Specifications

Instrument Setups
34
PERIODS=10. Start a scan by pressing the
START key. The counter should cycle through 10
count periods separated by the dwell time (1 s).
The display shows the current scan position in
brackets. Reset the scan using the STOP key.
Start another scan with the START key but press
the STOP key before all 10 count periods are
completed. This pauses the scan at the last
completed count period. START will resume the
scan and STOP will reset the scan.
In general, if the START key doesn't start a count
period it is because the counters are paused at the
end of a scan. In this case, the SCAN led will be
off and the N PERIODS display shows that the
programmed number of points has been taken. A
STOP is required to reset the scan.
Pressing the STOP key twice in succession always
resets the counters.
2 PHOTON PILE UP CORRECTION
If the pulse rate from the PMT is high enough,
significant number of 2 photon pulses will occur.
These pulses are larger than single photon pulses
and result when 2 photons hit the cathode at or
near the same time. A single discriminator level
will count 1 and 2 photon pulses equally giving an
erroneous count. By using 2 discriminators, the 2
photon pulses can be counted correctly.
Set both A and B inputs to INPUT 1 and apply the
signal to INPUT 1. Select the COUNT mode A+B
FOR T PRESET. The count will now be the
number of pulses that exceed the A DISC level
plus the number that exceed the B DISC level. Set
the A DISC level below the 1 photon level and the
B DISC level above the 1 photon level but below
the 2 photon level. 1 photon pulses will be counted
only by A while 2 photon pulses will be counted
by both A and B. A+B will be the number of 1
photon pulses plus twice the number of 2 photon
pulses which is the correct number of photons.
GATED PHOTON COUNTING
When the photons of interest result from a source
such as a pulsed discharge or laser, gated photon
counting may be required to discriminate against
photons which occur outside the time interval of
interest.
Assume that the pulse rate is a function of time as
shown below. The time scale may be nanoseconds
up to hundreds of milliseconds. The A counter will
be used to count the signal via INPUT 1.
Count Rate
Time
Trigger
Gate Width
Gate
Delay
Signal
By using A GATE, shown as the shaded region, to
enable the counter, the count rate versus time may
be measured. Only those photons which occur
within the gate time are counted. By scanning the
gate between count periods, the entire time
behavior is recovered. Set the T input to TRIG to
enable an equal number of gates during each count
period.
If background subtraction is required, the B
counter can be used along with the A counter. The
B input is INPUT 1 and the B DISC is set to the
same level as A DISC. Adjust B GATE to be
equal in width to A GATE and position it at a
FIXED delay either before the signal or after the
signal to count the background rate. Using the
COUNT mode A-B FOR T PRESET, the
background counts will be subtracted from the
signal counts.
SYNCHRONOUS OR "CHOPPED"
PHOTON COUNTING
In noisy environments or when the signal count
rate is very low, a mechanical light chopper may
be used to improve signal to noise. This is very
similar to lock-in detection of analog signals. Two
gated counters are used, one counts the signal plus