Fluke Biomedical 943-25 - Manual (Page 14)

Victoreen 943-25, 943-26
Operators Manual
4-2
5. Replace the four screws at the connector end of the detector.
4.2 Calibration
The beta scintillation detector is calibrated by adjusting the high voltage supply to the photomultiplier tube
(adjusting detector sensitivity to the photons that impinge on the scintillation disc).
This family of beta scintillation detectors essentially provides a linear output for all levels of beta radiation
until the disc starts to saturate. Setting the beta scintillation detector for the proper output count rate at
any point in its operating range assures an accurate measurement of any other point.
The beta scintillation detector is factory calibrated. It is usually shipped as part of a monitoring system and
the ratemeter or scaler module power supply is adjusted for an accurate reading over the entire operating
range.
After initial installation, at regular intervals after installation, if the detector is replaced, or if the power
supply output voltage changes, calibration should be repeated.
4.3 Calibration Where Calibration Data is Available
Calibration of the beta scintillation detector requires a standard button source of known beta activity for
which a known count rate is recorded in a standard geometry. The standard geometry (test fixture, P/N
844-36) is used to position the detector and button source with respect to each other (Figure 4-1). The
detector should be connected to the readout with all the cable that will be in place during normal
operation since the high voltage setting is slightly cable dependent. At the readout, set the discriminator
level to the level indicated on the data sheet (normally 0.2 VDC).
1. Insert the detector into the cavity provided in the standard geometry.
2. Place the 9O Sr button source, blank side up (P/N 844-36-14), on the slide in the standard geometry.
3. The expected count rate for the button source should be determined. It is recorded on the
calibration data sheet. Correction must be made for decay. (The half-life of the source is 29.12
years.)
4. Adjust the high voltage power supply to the detector so that the net count rate corresponds to the
expected count rate calculated in Step 3.
The net count rate is the indicated count rate minus
the background count rate.
5. The count rate will increase and decrease in conjunction with power supply voltage. If the high
voltage reaches 2000 VDC without achieving the necessary count rate, suspect a defect in the
detector, wiring, or readout device.
NOTE