Position-sensitive photo-detector

Figure 2 shows a schematic of a typical AFM instrument that consists of a cantilever-mounted tip, a Piezoelectric scanner, four position-sensitive photo detectors, a laser diode and a control unit. The process of operation of an AFM is relatively simple. The beam from the laser is directed against the back of the cantilever beam onto the quadrants of the photo detector. As the tip is moved across a sample, this causes the cantilever beam to bend or be twisted in manner that is proportional to the interaction force. This bending or twisting of the cantilever causes the position of the laser on the photo detector to be altered. The deflection of the cantilever beam can then be converted into a 3D topographical image of the sample surface (Gaboriaud and Dufrene, 2007 Kuznetsova et al., 2007 Lim et al., 2006). 

Fig. 14 The first experimental set-up with which it was possible to collect quasi-simulta-neously SAXS/WAXS/XAFS data. 1-position sensitive WAXS detector, 2-sample position and sample environment, 3-ion chamber for incoming beam intensity monitoring, 4-fluoresence detector, 5-optical bench, 6-SAXS detector, 7-beam stop in which a photo diode for transmitted beam intensity monitoring is integrated, 9-evacuated SAXS Sight tube. 

The photolysis of aromatic species with tetranitromethane in perfluoro alcohol solvent has been studied, in which the radical cations were observed by EPR spectroscopy.284 Photo-stimulated reaction of 1- and 2-haloadamantanes and 1,2- and 1,3-dihaloadamantanes with various carbanionic nucleophiles afforded products rationalized through an SrnI mechanism.285 286 Photolysis of the cycloadduct formed between a functionalized derivative of C6o and diazomethane has been shown to afford a pah of ling-opened structures (125) and (126) via a proposed biradical intermediate (127) (Scheme 19). The UV-photolytic fragments of /-butyl iodide (T and /-Bu ) have been ionized by resonance-enhanced multiphoton ionization for TOF mass spectro-metric analysis.287 A two-dimensional position-sensitive detector provided angular distribution and translational energy data. 

Two-Dimensional Position Sensitive DACecCor, Calibration. A number of tests were performed to assure the accuracy ot the data collected by the system. These Included testing the detector uniformity to Insure that all the photo-diodes on the chip are of equivalent sensitivity. Another calibration of the device was to determine the magnifying affects of the camera. This was done to allow conversion of the x-y pixel grid Into the dimensions of reciprocal space. Finally, the contrast setting was adjusted so that a S6 value of 0 was produced when the screen was not Illuminated, with the gain set at maximum and the f-stop wide open. Further details of the system calibration can be found In Effler s M.S. thesis (17). 

Another kind of linear solid state position sensitive detectors are the Photo-Diode-Arrays (PDA s), which are different from CCD s. A PDA consists of an array of separate photodiodes, each with an associated capacitance and a multiplexing read-out system (see Fig. 21). The charges collected in each cell are simply switched to the output, one by one. Unlike in the case of CCD s, the photosensitive elements are separated completely from the transfer circuity. 

The Position Reconstruction Algorithm developed for the SAX HP-GSPC instrument represents not only a performing solution for the on-board spectra data acquisition, but also a promising method for all the position sensitive detectors, especially for those detectors presenting a geometrical symmetry in the scintillation-photo-detection volume. 

Deflection detection schemes have been developed that use self-sensing piezoresistive cantilevers [104] or optical interferometry [105, 106]. But in nearly all practical systems cantilever deflection is measured with an optical lever system [107], as shown in Fig. 3.22. A small spot of laser light is reflected from the back of the cantilever and the reflected light is directed to an adjustable mirror. This in turn reflects the light onto a position-sensitive detector, a four-quadrant photo diode, shown schematically in Fig. 3.23. 

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