up to a factor of 10 and single neutron detection is readily achievable. 

A new environmental secondary electron detector (ESED) has been produced by Hitachi for its SEM. It provides an alternative to the traditional back-scattered imaging and closely mimics a conventional secondary electron detector to yield good surface information. The new ESED picks up ions as well as electrons, creates an avalanche effect with the ions and produces a better quality image. 

Intrinsic. In this mechanism, electrons in the conduction band are accelerated to such a point that they start to ionize lattice ions. As more ions are ionized and the number of free electrons increases, an avalanche effect is created. Clearly, the higher the electric field applied, the faster the electrons will be accelerated and the more likely this breakdown mechanism will be. 

In the case of the use of an uncooled cathode, with a current density of about 0.1A cm 2, an additional thermionic emission of electrons takes place. This results in another avalanche effect and since the output impedance of the supply limits the voltage, a discharge with low voltage and high current density commences. 

Sukhushin, et al. [19] observed gaseous and solid decomposition products in response to electric fields of intensity below that leading to detonation in PbNe, AgN3, and TIN3. The observations were made with both constant and pulsed fields under varied electrode configurations, and the nature and localization of decomposition varied from one material to another. The evidence appeared to preclude explanations involving purely thermal or electrochemical processes. It was proposed that decomposition resulted from the discrete development of (electronic) impact ionization avalanching (the chemical avalanche effect ). 

Joubert ME, Guy S, Jacquier B, Linares C (1994) The photon-avalanche effect review model and application. Opt Mater 4 43 9... 

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. 

Recently, some of the features of the avalanche effect have been observed at room temperature in Tm YA103 (Ni and Rand 1992), and in Pr in silica glass fibers (Gomes... 

In fluoride fiber there appear as yet not completely explained up-conversion spatial domains with periodic structure, with periods ranging from few cm to mm and 100 (tm (Chen and Auzel 1994a,b). Such avalanche effects have been found to be a way to obtain population inversions and consequently ESA and laser effects (Koch et al. 1990, Lenth and Macfarlane 1990, Hebert et al. 1992, Chen and Auzel 1994c). 

Auzel s chapter on coherent emission is different from many reviews on the subject, which are concerned with the laser effect itself, in that he concentrates on the broader issues. The emphasis of chapter 151 is on superradiance, superfluorescence, amplification of spontaneous emission by other stimulated emission than the laser effect, and coherent spontaneous emission. Also discussed are up-conversion by energy transfer, up-conversion by the avalanche effect, and recent advances in lanthanide lasers and amplifiers. 

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