PCM-2 Manual.pdf - Voss Associates

Setup and Calibration
High voltage is adjustable in 255 steps of slightly less than 10 volts per step. An
adjustment potentiometer is provided for more precise calibration of the
(nominally) 1000: 1 voltage divider.
Threshold Values
A 2-by-6 pin header located directly behind 11 configures the modular detector
board for operation or calibration. Placing a shorting jumper in position #1 (the
leftmost pair of pins) instructs the microprocessor to operate as a normal detector
board, while position #2 designates a hand probe board. Removing the jumper
entirely selects high voltage calibration mode, and jumpering position #6 disables
the high voltage power supply entirely. Positions 3, 4 and 5 should not be
jumpered. Jumper settings are read only when power is first applied to the board.
If the board is powered up with no jumper installed, the power supply will be set
for 1250 Volts. The actual output may be adjusted to this nominal value by
potentiometer RIO; this is the only calibration required. Use a high impedance
voltage divider probe to measure the output voltage, since the supply has an
impedance of one megohm.
Operation of the two amplifier stages may be monitored at test points 5 and 6,
respectively. DC levels at both of these points should be between 0.5 and 1.5
volts. Alpha and beta one-shot outputs are present on pins 2 and 3 of 12. If a
pulse generator is connected to the detector input, note that the input impedance of
the amplifier is low and may load the generator output significantly. As
mentioned above, placing a jumper in position #6 will disable the high voltage
supply for technician safety.
Alpha and beta pulse-height discriminator thresholds default to values of 45 % and
4.8 %, respectively. These levels may be changed via the detector parameter
menu, however this capability should be used only after careful evaluation. The
modular detector board is intended to be used with several types of radiation
detectors; its full adjustment range is not intended for anyone type.
Increasing the beta threshold will rapidly reduce detector response to low-energy
(beta and gamma) particles, both from contamination sources and background.
This will result in decreased beta/gamma detection efficiency, however
background count rates may actually decrease faster than those from
contamination.
The default alpha threshold was selected to almost eliminate alpha counts due to
beta particles. Lowering this value will increase alpha particle detection
efficiency, however false alpha-channel alarms may be caused by higher energy
sources. Raising the alpha threshold will slightly reduce the number of beta
PCM2.MANI REV AI April 1995 13-3