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4.8
NBR (natural background rejection)
For this additional feature the ratio between the detected
gamma rays and the corresponding dose rate is used to indi-
cate unusual radiation conditions. Low and high energy radia-
tion changes the ratio between count rate and dose rate, which
is essentially stable for background radiation from Natural
Occurring Radioactive Material (NORM). If the ratio exceeds
the adjustable (via PC-program) threshold, an alarm is released
and it appears a message "low energy" for low energy radia-
tion, respectively "high energy" for high energy radiation, in
the top line of the normal display. The alarm tone and vibrator
can be acknowledged by a short key depression. The NBR-
indicator can be activated by the menu function or the PC-
program RadEye.exe.
The purpose of this additional alarming algorithm is to allow
the setting of a significantly lower detection level (compared
the gross count rate alarm) for a vast number of artificial
gamma isotopes: Especially in an urban environment the
background count rate varies from a few cps up to 60-100cps
(~20 µR/h natural gamma background). Thus operating on a
Alarm 1 level of less than 60cps, natural background varia-
tions can cause a lot of annoying false alarms. Using the NBR
feature however, radiation fields with a energy distribution
that is sufficiently different from the natural environment can
trigger a alarm at 30cps or even below.
A typical detection limit for a background of 5 µR/h is about
0.4 µR/h for Am-241 and 0.8 µR/h for Co-57.
RadEye PRD-ER
DB-066 E
4-7
Iw/Tr/Ff 06.07.2015
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