9
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 input signal under the
same conditions. Again, a calibrated test 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) on the Input
connector of the 142, and with the ac voltmeter
connected to the main amplifier output. The noise
voltage indicated on the meter, designated E
rms
, is
read and noted. Then a test pulse of known energy,
E
in
(in keV), is applied to the Input and the
amplitude of the resulting output pulse, E
out
is
measured in volts with an oscilloscope. The noise
spread can then be calculated from the formula
where E
rms
is output noise in volts on the 3400A
meter, E
in
is input signal in keV particle energy, and
E
out
is output signal in volts corresponding to the
above input. If the gain of the shaping amplifier is
adjusted so that the output pulse height is 2.35 V for
an input of 1 MeV equivalent charge, then the rms
meter will be calibrated directly in energy (1 mV =
1 keV).
7. The noise performance of the preamplifier, as
measured by these methods, should not differ
significantly from that given in the specifications in
Section 2.
8. lf, during testing of the preamplifier and detector,
the noise performance of the preamplifier has been
verified as outlined in the preceding section or is
otherwise not suspected, a detector may be tested
to some extent by duplicating the noise
performance tests with the detector connected in
place and with normal operating bias applied. The
resulting combined noise measurement, made
either with an analyzer or by the voltmeter method,
indicates the sum in quadrature of the separate
noise sources of the amplifier and the detector. In
other words, the total noise is given by (N
tot
)
2
=
(N
det
)
2
+ (N
amp
)
2
.
9. Each quantity is expressed in keV FWHM. The
quantity N
det
is known as the "noise width" of the
detector, and is included as one of the specified
parameters of each ORTEC semiconductor
detector. By use of the above equation and with a
knowledge of the noise of the preamplifier, the
noise width of the detector can be determined. The
significance of this noise width in evaluating the
detector is subject to interpretation, but generally
the actual resolution of the detector for protons or
electrons will be approximately the same as the
noise width; the resolution of the detector for alpha
particles will be poorer than the noise width. The
most useful application of determining the noise
width of a detector is in the occasional monitoring of
this quantity to verify that the detector
characteristics have not undergone any significant
change during use.
10. Use an ORTEC 419 Precision Pulse Generator
with a matched charge termination to measure the
rise time of the 142 through the T (timing) or E
(energy) output. Connect the 419 output through the
charge terminator to the 142 Input and use an
oscilloscope with a fast (1-ns if possible) rise time.
The rise time of the preamplifier can then be
computed by:
(Total rise time)
2
= (Preamp rise time)
2
+ (Pulser rise time)
2
+ (Oscilloscope rise time)
2
.
The rise time of the 419 is typically 3 ns.
