Principles of Fluorescence Spectroscopy

46 INSTRUMENTATION FOR FLUORESCENCE SPECTROSCOPY
NIR fluorescence, the bialkali photocathode is becoming
less useful.
The sensitivity above 500 nm has been increased by the
introduction of multi-alkali and extended red multi-alkali
photocathodes, which provide good sensitivity to 700 or
800 nm (Figure 2.31). Red-sensitive PMTs typically have
higher dark current, but for most multi-alkali photocathodes
the dark current is not a problem. Sensitivity to still longer
wavelengths can be obtained using Ag–O–Cs or S-1 photocathodes.
However, their quantum efficiency is uniformly
poor, and it is often difficult to detect the signal above the
dark current with an S-1 PMT. In fact, these PMTs are
rarely used without cooling to reduce the dark current.
Table 2.2. Characteristics of Typical Photomultiplier Tubes a
R446 R2560 R3809 R3811
Type Side window Head-on Head-on Subminiature
Dynode chain Circular cage Linear plate Microchannel cage Circular cage
Photocathode Multi-alkali Bialkali Multi-alkali S-20 Multi-alkali
Wavelength
range (nm)
185–870 300–650 160–185 185–850
Amplification 5 x 106 6 x 106 2 x 105 1.3 x 106 Rise time (ns) 2.2 2.2 0.15 1.4
Transit time (ns) 22 26 0.55 15
Bandwidth (MHz)
(estimate)
200 200 2000 300
a The number refers to types provided by Hammamatsu Inc. 18
Figure 2.32. Types of photomultiplier tubes.
2.6.2. PMT Designs and Dynode Chains
The major types of PMTs and dynode chains used in fluorescence
are shown in Figures 2.32 and 2.33. A commonly
used PMT is the side-window or side-on tube. A large number
of variants are available, and all are descendants of one
of the earliest PMTs, the 1P-28. These side-on tubes used a
circular cage dynode chain, sometimes referred to as a
squirrel cage (Figure 2.33). The specifications of one sideon
tube are listed in Table 2.2. The multi-alkali photocathode
of the R446 is sensitive from 185 to 870 nm. This type
of circular cage PMT has evolved into the subminiature
PMTs. Because of their compact design the time response
is excellent (Table 2.2). PMTs are also available in the compact
TO-8 format, which is 16 nm in diameter. Small PMTs
are available complete with a high voltage supply and dynode
chain, all in a compact package. These compact highsensitivity
detectors have appeared in many research and
clinical instruments.
Figure 2.33. Types of PMT dynode chains. From [14].