Avalanche detector

Fission fragments were detected in both experiments by two position-sensitive avalanche detectors (PSAD) (Crouzen 1988) having two wire planes (with delay-line read-out) corresponding to horizontal and vertical directions. Protons were measured in coincidence with fission fragments. Typical proton energy spectra are shown in O Fig. 5.11 as a function of excitation energy. 

This chapter will concentrate on the very high quality detectors that are needed in scientific imagers and spectrographs, and other applications that require high sensitivity, such as acquisition and guiding, adaptive optics and interferometry. We limit our discussion to focal plane arrays - large two-dimensional arrays of pixels - as opposed to single pixel detectors (e.g., avalanche photodiodes). 

In real curvature sensors, a vibrating membrane mirror is placed at the telescope focus, followed by a collimating lens, and a lens array. At the extremes of the membrane throw, the lens array is conjugate to the required planes. The defocus distance can be chosen by adjusting the vibration amplitude. The advantage of the collimated beam is that the beam size does not depend on the defocus distance. Optical fibers are attached to the individual lenses of the lens array, and each fiber leads to an avalanche photodiode (APD). These detectors are employed because they have zero readout noise. This wavefront sensor is practically insensitive to errors in the wavefront amplitude (by virtue of normahzing the intensity difference). 

Nuclear scattering is counted by two avalanche photo diode (APD) detectors. The detector for NIS (Fig. 9.1) is located close to the sample. It counts the quanta scattered in a large solid angle. The detector for NFS is located far away from the sample. It counts the quanta scattered by the nuclei in the forward direction. These two detectors follow two qualitatively different processes of nuclear scattering ... 

A laser beam highly focused by a microscope into a solution of fluorescent molecules defines the open illuminated sample volume in a typical FCS experiment. The microscope collects the fluorescence emitted by the molecules in the small illuminated region and transmits it to a sensitive detector such as a photomultiplier or an avalanche photodiode. The detected intensity fluctuates as molecules diffuse into or out of the illuminated volume or as the molecules within the volume undergo chemical reactions that enhance or diminish their fluorescence (Fig. 1). The measured fluorescence at time t,F(t), is proportional to the number of molecules in the illuminated volume weighted by the... 

The experiment is performed with a spectrofluorometer similar to the ones used for linear fluorescence and quantum yield measurements (Sect. 2.1). The excitation, instead of a regular lamp, is done using femtosecond pulses, and the detector (usually a photomultiplier tube or an avalanche photodiode) must either have a very low dark current (usually true for UV-VIS detectors but not for the NIR), or to be gated at the laser repetition rate. Figure 11 shows a simplified schematic for the 2PF technique. 

Not only PMTs and other detectors such as avalanche photodiodes suffer from dead-time effects also the detection electronics may have significant dead-times. Typical dead-times of TCSPC electronics are in the range 125-350 ns. This may seriously impair the efficiency of detection at high count rates. The dead-time effects of the electronics in time-gated single photon detection are usually negligible. 

Often, experiments are carried out on specimens that emit only very weak fluorescence. For these cases, the most sensitive detectors should be used, for instance fast avalanche photodiodes or high quantum yield PMTs. These detectors may have somewhat longer dead-times causing longer exposure times but maximal sensitivity. 

The great boost of analytical CL appeared soon after the discovery of flow injection analysis (FTA) by Ruzicka and Hansen [3], The speed with which the solutions of reagents can be supplied to the detector proved to be the best for CL reactions. Various mixing coils were investigated and this was the beginning of an avalanche of research on CL [4],... 

The SPAD detector is similar in design to other photodiodes except the electric field of its junction is better separated than in other photodiodes. This design better endures the flow of the avalanche current triggered by a photogenerated carrier and provides better performance than normal SSPD. SPAD have detection limits several orders of magnitude lower than conventional PMTs.(35) In addition, they have been shown to have higher quantum efficiency (QE) values than PMTs in the NIR region.(57)... 

S. Cova, G. Ripamonti and A. Lacaita, Avalanche semiconductor detector for single optical photons with a time-resolution of60ps, Nucl. Inst. Meth. A253, 482-487 (1987). 

T. A. Louis, G. Ripamonti and A. Lacaita, Photoluminescence lifetime microscope spectrometer based on time-correlated single-photon counting with an avalanche diode detector, Rev. Sci Instrum. 61, 11-22(1990). 

Fig. 1.31 Discrete-dynode electron multiplier. When the ions hit the surface of the detector electrons are emitted to form an avalanche of electrons which generates the signal.

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