Photons, avalanche counting

Like all photon counting techniques, gated photon counting uses a fast, high-gain detector, which is usually a PMT or a single-photon avalanche photodiode. Due to the moderate time resolution of the gating technique, there are no special requirements to the transit time spread of the detector. However, the transit time distribution should be free of bumps, prepulses or afterpulses, and should remain stable up to a count rate of several tens of MHz. 

Currently available single photon avalanche photodiodes (SPADs) are not applicable to optical tomography. Although the efficiency in the NIR can be up to 80%, the detector area is only of the order of 0.01 mm. Diffusely emitted light cannot be concentrated on such a small area. A simple calculation shows that SPADs carmot compete with PMTs unless their active area is increased considerably. Another obstacle is the large IRF count-rate dependence sometimes found in single-photon APDs. 

TTS exists also in single photon avalanche photodiodes (SPADs). The source of TTS in SPADs is the different depth at which the photons are absorbed, and the nonuniformity of the avalanche multiplication efficiency. This results in differing delays in the build-up of the carrier avalanche and in different avalanche transit times. Consequently the TTS depends on the wavelength and the voltage. Moreover, if a passive quenching circuit is used, the reverse voltage may not have completely recovered from the breakdown of the previous photon. The result is an increase of the TTS width or a shift of the TTS with the count rate. 

T. Louis, G.H. Schatz, P. Klein-Bolting, A.R. Holzwarth, G. Ripamonti, S. Cova, Performance comparison of a single-photon avalanche diode with a microchannel-plate photomultiplier in time-correlated single-photon counting, Rev. Sci. Instram. 59, 1148-1152 (1988)... 

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. 

Fig. 13.16a. As an atom source, a magneto-optical trap (MOT) for cold Cs-atoms was used. The fluorescence of MOT atoms around the MNF was detected by the measurement of fluorescence photons with an avalanche photodiode connected to one end of the fiber. Signals are accumulated and recorded on a PC using a photon-counting.

H. Dautet, P. Deschamps, B. Dion, A. D. MacGregor, D. MacSween, R. J. McIntyre, C. Trottierand P. P. Webb, Photon counting techniques with silicon avalanche photodiodes, App. Opt. 32, 3894-3900 (1993). 

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). 

The low level comparator will reject pulses of low intensity which are generated by thermoionic emission in the dynodes. It is obvious that an electron emitted by dynode 5 for instance will produce a much smaller avalanche at the anode than an electron emitted by the photocathode but of course thermoionic emission from the photocathode itself would still appear as a genuine pulse, and for this reason PM tubes used in photon counting applications must be cooled down. 

Figure 2 Experimental arrangement for measurements of the Fe nuclear resonance at the Advanced Photon Source (APS). In the standard fill pattern, electron bunches with a duration of 100 ps are separated by 153 ns. X-ray pulses are generated when alternating magnetic fields in the undulator accelerate these electron bunches. The spectral bandwidth of the X-rays is reduced to 1 eV by the heat-load monochromator and to 1 meV by the high-resolution monochromator. At the sample, the flux of the beam is about 10 photons/s. APD indicates the avalanche photodiode used to detect emitted X-rays. The lower right inset illustrates that counting is enabled only for times weU-separated from the X-ray pulse, so that only delayed photon emission resulting from decay of the nuclear excited state contributes to the experimental signal...

Photon counting PMTs dominated Raman spectroscopy detection until about 1985, but their use decreased sharply after the introduction of CCDs. A brief review of PMTs is provided here, partly as background for their modem singlechannel analog, the avalanche photodiode. 

The ability to obtain single-photon counting using methods such as avalanche photon detectors and negative electron affinity photocathode photomultiphers has thus far been limited to the visible and infrared regions. The vertical QCD which utihzes a triple-well quantum (Q) dot system of the type illustrated in Fig. 9 offers a novel approach to sense THz radiation. Here, the detector is first primed into active-mode by tunnel injection into the top Q-dot (QDl) of the SES followed by an IR pulse that puts the electron into the middle Q-dot (QD2) of the THz-RDC. This electron will remain in the QD2 until a THz photon induces the electron s transition to QD3. Finally, an IR photon ejects the electron from QD3 thus resetting the detector. Since the electron injection into the QCD system and ejection from the detector are quick transitions, only the middle Q-dot (QD2) will be occupied for a significant period of time. Consequently, to successfully read-out the state of our triple Q-dot system one must be able to differentiate between the two possible states (1) if THz photons are present, the electron will quickly be ejected from the entire QCD system and no electrons will be present in QD2 or (2) if no THz photon is present, QD2 will remain occupied by an electron. 

Measurements at low light levels are routinely performed with photon-counting techniques. The development of ultrasensitive optical detectors has made great progress in the last couple of years. Integrated photon-counting modules with cooled avalanche photodiodes (APD) have been available for some years [31]. These detectors can have quantum efficiencies of 50% with less than 10 dark counts per second. The light sensitive area of such a device has a diameter of about 200 (im and can serve directly as a pinhole in a confocal detection channel. 

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