As Low As Reasonably Achievable (ALARA). This concept as it applies to Radiation Health and Safety, is more than a recommendation. This concept has been codified into law in the U.S. and other countries.
ALARA is addressed by the US NRC in 10CFR part 20.1101. "The Licensee shall use, to the extent practical, procedures and engineering controls based upon sound radiation protection principles to achieve occupational doses and doses to members of the public that are As Low As Reasonably Achievable (ALARA).
We addresses shielding in an earlier post, and we addressed distance to some extent when we did the post on the Inverse Square Law. The third leg of our protection against radiation is Time. The primary ways that we control exposure are; reducing our Time in a radiation area, increasing our Distance from the source, and increasing/using Shielding between us and the source.
Time of exposure is related to source activity and and indirectly linked to distance and shielding. If I have a source of a given activity (we tend to think of this as "source strength"), and a material of a given thickness, then I have a set exposure time to get an acceptable image on the film I have chosen, or is required by the specification for the job. Using the GE Exposure Calculator APP on my Android phone, I calculate an exposure for 40 Ci of Ir=192, through a steel thickness of 0.5 inches with a source to film distance of 20 inches and a target density of 2.7 on AGFA D4 film. The exposure time is 4 minutes and 36 seconds. Without changing any of the above parameters there is no changing the exposure time, the time is fixed. Parameters I can change are source strength, and film speed. Changing the film will effect the image quality, if I switch to AGFA D5 film the exposure time becomes 2 minutes and 57 seconds.
The shot time is much less, but our quality level has also been reduced, we gave up definition and contrast to gain speed. In some situations this will be unacceptable. The D5 may not be able to provide the sensitivity required by the job. D5 will be more grainy in appearance as well as having less contrast and definition. This is because the the emulsion on the film uses larger grains of silver bromide to gain the speed but increasing the grain size is exactly why it loses the definition and contrast and appears grainy.
Another method to reduce exposure time is to reduce the source to film distance. If we cut the distance in half we increase our dose by a factor of 4. This would reduce our shot time to 1 minute 9 seconds. If you increase the dose by a factor of 4 you reduce your shot time to 1/4 of what it was. This will cost you in image sharpness, again you lose definition as your geometric unsharpness increases. You may not be showing the required sensitivity. If you are shooting large bore pipe (20 inch OD) with a Double Wall Exposure/Single Wall Viewing setup you cannot reduce the Source to film distance and you are stuck with your shot time. If you can get inside the pipe to setup the exposure then you can reduce exposure time plus you get the benefit of added shielding.
If we increase the source strength, increased activity, it means the intensity/exposure rate at the control handle is greater and reduces the time we have to walk away from the source, gaining distance which would reduce our exposure rate.
So, here is the basic formula we will use:
Dose = Dose Rate x Time
If I am in an area that has a dose rate of 15 mR/hr, and I have 20 exposures that are 15 minutes each, what is my total dose at the end of the job?
15 minutes x 20 exposures is a total of 300 minutes of exposure = 5 hours. At 15 mR/hr =75 mR total dose for the day. For most of us that is completely unacceptable and we need to find a way to reduce that total dose. If I was standing at the crank handle getting that dose rate, the easiest thing to do is walk away from the source, if we double our distance we reduce our exposure to 1/4 what it was. In theory we would reduce our total dose to about 18 - 20 mR total. We can look at adding more shielding if there is something we can use.
The time we can be in an area is directly related to the intensity of the radiation. Time is affected by Dose rate which is related to distance from the source and the shielding in use.
Distance is the easiest way to reduce Dose Rate. The idea isn't to just reduce the time itself, but to reduce time spent in a radiation area. Walking away reduces the dose rate, we are spending less time in a higher radiation area by moving to the lower radiation area.
Because of the nature of our jobs, we are going to get occupational doses, that is given. Our job is to reduce our total exposure, and as we have seen dose rate is a matter of shielding and distance. The dose then becomes a matter of the time spent there. These 3 things are related each one affecting the other.
Always use your Survey Meter to verify dose rate/Intensity.
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