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Chapter 6
Gated Operation
57
values depends critically on the lack of stray light contamination, particularly at moderate
to high optical density levels. Selecting a 5 ns time window out of a 10 µs pulse is
already one part in 2,000. If UV leakage gives an on/off ratio of only 20,000:1,
contamination could be 10% of higher. This would limit the optical density to 1.0, and it
could make linear quantitation difficult beyond 0.1 OD.
MCP bracket pulse gating can substantially improve the on/off ratio in such an
experiment. Even with a 1 µs MCP pulse, the rejection of flash-lamp leakage can add
more than an order of magnitude of range, to 2.0 OD.
Limitations of Bracket Pulse Gating
MCP bracket pulse gating is most useful in rejecting background that lasts microseconds
up to CW. Fast transient backgrounds can be in the form of stray laser light scattering
(Raleigh, MIE, Raman) or unwanted fast fluorescence. Because these usually fall below
the MCP bracket pulsing 35 ns delay restriction, these measurements cannot be improved
much by MCP bracket pulsing in the PI-MAX3.
Electrically, gating the MCP will only reduce leakage at wavelengths where the MCP has
photoelectric response (primarily in the UV). Thus, for visible and NIR wavelengths
where leakage is primarily optical, the improvement will be minimal (although the on/off
ratio is already very good in these regions). Note that in some spectroscopic applications,
visible leakage may appear to be reduced by MCP pulsing. This is because the second
order UV spectrum overlays the first order visible spectrum in a grating spectrograph.
MCP pulsing can eliminate unwanted sensitivity to CW or quasi-CW second order UV,
causing the apparent improvement.
Also, keep in mind that MCP bracket pulsing adds 10 ns , delay to the photocathode gating.
Even though the bracket timing is controlled automatically by the software, in an
experiment where it is necessary to delay the arrival of the laser pulse at the sample, this
will mean inserting an additional delay of 10 ns (min gate delay = 25 ns, with bracket =
35 ns) to accomplish coincidence at the detector. MCP bracketing should only be used in
experiments where it is going to make a difference.
Also, MCP bracket gating limits the repetition rate to 6.25 kHz, without it the gate
repetition rate can be up to 1 MHz (more with option boards). This is not a significant
limitation in most cases. Note that with a 10 ns gate at 6.25 kHz, the on/off ratio =
16,000, close to the non-bracket value.
Note that background light need not be the limiting factor in measurements where MCP
bracket pulsing is unable to provide the required degree of rejection. In such measurements,
the option remains of installing an external shutter ahead of the PI-MAX3.
Impact of Bracket Pulsing on Delay
If operating in the UV
when bracket pulsing is activated (Gen II Intensifier only), the MCP
gate automatically brackets the photocathode gate pulse to further enhance the on/off ratio.
There is, however, a limitation of bracket pulsing that can complicate the coincidence of
the signal and gate at the camera. Because MCP bracket gating is slower than
photocathode gating (35 ns is required to gate the MCP on and another 1 µs to gate the
MCP off at the end of the photocathode gate). As a result, MCP bracket pulsing should not
be used in experiments where the delay between the trigger and the photocathode gate is
less than 35 ns.
Содержание PI-MAX 3 System
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Страница 110: ...110 PI MAX 3 System Manual Version 1 A Figure 68 Outline Drawing PI MAX3 with F mount Adapter ...
Страница 111: ...Appendix B Outline Drawings 111 Figure 69 Outline Drawing PI MAX3 with Spectroscopy mount Adapter ...
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Страница 113: ...Appendix B Outline Drawings 113 CoolCUBEII Circulator Figure 71 Outline Drawing CoolCUBEII Circulator ...
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