Fluke Biomedical 942A-200L-M4 - Manual (Page 46)

942A-200L-M4 & 942A-200L-M5 UDR
Operator Manual
3-6
3.4 Operational Modes
Normal Operation, Rate, Gross Mode
The UDR is in normal operation when the UDR function switch (see "Parameter Entry" Section 3.11) is in
any position except 8, the Trip Test function is not active and the Check Source button is OFF. During
normal operation, the display shows the radioactive activity data received from the detector in counts per
minute (CPM). The bargraph will also show the present CPM value. The color of the illuminated
segments is green when the CPM rate is below the WARN setpoint, amber when the CPM rate is above
the WARN setpoint and below the HIGH setpoint, and red when the CPM rate is above the HIGH
setpoint. The analog outputs are active while the UDR is in RATE, GROSS MODE and all alarms are
enabled. In GROSS MODE, all of the detector pulses that fall between the Low and High discriminator
are counted by the UDR.
Normal Operation, Rate, Single Channel Analyzer (SCA) Mode, 942A-200L-M4 only
The Models 942A-200L-M4 is equipped with a Single Channel Analyzer (SCA) function. The SCA
function is provided to permit monitoring of a specific radioactive isotope, as identified by the energy of
the gamma ray emitted as the radioisotope decays. By using a gamma scintillation detector with a Nal
crystal and a photo multiplier tube, a linear relationship between the energy of the gamma ray and the
voltage of the pulse produced in the detector may be established. The result is a pulse produced by the
detector is proportional to the energy of the gamma ray emitted by the radioisotope. During normal
operation in the SCA mode, the display shows only the detector pulses for the isotope of interest in
counts per minute (CPM). The bar graph will display the CPM value for Gross detector output. The color
of the illuminated segments will be green when the CPM rate is below the WARN setpoint, amber when
the CPM is above the WARN setpoint and below the HIGH setpoint, and red when the CPM rate is above
the HIGH setpoint. The analog outputs are active while the UDR is in RATE, SCA mode, and all alarms
are active. Only the detector pulses that fall between in the SCA window are displayed by the UDR.
To operate in the SCA mode, the Model 942A-200L-M4 is provided with a Model 942-200-90-M1 SCA
option circuit board. The 942-200-90-M1 provides an electrically adjustable voltage window that will count
only the pulses that fall into the voltage window established. By knowing the energy of the isotope of
interest and the gamma energy to voltage relationship of the detector, the appropriate window voltage
may be calculated and adjusted to respond to the gamma radiation emitted in the decay of a specific
radioisotope. The 942-200-90-M1 is provided with potentiometers to adjust the peak voltage and window
width as required to detect the presence of a specific isotope. When the SCA circuit is operating, the UDR
is said to be operating in the SCA mode. When the SCA circuit board is not supplied, or the SCA board is
not active, the UDR will respond to pulses over the full range of the gamma detector, approximately 70
KeV to 2.5 MeV. This is referred to as the Gross counting mode of operation, or Gross mode.