Chapter 4 Unit Setup
82
Plot Data
Collect and Dwell Time: Respective times are shown underneath the plot.
Board ID: Shows the type of Detector Board being examined for that zone.
Discriminator Voltage and Count Rate and DAC (value): Respective times are
shown underneath the plot.
High Voltage: Respective voltages are shown underneath the plot.
ULD: Respective voltage is shown underneath the plot.
F1 – Optimize Gamma HV
As mentioned before the default High Voltage for the detectors is 650 V. This
ensures some counts from the detector but does not guarantee optimum performance.
Higher voltages result in higher PMT gain, which amplifies the signal but also
increases the background counts and signal to noise ratio. The typical operating
voltage is usually between 500V and 1000V. On factory calibrated units the high
voltage is set to give approximately 100-200 cps, for large area Gamma detectors
with 25 mm (1 in.) lead shielding installed, assuming a normal background of 5 to 10
µR/hr.
To determine the high voltage setting that results in optimum signal to noise ratio
(including the background) for PMT based Gamma scintillator detectors a Figure of
Merit (FOM) technique is recommended.
Since the technique makes use of the level of ambient background radiation, it should
be performed only after the unit has been placed in its final location with lead
shielding installed. In addition, if an optional second layer of lead was purchased it
should also be installed before performing the Optimize Gamma HV procedure.
Refer to Equation 1, below.
The lower level discriminator (setup procedure described earlier) should also be set
(preferably to the default value) before optimization of the high voltage (as it affects
the results). The Figure of Merit (FOM) is given by the mathematical expression
below:
Equation 1 Figure of Merit (FOM)
(1)
…where, R
b
= Background count Rate, R
g
= Gross (source + background) count rate.
The Optimize Gamma HV procedure consists of a number of test screens that are
accessed in sequence to facilitate collection of data for this technique.
The first screen is accessed by selecting Shift+F2 - Additional Test Menu (Figure 40
on page 73) on the Service Menu (Figure 38 on page 72) and then F1 – Optimize
To Next Page
Loading...
Need help?
General
