4 preventing false alarms, Preventing false alarms – BNC SAM 935 Portable Gamma Spectroscopy System User Manual

Page 48

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SAM 935™ Instruction Manual

6.1.4 Preventing False Alarms

The Hysteresis items in the Monitor Setup menu allow you to mask out unnecessary alarms.

One of the most common “false alarm” scenarios occurs when a burst of radiation from a cosmic ray
causes a very short-term pulse of counts. To prevent this type of event from triggering an alarm, the SAM
935 has the ability to require that consecutive sample intervals all exceed a trigger level. Testing has
shown that false triggers can be almost entirely eliminated just by requiring that two consecutive time inter-
vals exceed trigger levels. You do this by configuring an On Hysteresis.

Hysteresis refers to the lag time between a cause and its result. On Hysteresis marks the start of a trigger
event, while Off Hysteresis determines how a trigger event is ended.



Set On Hysteresis on the Monitor Setup menu:


Use the


arrow keys to increase or decrease the value. This value will be the consecutive

sample intervals that must be exceeded before an alarm will be triggered.


Press E


when finished. C


will exit without changing the original value.

Another problem that can occur when the radiation source is very close to the trigger threshold is the gen-
eration of a stream of trigger events. The Off Hysteresis setting can help in this case.

Consider the case where the radioactive source is at a level where 80% of the time a trigger is generated.
This is always a possible situation since, even with a constant source, there is statistical variation in the
count rate.

In the example below, an X represents a trigger and a period represents a non-trigger. Consecutive trig-
gers are recorded as a single event. Thus, a source with an 80% trigger rate passing by might look like:

Sample Interval Results


On = 1, Off = 1

111 22 333 4 555 66 7 88 999

(9 events)

On = 2, Off = 1

11 2 33 44 5 6 77

(7 events)

On = 1, Off = 2


(1 event)

With an

On Hysteresis

of 1 and an

Off Hysteresis

of 1, the events generated would be as repre-

sented by the second line above. Note that 9 separate events are recorded, with the repeated numbers
representing consecutive triggers that are recorded as a single event. If On Hysteresis is increased to 2
and Off Hysteresis left at 1, then only 7 events are generated. The single-trigger intervals are eliminated.
When Off Hysteresis is set to 2, then the entire period is considered a single alarm.