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98 PI-MAX
®
4 System Manual Issue 4
6.4 Micro-Channel Plate (MCP) Bracket Pulsing
The primary advantage gained from incorporating gating is that it allows temporal
discrimination against background light. By allowing the photocathode to “see” only during
the event of interest, very high background illumination levels can be tolerated without
materially degrading experiment results. Limits for this technique are determined by the
light leakage of the intensifier, which, although very good, is not without bounds.
• Within the visible spectrum, the on/off ratio of a typical Gen II Intensifier with just
the photocathode gated is excellent, typically between 10
6
and 10
7
. For most
measurements, this ratio is more than adequate to guarantee that any signal reaching
the CCD during intensifier Off times is too small to affect the data.
• However, when working with wavelengths below 350 nm, optical leakage through
the photocathode to the UV-sensitive MCP occurs. At 200 nm, the on/off ratio is
reduced to approximately 2 × 10
4
. This is the dominant response of a Gen II image
intensifier to UV photons when the photocathode is electrically off. At an On/Off
ratio of 20,000, the ability of a camera with conventional photocathode (only)
gating to perform certain kinds of measurements in the UV is adversely affected.
MCP bracket pulsing
1
keeps the MCP biased OFF except for an interval that brackets the
timing of the photocathode gate as shown in Figure 6-1.
Figure 6-1: Timing Diagram: MCP Bracket Pulsing
For emitted photoelectrons to be accelerated in the MCP, the MCP must be biased ON. In
conventional intensified cameras, the MCP is biased ON continuously. In PI-MAX4,
however, when bracket pulsing is ON, the MCP is biased OFF until just before the
photocathode is gated ON and is biased OFF shortly after the photocathode is biased OFF.
By bracket pulsing the MCP off (in addition to photocathode gating,) the on/off ratio of the
Gen II PI-MAX4 in UV is improved by two to three orders of magnitude. The resulting UV
ratio exceeds even the high levels normally achieved in the visible. Applications that benefit
from this new approach include LIF and nanosecond pump-probe experiments.
NOTE:
Bracket pulsing does not help in the visible region. Under
extremely low duty-factor conditions, the only remedy is to
install an external shutter ahead of the camera.
1. Bracket pulsing is not available for cameras having a filmless Gen III Intensifier. Gen III Intensifiers do not
respond in the UV.
Off
On
Off
On
OffMCP Gating
Photocathode Gating
~1 μs
Off
4411-0137_0047