6
Fig. 4.1. Simplified Block Diagram of the ORTEC 142AH Preamplifier.
The corona blower that is furnished with the
completely. The stray capacity is about 0.1 to 0.2
142AH can be used to flush the interior connector
pF. If less conversion gain is desired, the value of
surfaces with air when corona effects are
the feedback capacity can be increased, but this
observed. It can be used on both the Input
may affect the stability of the preamplifier. The
connector on the preamplifier and on the
maximum recommended additional capacity is
connectors of the cables that are to be attached.
1.5 pF.
It can also be used to clean the connectors at the
detector end of the input cable. Simply insert the
The energy output signal from the preamplifier is
glass tube into the connector and squeeze the
a fast-rise-time voltage step with an exponential
bulb to blow air into the connector.
return to the baseline is about 500 µs. The
With the protection circuit in, the input cable can
the signal polarity at the detector output. When
be removed and reconnected without catastrophic
the (normal) positive bias polarity is used for the
damage to the preamplifier, but the user must be
detector, the detector output pulses are negative
cautious to prevent touching the interior of the
and the E output of the preamplifier is positive, as
connector with anything other than a good
shown in Fig. 4.2.
insulator, such as the glass tube of the blower;
lethal high voltages are present on the center pin
of the Input connector under these conditions.
4.3. ENERGY OUTPUT
The charge-sensitive loop is essentially an
differentiation removes low-frequency noise for
operational amplifier with a 1-pF capacitive
better timing results. Due to the differentiation of
feedback. The conversion gain is nominally 45
the charge loop output, any overshoot present will
mV/MeV, and can be increased by decreasing the
appear to increase the rise time of the timing
value of the feedback capacity but a subsequent
output as shown in Fig. 4.3. This, however, does
increase in rise time will result. The upper limit
not affect typical timing experiment results since
on the conversion gain is the stray capacity in the
it is the initial slope of the waveform that carries
circuit with C4, the 1-pF capacitor, is removed
the information of importance in timing. Similarly,
polarity of the E output signals is inverted from
4.4. TIMING OUTPUT
As indicated in Fig. 4.1, the T output from the
preamplifier is a transformer differentiated and
inverted version of the E output. This
