Auxiliary Circuits
The calibration pulser input includes potentiometer RV10 which is factory set so that
the output pulse amplitude represents the energy displayed in MeV. Thus you can
pre-calibrate a region of interest without having to use an open source.
Detector current is filtered by C39, C301, C302, R1 and C38, then sensed by R191
which generates a voltage that is measured by the A/D. The result is displayed when
LEAKage Function has been selected by the front panel controls.
Following the charge integrator is a pole/zero compensated differentiator (C6, R20
and R21) set for a nominal 0.5
µ
s decay time constant. The time-clipped tail pulse is
amplified by a fixed gain non-inverting feedback amplifier formed by differential
pair Q17 and Q25, driver Q18 and push-pull outputs Q16 and Q15. The gain of this
amplifier stage is fixed at X30.
Preliminary integration is provided by R30, L2, and C21 before the signal is fed to
the split-load inverter Q21, which permits polarity selection by the internal jumper
plug J12. Q22 buffers the signal before the second integration in R39, L3 and C24.
The filtered signal is then amplified again by A25, which incorporates the front-
panel-mounted gain control in its feedback path. The signal is a positive-going pulse
at this point, and D4 limits the excursion by clamping the output of A25 to enhance
prompt overload recovery.
Capacitor C120 couples the positive pulse into the simple restorer provided by Q23.
This circuit stabilizes the dc reference level for the pulse in order to maintain a
proper baseline from which the output may be quantized in an ADC or SCA.
Output amplifier A26 provides the final stage of gain and permits the introduction of
a dc offset/zero-adjust, controlled by a front panel potentiometer. D5 limits the
positive excursion to prevent saturation in either A26 or output transistor Q24 and
insures prompt overload recovery.
The output is a unipolar semi-Gaussian pulse with a shaping time constant of 0.5
µ
s.
This integration has been found to be nearly optimum for the noise spectrum of
typical surface barrier alpha particle detectors to provide the best energy resolution.
Auxiliary Circuits
The auxiliary circuits include the power supplies for the detectors and preamp/amp,
the thermocouple vacuum gauge circuitry, the calibration pulser, discriminator and
A/D. As can be seen on the schematic, interlocking, switching, and sensing is
necessary for the proper integration of these subsections.
Power Supplies
The internal 12 V and 5 V supplies are derived from inductor-capacitor filters, three-
terminal regulators and Zener diode regulators. The detector high voltage bias is
derived from the 115 V ac supplied by the NIM power supply which is boosted by
transformer T1 then full-wave rectified, filtered by capacitor C3, and controlled by a
discrete component series regulator. The output polarity is selected by a board-
7401/7401VR Alpha Spectrometer User’s Manual - 9231603C
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