Quantum detection efficiency

MicroChannel plates are becoming widely used in spectroscopy and two dimensionaj imaging at EUV (100 - 1000 A) and soft X-ray wavelengths (10 - 100 A) for astronomy and microscopy. Although bare microchan-nel plates have low sensitivity (5% - 10% quantum detection efficiency) in this spectral region, use of photocathodes can increase substantially (to 30%-40%) microchannel plate performance. This, combined with the high spatial resolution (<50/rm), fast time response (<300 ps), and large effective area (up to 100 mm diameter) achievable with microchannel plates make the latter a very attractive and versatile tool. We discuss the properties of microchannel plates, photocathode materials, and various microchannel plate detector readout schemes that have been used for soft X-ray and EUV detection. 

Figure 8. Quantum detection efficiency of Cslcoatings as a function of grazing angle of incidence. Key , X = 67 A (48) and, X = 44 A (17).

The quantitative determination of X-ray intensities with a photon energy up to 150 keV can be realized by three types of detectors semiconductor-based detectors which call for efficient cooling, ionization chambers which have a low quantum detection efficiency (< 50%), and detectors based on luminescent materials. The latter type of detector is of interest here, because it is realized by the combination of a luminescent material with a photodiode. 

To form an image, the transmitted X-rays must be detected and their energy converted into a usable signal. Quantum detection efficiency, rj, describes the fraction of the X-rays incident on the detector, that... 

These operations must be optimized if the detector is to provide high-quality images at appropriate dose levels. Detectors are characterized by their quantum detection efficiency,sensitivity,spatial resolution properties, noise, dynamic range, and linearity of response. 

As discussed in Chap. 1, quantum detection efficiency, r ( ), describes the fraction of the X-rays falling on the detector, that interact with it, producing at least some signal. Some calculated values of tj for... 

Furthermore, for the face detected emission (as usually is the case), the light output coupling factor (287) reduces the measured Oel to OgE, so that we deal with the external quantum EL efficiency... 

In a model proposed by Lewis [228] the effect of the excited state of retinal on the conformational state of the protein is considered to be the first step of the excitation mechanism. Charge redistribution in the retinal by excitation with light would have the consequence of vibrationally exciting and perturbing the ground state conformation of the protein, i.e., excited retinal would induce transient charge density assisted bond rearrangements (e.g., proton translocation). Subsequently, retinal would assume such an isomeric and conformational state so as to stabilize maximally the new protein structure established. In this model, 11-m to trans isomerization would not be involved in the primary process, but would serve to provide irreversibility for efficient quantum detection. It was also proposed that either the 9-m-retinal (in isorhodopsin) or the 11-m-retinal (in rhodopsin) could yield the same, common... 

After reaching the conical intersection, the system may evolve to form the radical pair that leads to the final products, or revert back to the ground-state precursors. These processes cannot be observed in the jet, since the products of both routes are formed in a huge number of quantum states, and the detection efficiency of a single product is prohibitively small. This mechanism is also in line with available data in liquid solution. It supports the notion, suggested by some workers [43], that the formation of the exciplex is in competition with formation of stable products. At the same time, it agrees with the important role of the charge-transfer state (or some polar structure), denoted in the scheme as the CT,FC state. The complicated Stern-... 

Figure 16.9 Results of Aspect experiment. Dotted curve shows prediction of quantum mechanics (multiplied by 0.955 to correct for detection efficiency). Shaded regions show where Bell s inequality is violated. [From A. Aspect, P. Grangier, and G. Roger. Phys. Rev. Lett. 49,91 (1982)].

Investigations (51., 52) of the variation of quantum efficiency as a function of the Csl layer thickness show that (Figure 7) the quantum detection o efficiency, for wavelengthso under 100A, is nearly constant for layers above 1000A in thickness. Below 1000A thick, the QDE fails off, due to the increasing transmission of the photocathode layer. 

Figure 6. Quantum detection of efficiency of Csl-coated MC Ps and flat samples, as a function of wavelength. Key —. Reference 48 (10°) —, Reference 49 (20 and 23°) — and —. Reference 17 Reference 15 (8°) and o. Reference 47.

Ion tr experiments can achieve high detection efficiency of the ion s internal states. Unit detection efficiency has been achieved in previous experiments on quantum jumps [13] where the internal state of the ion is indicated by light... 

From Eq. (9) it is clear that correct determination of QY depends on accurate knowledge of the overall detection efficiency/, which is generally about 0.5. It can be determined in two ways. One is to compare the measured ratio C2+/C+ with the calculated ratio for an atomic rare gas such as Xe at a wavelength where the true relative production ratio A2+/N+ is known from photoionization mass spectrometric measurement of the ion yields. This method has been used in deriving the Xe data of Fig. 15. As expected, the quantum yield in the atomic case is unity (100%) within experimental error this must be generally true because deactivation of superexcited states by light emission is very rare. Thus the second and quicker method is simply to measure the apparent QY for an atomic gas and determine / accordingly. 

Usually quantum efficiencies given for detectors refer to the emission of a photoelectron or, in avalanche photodiodes, the generation of an electron-hole pair. The detection efficiency in PMTs is smaller for the reasons mentioned above Not all photoelectrons cause a detectable anode current pulse in a PMT, and not all electron-hole pairs trigger an avalanche in a SPAD. 

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