Fluke Biomedical 943-35 - Manual (Page 17)

Maintenance, Calibration, and Troubleshooting
Calibration
4
4-3
4.2 Calibration
The 943 series Gamma Scintillation Detector is factory calibrated and is shipped with a calibration data
sheet. It is usually shipped as part of a monitoring system and the ratemeter or scaler (960 DRMS)
module high voltage supply, upper and low discriminator are adjusted for a specific response over the
entire operating range. With the factory calibration, a dead time correction factor (Tau) is also provided.
At regular intervals after initial installation of the detector, if the detector is replaced, if a dramatic change
in detector output is noticed, or if the power supply output voltage changes, calibration should be
repeated.
The count rate will increase and decrease in
conjunction with detector and pulse height voltage.
If the high voltage reaches 1000 VDC without
achieving the necessary
137 Cs pulse height, suspect
a defect in the detector, wiring, or readout device.
The 943-series Gamma Scintillation Detectors are calibrated by adjusting the high voltage supply to the
photomultiplier tube (adjusting photomultiplier tube gain based on the photons that impinge on the Nal
scintillator).
The detector provides a linear output for all levels of gamma radiation. Although the output is essentially
linear, at high counting rates the dead time of the crystal and preamplifier combination causes pulses that
are produced at the same time or very nearly the same time to appear as one pulse, reducing the actual
pulse output of the detector. Linearity can be improved by using a dead time correction. In digital systems,
an algorithm automatically compensates for dead time based on an operator-entered constant. Setting
the detector for the proper output count rate at any point in its operating range assures an accurate
measurement at any other point.
Victoreen detectors are factory calibrated to provide a fixed output count rate and pulse height over an
energy range of 60 keV to over 2 MeV. All Victoreen Primary calibrations are based on detectors that are
set-up to provide the same gain, using a master set of button sources and a standard geometry fixture
retained permanently at Fluke Biomedical. This permits the use of secondary, or transfer button sources,
to reset the high voltage (i.e. re-calibrate) in the field to obtain the same detector efficiency used in the
original detector/sampling geometry primary isotopic calibrations. Transfer button sources and standard
geometry fixtures are normally provided with Victoreen radiation monitoring systems. The factory
calibration procedure is contained in the each applicable system manual.
Calibration Where Calibration Data and Button Sources are Available
Calibration of the gamma scintillation detector requires a standard button source (s) of known gamma
activity and P/N 844-36 standard geometry. The button source is used to reproduce a known count rate
using the standard geometry.
The standard geometry is used to consistently position the detector and button source with respect to
each other. The detector should be connected to the readout device with the cable that will be in place
during normal operation (high voltage setting is slightly cable dependent). At the readout device set the
discriminator level to the level indicated on the calibration data sheet (normally 0.2 VDC).
1. Insert the detector into the cavity provided in the standard geometry. Allow detector to warm up for
approximately 1 hour.
2. Determine the background count rate by allowing the detector readout device to run for ten
minutes. Record this rate on the calibration data sheet. The following Max count rates should be
displayed on the readout device (detector dependent):
NOTE