Noise thermal emission

Keywords coherent detection, incoherent source, thermal emission, Shottky noise, photon... 

The major characteristics of the photomultiplier with which the user is generally most concerned include (1) sensitivity, spectral response, and thermal emission of photocathodes (2) amplification factor and (3) noise characteristics and the signal-to-noise ratio. 

Dark noise in photomultipliers is caused by (1) leakage current across insulating supports (2) field emission from electrodes (3) thermal emission from the photocathode and dynodes (4) positive ion feedback to the photocathode and (5) fluorescence from dynodes and insulator supports. Careful design can eliminate all but item (3). Associated with the photocurrent from the photocathode is shot noise. There is also shot noise from secondary emission in the multiplier structure. 

The pulse amplitude distribution consists of three major eomponents. There are the regular photon pulses, i.e. the pulses originating from eleetron emission at the photocathode, which form a wide peak at relatively high amplitudes. Thermal emission, photoelectron emission, and reflection of primary electrons at the dyn-odes forms a secondary peak at lower amplitudes. At very low amplitudes electronic noise, either from the preamplifier or from the environment, causes a third peak of extremely high eount rate. 

Radiometer effect Thermal (Johnson) noise Radiant emission... 

ABSTRACT The principle of electrical spectrography and its measurement system is discussed. The phenomenon of noise in electrolytes and interfaces receives attention. Noise spectrography is found to have applications in some biomolecular systems, viz., DNA helix-to-coil transition, thermal transconformation, and salt-free premelting effects. Noise conductivity emission spectra of collagen solutions gave information on permanent dipole fluctuations and hydrodynamic properties of the system. 

Flicker noise An electrical noise current contained in a thermal emission current due to stochastic change on conditions of cathode surface. For example in case of the 1 // noise, the noise power is inversely proportional to the operating frequency of interest. 

From the ground, the sensitivity of near infrared observations is usually severely limited by the noise from terrestrial sources of photons. At various points in the spectrum, the prime contributors can be molecular trsmsitions pumped by sunlight, thermal emission from species in the atmosphere, thermal emission from the telescope optics, scattered light from the telescope, thermal emission from particulates in the atmosphere (volcanic dust, ice, water droplets, etc.), auroral airglow, and scattered moonlight, to name a few of the brightest candidates. 

The first commercial supersonic transport, the Concorde, operates on Jet A1 kerosene but produces unacceptable noise and exhaust emissions. Moreover, it is limited in capacity to 100 passengers and to about 3000 miles in range. At supersonic speed of Mach 2, the surfaces of the aircraft are heated by ram air. These surfaces can raise the temperature of fuel held in the tanks to 80 °C. Since fuel is the coolant for airframe and engine subsystems, fuel to the engine can reach 150°C (26). An HSCT operated at Mach 3 would place much greater thermal stress on fuel. To minimize the formation of thermal oxidation deposits, it is likely that fuel deflvered to the HSCT would have to be deoxygenated. 

Even in the absence of illumination (darkness) some electrons, excited by thermal energy, are emitted from the photocathode. Since photocathodes are materials with low working functions, the thermal energy can be high enough to induce the emission of electrons. These emitted electrons give rise to what is known as the dark current or, sometimes, the thermo-ionic current. The dark current varies randomly with time, so that it is considered as noise. It has been experimentally determined that the thermo-ionic current, U, due to photoelectrons emitted by a photocathode in the absence of illumination is given by... 

In the first model, the mnneling electron mainly interacts with the electronic polarization of water ( = 1.88) since tunneling was assumed to be fast in comparison with the orientational response of the dipolar molecules of the liquid. Considering water as a dielectric continuum between a jellium spherical tip and planar substrate yields an effective barrier for tunneling that is about 1 eV lower than that for the vacuum case [95]. This result is consistent with photoemission studies of metal/aqueous interfaces, which reveal electron emission into water at 1 eV below the vacuum level [95-97]. Similar models have been employed to examine the effect of thermal fluctuations on the tunneling current [98-100]. Likewise, a related model assessing the noise associated with the reorientation of adsorbed molecules has been presented [101]. 

Quantum-state decay to a continuum or changes in its population via coupling to a thermal bath is known as amplitude noise (AN). It characterizes decoherence processes in many quantum systems, for example, spontaneous emission of photons by excited atoms [35], vibrational and collisional relaxation of trapped ions [36] and the relaxation of current-biased Josephson junctions [37], Another source of decoherence in the same systems is proper dephasing or phase noise (PN) [38], which does not affect the populations of quantum states but randomizes their energies or phases. 

A second way to help resolve the thermal noise problem is to use two photomultiplier tubes for detection of scintillations. Each flash of light that is detected by the photomultiplier tubes is fed into a coincidence circuit A coincidence circuit counts only the flashes that arrive simultaneously at the two photodetectors. Electrical pulses that are the result of simultaneous random emission (thermal noise) in the individual tubes are very unlikely. A schematic diagram of a typical scintillation counter with coincidence circuitry is shown in Figure 6.2. 

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