Principles of Fluorescence Spectroscopy

PRINCIPLES OF FLUORESCENCE SPECTROSCOPY 117
are decreasing because the higher-repetition-rate lasers provide
high data rates even with a 1% count rate. Additionally,
multichannel systems with multiplexing have become
available for TCSPC (Section 4.7).
4.6. DETECTORS FOR TCSPC
4.6.1. Microchannel Plate PMTs
Perhaps the most critical component for timing is the detector.
The timing characteristics of various PMTs have been
reviewed for their use in TCSPC. 22,27,73–74 At present the
detector of choice for TCSPC is the microchannel plate
PMT. An MCP PMT provides a tenfold shorter pulse width
than any other PMT, and displays lower intensity afterpulses.
Also, the effects of wavelength and spatial location of
the light seem to be much smaller with MCP PMTs than
with linear-focused or side-window tubes. While good time
resolution can be obtained with linear-focused and sidewindow
PMTs, the high-speed performance and absence of
timing artifacts with an MCP PMT make them the preferred
detector for TCSPC.
Development of MCP PMTs began in the late
1970s, 75–78 with the first useful devices appearing in the
early to mid-1980s, 79 and their use for TCSPC beginning at
the same time. 80–88 The design of an MCP PMT is completely
different from that of a dynode chain PMT (Figure
4.24). The factor that limits the time response of a PMT is
Figure 4.24. Photograph and schematic of an MCP PMT. The diameter
of the housing is about 45 mm. Revised from [90–91].
its transit time spread (TTS), which is the distribution of
transit times through the detector. The overall transit time of
the electrons through a PMT is not important, as this is just
a time delay corrected for in the measurements. However,
the distribution of transit times or TTS is important because
this spread limits the time resolution of a PMT. One cannot
do timing measurements more accurately than the uncertainty
in the time it takes a signal to pass through the detector.
In a linear-focused PMT the TTS is minimized by
designing the dynodes so all the electrons tend to travel
along the same path. The TTS of most phototubes is near 2
ns, and it can be less than 1 ns with carefully designed
PMTs (Table 4.1).
The design of an MCP is completely different because
it does not have dynodes. Instead, the photoelectrons are
amplified along narrow channels lined with the dynode
material (Figure 4.24). Because these channels are very narrow,
typically 4 to 12 microns in diameter, the electrons all
travel the same path and hence have the same transit time.
Smaller channels result in less transit time spread. There are
a few additional features in the MCP PMT design that provide
improved time response. The channels are angled relative
to each other, which prevents feedback between the
channels and broadening of the time response. 77–78 Also, the
first MCP surface is typically covered with aluminum,
which prevents secondary electrons emitted from the top of
the MCP from entering adjacent channels.
MCP PMTs provide very low TTS. Figure 4.25 shows
the response of an R3809U to femtosecond pulses from a
Ti:sapphire laser. The R3809U is now widely used for
TCSPC. The FWHM is only 28 ps and the afterpulse a factor
of 100 smaller then the primary response. MCP PMTs
do have some disadvantages. The photocurrent available
from an MCP PMT is typically 100 nA (R2908), as compared
to 0.1 mA (R928) for a dynode PMT. This means that
the MCP PMT responds linearly to light intensity over a
smaller range of incident intensities than does a dynode
PMT. The current carrying capacity of an MCP PMT is less
because the electrical conductance of the MCPs is low. It is
known that the pulse widths from an MCP PMT can depend
on the count rate, 88–89 presumably because of voltage
changes resulting from the photocurrent. Another disadvantage
of the MCP PMTs is their presumed limited useful lifetime
which depends on the photocurrent drawn from the
tube. In our hands this has not been a problem, and it is difficult
to know if an MCP PMT has lost gain due to the total
current drawn or due to the inevitable overexposure to light
that occurs during the lifetime of a PMT. MCP PMT are