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 recorded.
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.