Model 4525 Gate Monitor
Operator’s Manual
Ludlum Measurements, Inc.
Page 8
May 2018
level varies as the inverse square of the distance from the source. For
example, a source of a certain size will read a certain amount at a
distance of one foot. If the distance is doubled (two feet from the
source), the reading will only be 25% of that obtained at one foot.
2.
The more time the detectors have to look at the source, the better the
reading. For this reason, the slower the vehicle passes through the
Radiation Portal Monitor (RPM), the better the chance of seeing a
potential source.
3.
The amount of scrap and its density play a large role in the detectors’
ability to see a potential source. The more material there is between the
source and the detectors, the harder it is for the detectors to see it. The
higher the density, the harder it is for the detectors to see it as well.
4.
The larger the detector, the more sensitive it will be.
5.
A well shielded detector will be able to detect smaller changes in
radiation levels than an otherwise equivalent unshielded detector. By
shielding the sides and back of the detectors, we are able to block out a
large part of the background radiation, thus increasing the sensitivity of
the detector to the small changes in radiation levels caused by a source.
When taking all of the above circumstances into consideration it can be
determined that the best way to monitor loads is to have large detectors as close
to a lightly packed small load that is moving through the system as slowly as
possible. Another factor to consider is that the system should be able to monitor
background radiation at all times and make adjustments to the alarm to keep it
as close to the background as possible without causing a false alarm.
Naturally Occurring Radioactive Material (NORM) is a big problem. Oilfield
pipe, hot water heaters, and industrial piping will sometimes contain scale that is
radioactive. Most alarms are the result of NORM. Medical tests that use a
radioactive dye or tracer are yet another problem. Patients can be released from
the hospital reading several thousand µR/hr or less than 100µSv/hr, and set off
RPMs 30 m (100 ft) away. A third problem is that of radiographers and certified
welders who use a powerful radiation source to check their material or welds for
cracks. This radiation is powerful enough to set off RPMs 3 km (2 miles) away.
When installing your RPM, check with your neighbors and ask them to point
their source(s) in another direction.