7
5. MAINTENANCE INSTRUCTIONS
5.1. TESTING PERFORMANCE
As ordinarily used in a counting or spectroscopy
system, the preamplifier is one part of a series
system involving the source of particles to be
analyzed, the detector, the preamplifier, the main
amplifier, and the pulse height analyzer. When the
proper results are not being obtained and tests for
proper performance of the preamplifier and the
other components are indicated, it is important to
realize that rapid and logical testing is possible only
when the individual components are separated from
the system. In proving the performance of the
preamplifier, it should be removed from the system
and be dealt with alone by providing known
electrical signals through the input and testing for
the proper output signals with an oscilloscope as
specified in the following steps:
1. Furnish a voltage pulse to the Test Pulse
connector as outlined in Section 3.4. The polarity of
the test pulse signal should agree with the expected
signal input polarity from a detector.
2. Using a calibrated pulser, the 142IH output,
either E or T, should be inverted from the input and
have a nominal scale factor of 45 mV per 1 MeV
equivalent energy (Si).
3. The noise contribution of the preamplifier may be
verified by two basic methods. In either case, the
normal capacity of the detector and associated
cables should be replaced by a capacity of equal
value placed across the Input connector, and no
bias should be applied. This is necessary because
the noise contribution of the preamplifier is
dependent upon input capacity. The only meaningful
statement of the noise level is one that relates to the
spread caused by the noise in actual spectra. This
can be measured and expressed in terms of the full
width at half maximum (FWHM) of a monoenergetic
signal after passing through the preamplifier and the
main amplifier in the system.
The noise performance referenced in the
Specifications in Section 2 is stated in these terms,
and verification methods will be described. If
desired, the preamplifier can be tested with no
external capacity on the Input connector, in which
case the noise width should be approximately that
shown for zero external capacity. In any case, the
Input connector and capacitors, when used, should
be completely shielded electrically. A wrapping of
aluminum foil around the Input connector or a
shielding cap attached to the connector will suffice
for testing at zero capacity.
4. The preamplifier must be tested in conjunction
with an associated main amplifier that provides the
required pulse shaping. The typical noise
performance given in Section 2 is obtained using an
ORTEC 572 Spectroscopy Amplifier on which 2-µs
time constants have been selected as specified. For
comparison of these tabulated values, it is
preferable to test the preamplifier under identical
pulse-shaping conditions. It is also important to
ensure that the noise level of the input stage of the
associated main amplifier does not contribute
materially to the total noise. This is usually no
problem provided that the input attenuator, if any,
on the main amplifier is set for minimum
attenuation.
5. If a multichannel analyzer is used to measure
the main amplifier output pulses, testing of the
noise performance can be accomplished by merely
using a calibrated test pulse generator with a
charge terminator. With only the charge terminator
connected to the 142IH Input, the spread of the
pulser peak thus analyzed will be due to only the
noise contribution of the preamplifier and the main
amplifier. The analyzer can be calibrated in terms
of keV per channel by observing two different
pulser peaks of known energy, and the FWHM of a
peak can be computed directly from the analyzer
readout.
6. It is also possible to determine the noise
performance of the preamplifier by use of a wide
bandwidth rms ac voltmeter such as the Hewlett-
Packard 3400A, reading the main amplifier output
noise level and correlating it with expected pulse
amplitudes per keV of signal through the input
under the same conditions. Again, a calibrated
pulse generator is required for an accurate
measurement.
In this method the preamplifier and main amplifier
are set up as they would be used normally, but with
a dummy capacitor (or no capacity) in the Input
connector of the 142IH, and with the ac voltmeter
connected to the main amplifier output. The noise
level indicated on the voltmeter, designated E
rms
, is
then read and noted. Then a test pulse of known
energy, E
in
(in keV), is applied to the input and the
Содержание 142IH
Страница 6: ...vi...
