Model 4525 Gate Monitor
Operator’s Manual
Ludlum Measurements, Inc.
Page 11
May 2018
Therefore, if this vehicle drives off at the same time that another
vehicle is within the portal, the radiation detector sees the increase in
background radiation, and alarms. Our recommendation is for
vehicles to stay out of the 3 m (10 ft) immediately before, after, and
behind the detectors. Vehicles should only enter the buffer zone
while driving through at a constant, slow speed.
NOTE:
Either of these first two alarms may be verified by looking
at the radiation profile of the vehicle. The first and last samples
should be very similar if the background radiation has not changed
while the vehicle was present.
3.
Voids
: If the scrap metal has been loaded unevenly on the trailer,
especially when dense bales are set on the axles with empty space
between axles, an alarm may occur when the radiation detector sees
the drop in radiation level as caused by the dense bale, and then sees
the increase in radiation level as the empty space is encountered.
Inspection of the scrap loading or screening with a portable detector
may allow this determination to be made. Our portal system can
avoid this problem if the bales are stacked on a flat-bed trailer, but
not if the bales are inside a roll-off trailer.
4.
Driver with Medical Tests:
If the radiation profile shows a large
increase at the very beginning of the vehicle, the driver should be
screened with a portable radiation detector to determine if he is
“hot.” This will not happen with X-rays, but with stress tests,
colonoscopies, and other tests involving radioactive “dyes.” These
radiation levels can be very big. A reading with a hand-held radiation
detector may be 1000 times the background reading. Such an
individual may be "hot" for weeks, with the radiation reading
declining a little each day.
5.
Drive-By Medical:
Many sites are located very close to a major
road, and the alarm may be triggered by drive-by vehicles with
people who have had medical tests. Again, because they are so "hot"
they only need to be within 46 m (50 yd) or so to set off the alarm.
Alternatively, a worker on-site will have a test of this nature, and the
alarm will be triggered intermittently until that worker is identified.
This kind of alarm may be very aggravating and difficult to diagnose.
6.
Welds
: Some welding rods are “thoriated,” usually containing about
2% thorium. Thorium has an isotope that is radioactive, and the
weld can thus be “hot.” This is also difficult to diagnose, but can be
done if the vehicle is surveyed with a portable detector once the load
has been removed from the trailer.
