Teledyne Princeton Instruments PI-MAX 3, Системное руководство

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128 PI-MAX
®
3 System Manual Issue 2
For this mode, the following parameters are automatically configured by the
application software and cannot be changed:
• Exposure Time is set to 0;
• Continuous Cleans is disabled;
• Shutter Control is set to Disabled Opened.
9.3 MCP Bracket Pulsing
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
6
and 10
7
. In 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 x 10
4
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
1
keeps the MCP biased OFF except for an interval that brackets the
timing of the photocathode gate as shown in Figure 9-1 . 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-MAX3, 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.
Figure 9-1: Timing: Bracket Pulsing
Table 9-1: Timing Mode, Shutter Control, and Ext. Trigger Input when Using the
Internal Timing Generator
Timing Mode Shutter Control External Trigger Input
Controlled by Internal Timing
Generator
Controlled by Internal Timing
Generator
Trigger In BNC
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
Off MCP Gating
Photocathode Gating
~1 μs
Off
4
4
11
-01
29
_005
0