4 taking readings in surveillance mode, Taking readings in surveillance mode – BNC SAM 935 Portable Gamma Spectroscopy System User Manual

Page 28

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SAM 935™ Instruction Manual


Taking Readings in Surveillance Mode

The monitoring display of the SAM 935 will look different depending on the operating mode of the instru-
ment. The default operating mode for the SAM 935 is Surveillance mode, also called dose rate monitoring.
In Surveillance mode, the SAM 935 continuously takes readings and analyzes isotopes in dose rate units,
saving and reporting any alarm conditions (when the dose rate exceeds the dose rate trigger).


The blank Surveillance monitoring screen will look like this:


Point the detector at the sample to be monitored and watch the front panel display:


Press C


(F3) to save the currently displayed reading. The current alarm will be terminated

when you press C


. If the condition continues to alarm, a new alarm will be created and you

will again have the choice to capture it or not.

An alarm condition is triggered when any of the items being monitored exceeds the trigger level for at least
the On Hysteresis number of sample intervals. The trigger level can be dose rate or peak-to-background
ratio. The alarm condition continues until the radiation level drops below the trigger level for at least the Off
Hysteresis number of samples.

During an alarm, the SAM 935 acquires and stores an integrated spectrum for the entire alarm period. The
alarm level displayed for each monitored item is the highest level encountered during any time interval of
the alarm. This allows the SAM 935 to detect a transient trigger on one isotope that might otherwise be
masked by a longer alarm generated by another isotope.

If you suspect that isotopes are being reported incorrectly, you may want to temporarily
increase the sample count time to get more stable readings. If this does not resolve the issue,
re-do the coarse and fine calibrations. Note that if the Cs137 source is too “hot”, or if it is posi-
tioned at a distance from the detector, the X-ray peaks will not register correctly. The SAM
935 may think it is calibrated without using the X-ray peak. The other cause of incorrect iso-
tope reporting may be poor detector resolution, or the parameters in the Analysis Tools Setup
(energy match windows) may be adversely affecting the readings.