Quantum efficiency of the detector

SPC techniques are hardly affected by additive noise and multiplicative noise is absent. However, subtractive noise due to the collection efficiency and transmission of optics and the quantum efficiency of the detector do play a role. In addition, at high count rates, the efficiency goes down due to pileup effects. 

When a luminescence spectrum is obtained on an instrument such as that used to produce the spectra in Figure 7.23, it will depend on the characteristics of the emission monochromator and the detector. The transmission of the monochromator and the quantum efficiency of the detector are both wavelength dependent and these would yield only an instrumental spectrum. Correction is made by reference to some absolute spectra. Comparison of the absolute and instrumental spectra then yields the correction function which is stored in a computer memory and can be used to multiply automatically new instrumental spectra to obtain the corrected spectra. The calibration must of course be repeated if the monochromator or the detector is changed. 

Another problem with RS measurements is that the corresponding thermometry is subject to calibration. This implies careful evaluation of the experimental parameters (quantum efficiency of the detector, collection efficiency, laser energy, total number density, solid angle of the collection optics, optical path length, and so on), but a typical procedure relies on the ratio between the measured RS signal and a reference signal obtained from a gas of known RS cross section and temperature. 

Both rates, and are proportional to the efficiency of the optical system and the quantum efficiency of the detectors. Consequently the eoineidence rate, r. ... 

In an ideal imaging system, the only source of noise in the image should be quantum noise. Quantum noise is fundamental and unavoidable in an X-ray image, but for a given exposure, its effect should be minimized by ensuring that the quantum efficiency of the detector is as close to 100% as possible. In real imaging systems, there are also other noise sources, mainly associated with the detector, and efforts should be made in the system design to ensure that these are much smaller than the quantum noise. 

It is also essential that emission spectra are corrected for the instrumental function estabUshed with a standard calibrated lamp. It is wise not to use the calibration curve given by the manufacturer of the spectrometer and to re-measure this instrumental function at regular intervals because many items influence it, particularly the emission intensity of the excitation lamp and the quantum efficiency of the detector (which both decrease with time). In case (27a) is used, the excitation instmmental function has to be known as well. Regarding the standard, it is best when its emission spectrum overlaps the emission spectmm of the unknown... 

This is actually the increase of the dark current due to illumination and is a dc value (valid for/= 0 Hz). Here rj denotes the quantum efficiency of the detector, < ) is the incident photon flux density, A is the detector active area. The factor T denotes the photoelectric gain or photogain (the ratio between the number of electrons flowing through the electric circuit and the number of absorbed photons). The fundamental equation of photoconductivity is also valid without changes for the short circuit current of a photovoltaic detector (photodiode operating in photo-conductive mode). In that mode of operation T = 1 in most of the cases. 

We can reduce loss of excitations and obtain enhancement of the quantum efficiency of the detector, employing quasi-particle trapping. The Nb layer in an Nb/AlO /Nb junction can... 

As in heterodyne detection at r.f. frequencies, the optical heterodyne technique eliminates the problems caused by intrinsic or dark current noise in the detector and the effective signal-to-noise level is limited only by the quantum efficiency of the detector. On the other hand... 

The absorbing surface of the photomultiplier, its photocathode material, defines the spectral range and quantum efficiency of the detector. Photocathodes that respond to longer... 

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