Photon-counting detector

The 5 ns pulses of about 10 electrons released at the anode by a photon absorbed by the photocathode of a PM tube can be used to count photons. In such instruments the intensity of light is displayed as a count per second which varies between about 15 (dark count) and 105. A photon-counting detector system is of course much more complex than the simple PM/ampli-fier used in conventional spectrofluorimeters. Figure 7.27(a) is a block diagram of such a photon counter (b) gives a simple illustration of the important process of pulse selection through a discriminator. The output of... 

Because a 2-D photon-counting detector was not available at the time of construction of the demonstrator, the detector was realized as a single 1-D vertical detector column based on scintillator crystals and photomultipliers, which can be rotated around the focus point, thereby acquiring one projection. Spatial resolution was ensured by a collimator made of thin tungsten lamellae placed in front of the detector. 

There are several types of photon counting detector front-ends that can now handle the desired count rates, ranging from the new types of gas detectors (Microstrip gas chambers and MicroGaps) to silicon arrays. Crucial to the success of these detectors is the design and fabrication of a readout and memory system which can handle both the count rate and framing rate of the next generation of time resolved diffraction experiments. 

A complication to the frequency dependency of aj arose when photon counting detectors were introduced into Raman instrumentation. Virtually all modern spectrometers count photons rather than measure watts, and the two differ by a factor of hv. Since the incident and scattered photons differ in energy, the ratio of scattered to incident power differs from the ratio of scattered to incident photon flux. The consequences of this difference are quantitatively fairly minor, but conceptually important. Derivations based on photons/second and photon counting are more consistent with a quantum mechanical treatment where a cross section is effectively a statement of probability, whereas the classical treatment is based on induced dipoles. If Eq. (2.9) is rewritten for photon counting systems, Eq. (2.11) results ... 

The experimental station contains manipulators to positions the sample, optics to view the sample, and detectors to measure the fluorescent, scattered or diffracted X-rays. The details of these apparatus depend entirely on the type of experiment to be performed. For example, an XRF microprobe requires a precision X-Y-Z sample stage, high quality microscope and multi-element fluorescence detector. A surface scattering experiment, on the other hand requires a 4-circle goniometer and a low-noise photon counting detector. 

Transit-Time Spread. In a photon counting detector, the transit time for the individual photons varies. The TTS is the distribution of the observed times of the output pulses for infinitely short input light pulses. 

Photon-counting detectors have an increased probability of producing background pulses within the microseconds following the detection of a photon. These afterpulses are detectable in almost any conventional PMT. It is believed that they are caused by ion feedback, or by luminescence of the dynode material and the glass of the tube. 

A technique for measuring the quantum efficiency of a photon counting detector without a calibrated reference detector is described in [301, 356, 357, 358, 423, 536]. The technique is based on the generation of photon pairs - or entangled photons" - by parametric down-conversion. The principle is shown in Fig. 6.28. 

L.A. Kelly, J.G. Trunk, J.C. Sutherland, Time-resolved fluorescence polarization measurements for entire emission spectra with a resisitive-anode, single-photon-counting detector The fluoreseence omnilizer, Rev. Sci. Instrum. 68, 2279-2286 (1997)... 

Note that Secs. 2.2 to 2.6 dealing with scattered intensities are the only places in this book where the strict definition of intensity is implied. Most diffraction and scattering theories have been developed around the strict definition of intensity [48]. This must be kept in mind when applying the results of such theories to single-photon-counting detectors. 

Roessl E, Proksa R (2007) K-edge imaging in x-ray computed tomography using multi-bin photon counting detectors. Phys Med Biol 52 4679-4696... 

Faint Object Camera A narrow field of view camera able to utilize the full resolution capabilities of the LST. Provided with a photon counting detector, it would be able to reach the faintest possible stars and galaxies. 

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