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100 PI-MAX
®
4 System Manual Issue 4
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 camera. 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-MAX4.
6.4.4 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.