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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
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 a known electrical signal through the
input and testing for the proper output signals with
an oscilloscope as specified in steps 1 through
10.
1. Furnish a voltage pulse to the Test connector,
as outlined in Section 3.5. The polarity of the test
pulse signal should agree with the expected
signal input polarity from a detector.
2. Using a calibrated pulser, the 142AH E output
should be inverted from the input polarity and
should have a nominal scale factor of 45 -mV
output per 1-MeV equivalent energy (Si). The T
output should have the same polarity as the
inputs with a scale factor of about 20% less than
the signals through the E output.
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
capacitor of equal value connected to the Input
connector. This is necessary because the noise
contribution of the preamplifier is dependent upon
input capacity, as can be seen from the noise
specifications given in Section 2. The only
meaningful statement of the noise level of the
preamplifier 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
main amplifier system.
The noise performance referenced 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 472A Spectroscopy Amplifier on
which the time constants have been set 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 input
attenuators, if any, on the main amplifier are set
for minimum attenuation.
5. If a multichannel analyzer is used following the
main amplifier, testing of the noise performance
can be accomplished by merely using a calibrated
test pulse generator with charge terminator, as
outlined in step 1. With only the charge terminator
connected to the Input of the 142, the spread of
the pulser peak thus analyzed will be due only to
the noise contribution of the preamplifier and
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 the use of a
wide-bandwidth rms ac voltmeter such as the
Hewlett-Packard 3400A, reading the main
amplifier output noise level and correlating with
the expected pulse amplitudes per keV of signal
under the same conditions. Again, a calibrated
test pulse generator is required for an accurate
measurement.
