Mcp Bracket Pulsing; Introduction - Princeton Instruments PI-MAX System Manual

Chapter 7

MCP Bracket Pulsing

Introduction

The principal utility of 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. The limit on this technique is set by the light leakage of the intensifier, which,
although it is very good, is not without limit. In the visible, the on/off ratio of a typical Gen II
Intensifier with just the photocathode gated is excellent, typically between 10
most measurements, this ratio is sufficient to assure that the signal reaching the CCD during
the intensifier Off times will be too small to affect the data.
Below 350 nm, however, a second leakage mechanism occurs, optical leakage through
the photocathode to the UV-sensitive MCP, which reduces the On:Off ratio to about 2 ×
4
10 at 200 nm. 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
timing of the photocathode gate as shown in Figure 34. 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 the PI-MAX, however, when bracket
pulsing is ON, the MCP is biased OFF until just before the photocathode is gated ON.
The MCP is then biased ON and remains biased ON until just after the photocathode is
biased OFF.
Traditionally, intensified detectors discriminated against background signal by gating the
photocathode. Although this technique yields very high peak Off/On ratios, on the order
of 5 × 10
factor measurements, particularly in the UV region where the rejection is only ~10
By bracket pulsing the MCP off (in addition to photocathode gating), the on/off ratio of
the PI-MAX in UV is improved by 2-3 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.
*
Bracket pulsing is not available for cameras having a Gen III Intensifier. Gen III Intensifiers do
not respond in the UV.
Gated Operation with a PTG
*
keeps the MCP biased OFF except for an interval that brackets the
Figure 34. Timing: Bracket Pulsing
6
:1 in the visible, background signal can still prove troublesome in low-duty
85
6 7
and 10 . In
4
:1.