Page 118 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Figure 4.15
Photomultiplier sensitivity curves.
of caesium, may be preferred. Gallium arsenide cathodes offer a reasonable uniform response over a
wide range, up to 850 nm. Response at the low wavelength end (e.g. the arsenic line at 193.7 nm) is
often dependent on the material used for the envelope and its transmission at short wavelengths. For
certain applications, a photomultiplier which operates only in the ultraviolet region may be preferred.
Such a photomultiplier does not respond to daylight and is known as a solar blind. Ordinary
photomultiplier tubes can be damaged if operated without protection from daylight.

It is important that a photomultiplier gives low noise and a low dark current, i.e. low background
signal in the absence of photons, usually caused by thermionic emission of photons from the cathode
material.