Geoscience Reference
In-Depth Information
REFLECTION MODE PHOTOCATHODE
10
8
60
40
452 U
350 S
350 K
20
456 U
1
8
6
4
250 M
350U
250 S
2
1.2
0.8
0.6
0.4
0.2
0.1
100
200
300
400
500
600
700
800
1000
1200
WAVELENGTH (nm)
Figure 5.15. Radiant sensitivity of a selection of photomultiplier photocathode types; the dotted lines
refer to a specific gain in the dynode chain.
wavelengths have lower work functions and are therefore more susceptible to noise signals
from thermally generated electrons. Thus, if working above 650 nm in photon-counting
mode, it may be necessary to cool the detector to -20°C or less in a suitable housing.
Photomultiplier tube detectors are still the detector of choice for fluorimeter application
owing to their excellent sensitivity and operational gain.
Figure 5.15
shows typical spectral
responsivity of a selection of photomultiplier detectors.
Photodiode detectors are based upon the absorption of a photon in a P-N junction diode
(Skoog,
1998
) and the creation of an electron-hole pair in the depletion layer by promotion
of electrons across the valence band to the conduction band. The spectral response of sili-
con photodiodes extends from the UV to the near-IR (1.1 μm), and the responsivity of such
photodiodes is often much less than that of photomultipliers because they have no internal
gain. However, single-photon avalanche diode (SPAD) identifies a class of solid-state pho-
todetectors based on a reverse biased P-N junction in which a photogenerated carrier can
trigger an avalanche current owing to the impact ionization mechanism. This device is able
to detect low-intensity signals (down to the single photon) and to signal the arrival times of
the photons with a jitter of a few tens of picoseconds (Niclass et al.,
2005
). Even so, silicon
photodiodes are rugged, exhibit excellent linearity over seven decades of incident radiant
power, and can be small in size with excellent time responses. As a result they are ideal as
the detector used as the excitation channel monitor.
Figure 5.16
demonstrates typical spec-
tral responsivity of such detectors.
