Radiant sensitivity

Wavelength dependence of the radiant sensitivity of various photocathode materials. S-1 = AgOCs,... 

Fig. 5.91 Left Relative radiant sensitivity (relative counts per incident power) of the bialkali and the multialkali cathode. Right Transmission of a BG39 NIR blocking filter, and relative sensitivity of the cathodes with the filter. Curves calculated from Hamamatsu sensitivity curves and Schott filter data...

Single photon avalanche photodiodes (SPADs) achieve the highest radiant sensitivity of all detectors in the NIR. Currently available APD detectors have ex-... 

The cathode radiant sensitivity" is the cathode current per watt of incident power at a given wavelength. It is usually given as a plot versus the wavelength. The cathode radiant sensitivity does not include the efficiency of the electron transfer from the cathode into the dynode system or the possible loss of photon pulses due to poor resolution of the pulse height distribution. Nevertheless, the cathode radiant sensitivity is useful for comparing different detectors and different cathode versions. 

The most useful parameter for characterising the efficiency of a photoncounting detector is the quantum efficiency. The quantum efficiency, QE, of a photocathode is the probability of the emission of a photoelectron per incident photon. It is directly related to the radiant sensitivity, S ... 

The general wavelength dependence of the radiant sensitivity for some commonly used photocathodes is shown in Fig. 6.16. 

Unfortunately, GaAsP and GaAs cathodes are intrinsieally slow and contribute with typically 100 ps to the TTS of the PMT. A short TTS in combination with sensitivity up to 1,100 nm is obtained from the SI eathode (Fig. 6.16, right). Flow-ever, the SI cathode delivers an extremely high dark eount rate and is therefore rarely used for TCSPC. A good solution to fast measurements in the NIR is the extended red , or S25 cathode. Recently a high efficiency extended red cathode has become available. Up to 750 nm the specified radiant sensitivity is almost the same as for the GaAs cathode. 

The radiant sensitivity of different PMT types of the same cathode material can vary considerably, especially for NIR-sensitive tubes. Reflection-type cathodes are usually a bit more efficient than transmission type photocathodes. Even tubes of the same type and cathode material differ noticeably in their radiant sensitivity. 

The radiant sensitivity of a detector - or its quantum efficiency - is one of the most important parameters for TCSPC application. Unfortunately absolute measurements of the radiant sensitivity or the quantum efficiency are extremely difficult. The problem is not only that a calibrated light source or a calibrated reference detector are required but also that extremely low light intensities have to be used. However, accurate attenuation of light by many orders of magnitude is difficult. 

The Cathode Radiant Sensitivity" is the current of the photocathode divided by the power of the incident light at a given wavelength. Measuring the Cathode Radiant Sensitivity requires a lamp, a monochromator and a reference detector, e.g. a calibrated photodiode. The setup is difficult to calibrate due to the various error sources. 

Fig. 6. 33 H7422 cooled PMT module (left) and radiant sensitivity of different cathode versions (right), from [214]...

Fig. 6.39 H5773 and H5783 photosensor modules and radiant sensitivity of the different cathode versions. From [213]...

The most common cathode types for PMTs are the bialkali, the multialkali, the extended multialkali, and the GaAs and GaAsP cathodes. Typical curves of the cathode radiant sensitivity are given in Fig. 6.16, page 230. The selection of the cathode is often a tradeoff between red and NIR sensitivity and dark count rate. 

The eathode luminous sensitivity" is the eathode current per watt incident light power from a tungsten lamp operated at 2,856 °C. The cathode luminous sensitivity is the integral of the product of the eathode radiant sensitivity and the lamp speetrum. Beeause the lamp has its emission peak in the NIR the cathode luminous sensitivity lets the sensitivity of NIR-sensitive cathodes appear higher than it aetually is. 

Radiant sensitivity noise equivalent irradiance (NEI) 1 x lO ph/cm /sec (because the applications for this type of instrument measure radiant power rather than temperature, sensitivity is expressed in NEI rather than NETD)... 

Figure 5.15. Radiant sensitivity of a selection of photomultiplier photocathode types the dotted lines refer to a specific gain in the dynode chain.

Figure 5.16. Radiant sensitivity of a large area silicon photodiode detector.

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