Cathode radiant sensitivity

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 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. 

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. 

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...

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. 

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 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. 

The half width of elemental lines is of the order of 0.002 nm when observed by emission spectroscopy with flame or electrothermal atomisation. A number of reasons can cause broadening of the linewidth, of which the most important and best understood are natural, pressure, resonance, and Doppler broadening. If a stable and sensitive detection is to be achieved, the linewidth of the excitation radiation must be narrower than the full width at half maximum (FWHM) of the analyte line. Under these conditions, the entire radiant energy produced by the excitation source will be available for absorption by the analyte. The typical line sources used for atomic absorption are element specific excitation sources such as the hollow cathode lamp or the electrodeless discharge lamp. But even continuum sources can be used with appropriate instrumental designs. 

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