15
Note that the High Voltage has a maximum of 1200 volts. If a value higher than this is set, 1200 V will instead
be applied
The Set Threshold Screen
Figure 4(4)
The threshold sets the minimum size a detector pulse must be, in units of mV, in order to be registered as a
count. From the calibration Settings screen, tap the “Threshold…” button to open the Set Threshold screen.
Use the keypad to enter the desired threshold. Any integer value may be entered. The modified threshold
setting will not be applied until the “Set” button is tapped, at which point the display will return to the
Calibration Settings screen. Tap the “Back…” button instead to cancel any modified value and return to the
Calibration Settings screen.
The threshold can be set between 0 and 2000 mV . If a value higher than 2000 mV is set, 2000 mV will instead
be applied.
If the threshold is set too low, excessive counts (due to electronic noise) may occur. If it is too high, counts may
be missed. In gamma scintillation detectors, a threshold setting that is too high could also reduce the response
to low-energy gamma radiation. Generally, a setting of approximately 50 mV is appropriate.
The Set Dead Time Screen
Figure 4(5)
Dead time is used to correct count rates for pulses that are
“missed” due to pile up (in the case of scintillation detectors)
or from gas recombination time (in the case of GM detectors).
Dead-time correction keeps responses linear over the entire
useful range of radiation intensities. From the Calibration
Settings screen, tap the “Dead Time…” button to open the Dead
Time screen.
Use the keypad to enter the desired dead time. Any value,
including fractional, may be entered. The modified dead time setting will not be applied until the
“Set” button is tapped, at which point the display will return to the Calibration Settings screen. Tap the
“Back…” button instead to cancel any modified value and return to the Calibration Settings screen.
There are no restrictions on values to which the dead time can
be set. However, note that the absolute maximum count rate
that can be displayed is equal to 1/dead time.
Generally, scintillation detectors have dead times in the range of
1 to 4 µSec while GM tubes have dead times in the range of 10 to
100 µSec.
Setting the Dead Time to zero will disable dead-time correction.
Figure 4(5)
Figure 4(4)