4
3.5. TEST PULSE
3.7. CORONA ELIMINATION
A voltage test pulse for energy calibration can be
Because the normal range of bias voltages that
accepted through the Test input connector on the
are used for the detectors appropriate to the
142AH without the use of an external charge
142AH are extremely high, dust particles that
terminator. The Test input of the preamplifiers
settle within the Input connector tend to cause
has an input impedance of 93
, and its circuitry
spiking when the high voltage is applied. The
provides charge injection to the preamplifier
effect can be observed directly at the E output
input. The shape of this pulse should be a fast
connector on the 142AH with no input signal.
rise time (less than 40 ns) followed by a slow
Figure 3.1 shows the typical output signal with
exponential decay back to the baseline (200 to
large noise spikes due to a "dirty" Input connector.
400 µs). While test pulses are being furnished to
This can usually be eliminated, so that the output
the Test input, connect either the detector (with
has an appearance like Fig. 3.2, by disconnecting
bias applied) or its equivalent capacitance to the
the input cable (with the protection circuit
Input connector on the 142AH.
installed) and blowing out the inner surfaces of
The Test input may be used in conjunction with a
is furnished with the 142AH. In particularly
pulser such as the ORTEC 419 or 448 to calibrate
stubborn cases, an orderly procedure must be
the preamplifier E output amplitude in terms of
used to determine the cause of spiking; a
energy or for multichannel analyzer calibration.
recommended procedure is as follows:
However, due to stray coupling between the test
circuit and other portions of the preamplifier
1. With no cable connected to the preamplifier
circuitry, the transient performance of the
Input, operate the preamplifier at the desired bias
preamplifier is best determined by connecting the
voltage while observing the E output, through a
actual detector signal through the Input connector
shaping amplifier, with an oscilloscope.
instead of using the pulse generator signals.
A voltage test pulse for transient response in the
connector with the blower several times until the
142AH can be accepted through a charge
spiking disappears.
terminator and into the Detector Input connector.
If external capacitance is to be included for these
3. Attach the input cable from the detector. If
tests, an SHV tee can be inserted between the
spiking appears, disconnect the cable and blow
Input connector and the charge terminator, and
out the inner surfaces of the high-voltage
this will then accommodate the test capacitances.
connectors. Then attach the cable again and
Do not furnish any bias during these tests.
check for spiking.
3.6. DETECTOR BIAS INPUT
Operating bias for the detector is supplied to the
Bias connector on the 142AH and through a filter
and large bias resistance to the Input signal
connector. From there it is furnished out through
the signal input cable to the detector.
Connect a cable from the detector bias supply
(ORTEC 459 is typical) to the Bias connector on
the 142AH. The connectors used in this high-
voltage circuit are type SHV, and the mating
cable should be furnished with the bias supply
module
the SHV connector with the "corona blower" that
2. If spiking is present, blow out the Input
4. Apply this procedure to the connectors of each
individual cable section that has high-voltage
connectors.
5. In laboratory areas that are particularly dusty,
this procedure may have to be repeated several
times to completely eliminate all traces of spiking.
.
