The majority of over
exposures in industrial radiography are the result of the radiographer
not knowing the location of a gamma emitter and failing to conduct a proper radiation survey. Exposure vaults are equipped with warning lights
and safety interlock switches which provide a margin of safety for workers. A survey must be performed occasionally to verify that vaults are not "leaking" radiation and that the safety devices are performing properly. However, when conducting radiography with gamma emitters in the field, the radiographer must rely heavily on measurements with a survey meter since other safety devices are uncommon. A series of surveys must
be taken and some of the results from these surveys must be documented when transporting and working with
gamma emitters in the field.
Approaching the Exposure Device
A technician should be thoroughly familiar with the operation of a survey meter since proper use of the device is essential. Before removing the exposure device (camera) from storage, the calibration of the survey meter
must be verified and the
battery level must be checked. When approaching the exposure device to remove
it from the storage location, the survey meter should be in hand and operational. The survey meter
should be placed next to the exposure device to verify that the source
is contained inside the projector, and to verify that the survey meter is
working properly. Survey meter readings should be compared to previous readings and
Transporting the Exposure Device
When transporting the exposure device, it must be stowed securely in the vehicle. A lockable metal box is often bolted
in the rear of the vehicle. A survey of the over pack, the outside
of the vehicle, and the drivers compartment is then conducted and documented.
Preparing for an Exposure
Once on the job site, the exposure area will be assessed, distance calculations made
for restricted area boundaries, and ropes and signs placed appropriately. Once this is complete, the radiographer
is ready to remove the exposure device from its storage compartment in the vehicle. The survey meter should be monitored as the storage compartment is approached and when removing the exposure device from the compartment. Daily safety checks should then be made. Once these checks are completed, the radiographer and assistant may then move the exposure device
to the exposure location. As the cranks and guide tubes are attached in preparation
for the first exposure,
the survey meter should be monitored. Before the source is exposed, the assistant should check
the area for persons who may have crossed into the restricted
area, and then move outside the rope boundary.
Making an Exposure
The radiographer should be at the maximum distance from the exposure device that the guide tube will allow as he or she quickly cracks the source out of the exposure device and into place. As the source moves out of the exposure device, the survey meter will increase to a very high level and
then reduce once the source is inside the collimator. During the exposure, the assistant will survey the established
boundary to determine the levels of radiation present. If the survey meter indicates levels are higher than calculated,
the boundary must be extended.
Retracting the Source
On retraction of the source, the radiographers will see a rise
in readings as the source moves from the collimator and is retracted
into the projector. When the source is inside the exposure device, the radiographer should approach it while monitoring the survey meter. If the source is properly retracted, no increase in
the survey meter reading should be seen when approaching the exposure device. The exposure device should be
surveyed on all sides, paying special attention to the front of
the device. The entire length of the guide tube must then be surveyed.
This process is repeated for each exposure. The survey results must be documented when the exposure device is returned to the vehicle
for transportation, and when it is placed back into its storage location.