ORTEC 109A - Installation Instructions; Connection to Detector; Connection to a Shaping Main Amplifier; Input Power

3. INSTALLATION INSTRUCTIONS
3.1 Connection to Detector
A direct connection with shielded coaxial cable should be
made between the detector and the BNC connector labeled
DETECTOR on the front panel.
The performance of the 109A Preampl ifier, like that of all
other such low-noise nuclear amplifier, is degraded as the
capacity at the input of the amplifier increases; for this
reason, it is important that the length of coaxial cable used
between the amplifier and the detector be kept at the
minimum necessary. Also, it is preferable to use 93- or
100J2 impedance cable rather than 75- or 5012 cable, since
the capacity per foot is less for the higher impedance
cable. Type RG-62/U cable, which has 9312 impedance
and a 13.5-pF/ft capacity, is recommended. Type UG-
260/U connectors fit both this cable and the BNC input
connector. Microdot cables and fittings of the series
stocked and suppl ied by ORTEC are also suitable. The
cable is of 10012 impedance, 13 pF/ft. An adapter, ORTEC
No. C-17, may be used on the input BNC connector to
permit use of the Microdot cables. See ORTEC 1970
Nuclear Price List for a complete l isting of compatible
Microdot cables, connectors, and adapters.
Once the input cable has been installed, the electronic noise
performance of the preamplifier can be predicted by
calculating the cable capacity from the above information,
adding the capacity expected from the detector, and
referring to the table of typical performance versus input
capacity (Section 2).
3.2 Connection to a Shaping Main Amplifier
The preampl ifier can be used to drive long 9312 l ine to a
shaping main ampl ifier and is designed to be directly com
patible with the ORTEC transistor main ampl ifiers. It can
be used with any shaping main amplifier if a power supply
is used to power the preamplifier.
3.3 Input Power
Power for the 109A is supplied through the Amphenol
connector (17-20090) on the rear of the chassis. Power may
be supplied by a single 45-V battery with a tap at 22.5 V
(the tap is used as ground, providing -t22.5 V and -22.5 V;
current drain is 36 mA), or any well-filtered power supply
such as the ORTEC 115 Preampl ifier Power Supply that
furnishes both +24 V and -24 V.
If the preampl ifier is used with ORTEC transistor main
ampl ifiers, its power will automatically be supplied from
the main ampl ifier via the interconnecting cable suppl ied
with the 109A.
3.4 Test Pulse
A voltage test pulse can be inserted at the TEST PULSE
connector on the rear of the 109A without the use of an
external charge terminator, since the preamplifier has a
built-in charge terminator. The shape of this voltage pulse
must have a fast rise (less than 10"® sec) followed by a slow
exponential decay back to the basel ine (2 to4 x 10"^ sec).
The input ampl itude can be set to any desired level with
the knowledge that 46 mV amplitude at the TEST PULSE
connector is equal to approximately 1 MeV energy loss in a
silicon detector.
Also, the test pulse can be inserted into the DET. INPUT
connector simultaneously with an operating detector by
using an external charge terminator, provided the charge
terminator will withstand the detector bias voltage.
NOTE; In most experimental situations the optimum
signal-to-noise ratio occurs with the preampl ifier gain
switch in the XI0 position. When this switch is in the XI
position, the signal-to-noise ratio at the preamplifier output
is the same as that of the XI0 position, but the signal level
is only 1/10. At this low signal level, main amplifier noise
contribution becomes more significant. The signal-to-noise
ratio at the output of the main amplifier can be degraded
by the amount of the main ampl ifier noise contribution,and
resolution somtimes suffers.
4. OPERATING INSTRUCTIONS
4.1 Detector Bias
Detector bias is appl ied to the Preampl ifier through the
high-voltage SHV connector.
NOTE: The detector bias components and connectors are
rated and tested at 1000 V. Inside the 109A the detector
bias is appl ied to the DET. INPUT connector through a
decoupling network consisting of R1 (1M12), R2 (100M12)
and C1 (1500 pF) so that the total resistance in series with
the detector bias is approximately 100M12. This resistance
must be taken into account when considering the bias
voltage actually appl ied to a leaky detector. The voltage
dropped in this network wil l be 100 V for each microampere
of detector leakage current. It will be necessary to increase
the bias supply output voltage to compensate for this drop.
Example: It is desired to operate a detector at 100 V bias.
Detector leakage at 100 V = 0.5 fxA. The drop
in the bias network = (100 x 10®) (0.5 x 10'®) =
50 V. Voltage required at bias supply =
100 -I- 50 = 150 V.