10
In this method the preamplifier and main amplifier
The significance of this noise width in evaluating
are set up as they would be used normally, but
the detector is subject to interpretation, but
with a dummy capacitor (or no capacity) on the
generally the actual resolution of the detector for
Input connector of the 142AH, and with the ac
protons or electrons will be approximately the
voltmeter connected to the main amplifier output.
same as the noise width; the resolution of the
The noise voltage indicated on the meter,
detector for alpha particles will be poorer than the
designated E
, is read and noted. Then a test
noise width. The most useful application of
rms
pulse of known energy, E (in keV), is applied to
determining the noise width of a detector is in the
in
the Input and the amplitude of the resulting output
occasional monitoring of this quantity to verify
pulse, E is measured in volts with an
that the detector characteristics have not
out
oscilloscope. The noise spread can then be
undergone any significant change during use.
calculated from the formula
where E
is output noise in volts on the 3400A
on the 142AH and use an oscilloscope with a fast
rms
meter, E is input signal in keV particle energy,
rise time (1 ns if possible). The rise time of the
in
and E is output signal in volts corresponding to
preamplifier can then be computed by:
out
the above input. If the gain of the shaping
amplifier is adjusted so that the output pulse
(Total rise time) = (Preamp rise time)
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 ) = (N ) + (N
) .
tot
det
amp
2
2
2
9. Each quantity is expressed in keV FWHM. The
quantity N is known as the "noise width" of the
det
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.
10. Use an ORTEC 419 Precision Pulse
Generator with a matched charge termination to
measure the rise time of the 142AH through the T
(timing) or E (energy) output. Connect the 419
output through the charge terminator to the Input
2
2
+ (Pulser rise time) + (Oscilloscope rise time) .
2
2
The rise time of the 419 is typically 3 ns.
5.2. CLEANING
If it is necessary to clean the components and/or
the printed circuit in the 142AH at any time, use
only methanol as a cleaning solvent. Do not use
compressed air or other source of pressurized gas
unless it is known to be clean and free of
compressor oil, and do not use any cleaning
agent other than methanol.
5.3. FACTORY REPAIR
This instrument can be returned to ORTEC for
service and repair at a nominal cost. Our
standard procedure for repair ensures the same
quality control and checkout that are used for a
new instrument. Always contact the Customer
Service Department at ORTEC, (865) 482-4411,
before sending in an instrument for repair to
obtain shipping instructions and so that the
required Return Authorization Number can be
assigned to the unit. Write this number on the
address label and on the package to ensure
prompt attention when it reaches the factory.
