Answer to Question #12646 Submitted to "Ask the Experts"
Category: Industrial Radiation — Industrial Radiography
The following question was answered by an expert in the appropriate field:
We have a bunker for industrial radiography, which was built by our client, not by our company. The bunker has a 1.5 m thick concrete wall in a 20 m by 20 m area, and the height of the shield walls is 3 m. We performed radiation safety surveys for confirmation of the shielding. We found the reading just outside the wall is about 10 µSv-h-1, while at 20 m outside the shield wall it was about 40 µSv-h-1. Is there any relation between radiation buildup and the thickness of the wall? What is your advice for the shielding bunker design?
What you are experiencing is radiation skyshine, which is radiation scattered from the air above the opening of the open-top radiography shield (bunker). Skyshine happens both when the shielding is open at the top or has a minimal amount of top shielding. Both the direct radiation from the radiography source and the upwardly scattered radiation from the shield walls or equipment can scatter in the air above the shielding wall. A portion of this scattered radiation will be scattered downward toward the ground surrounding the shielding. The radiation dose rate measured at the surface of the shielding wall may drop, then begin to increase the further you move away from the wall. The dose rate will reach a maximum and then will reduce again, as you get even further away from the shielding wall. The more dense the atmosphere, such as in a sandstorm, the more the skyshine will increase. A way to calculate the skyshine for an open topped shield is provided on the American Association of Physicists in Medicine website.
Adding additional shielding to cover the open top of the bunker will reduce the skyshine. As you have learned, measuring the skyshine dose rate is a very effective method to access and control the exposures for the personnel around the shielding.
John Hageman, MS, CHP