Dear Colleagues,
in order to shield a dose rate (NORM in drums) from about 250 micro Sievert per hour, down to about 5 micro Sievert per hour, I have calculated a lead sheet shielding of about 17 mm thickness. Is there anybody to confirm this result, respectively giving a more detailed input on this topic such as literature or examples, standards etc.
Your kind support is highly appreciated
Ruediger
You want to reduce the dose rate by 50. The Radium-226 Half Value Layer (HVL), the amount of lead to reduce the dose rate by 1/2, is 4 mm. This suggests that you will need much more. Information is from Radiological Health Handbook 1970 (out of print but available on web).
How much of the material do you have? What is the configuration? What is the reason for the needed dose reduction?
http://www.radprocalculator.com/Gamma.aspx
try to use this calculator (but it is for point source)
Dear Joseph,
Reason is to fulfill requirements of UN2910, which is 5 micro Sievert per hour (basically HSE and transport related). The material is in 200 l drums.
HVL for lead is 0.4 mm?
Ruediger
Ruediger
The majority of the dose comes from the higher energy gammas from the Ra progeny. Calculation is difficult. Dose rate depends upon shape, size, the matrix containing the Ra and distribution of Ra in material. The HVL of 4 mm is for a point source. Many medical sources have a listed HVL of 12 - 14 mm. See attached.
What type instrument are you using for measuring dose rate? Some instruments over respond to low energy gammas making the measurement too high. Check with simple shielding. If the dose rate is easily reduced by shielding you have over response.
Consider repackaging. Is the Ra uniformly distributed? If not, place higher dose rate material in the center. Can you add filler, for example, wet soil? If uniformly distributed place material in smaller containers and then filler in the 200 L drums to hold them in the center.
How you shield will come down to cost.
Thank you very much Joseph,
Your kind input is entirely helpful and exactly the answer to my question.
kind regards
Ruediger
Dear Joseph,
all oil companies use Tracerco T202 dose rate monitors: http://www.tracerco.com/monitors/T202-doserate-monitor
Do you have a comment on that equipment or any recommendation?
Thank you in advance
Ruediger
The Tracerco T202 uses a compensated GM detector. Typical compensated GM do not over respond. I am not familiar with the Tracerco T202 and could find no published response curves. Likely, the dose rate measurement is acceptable.
All you need is an MC simulation of doses with or without different layers of shielding.Maybe in 70s the handbooks were giving fast results. Nowadays it is preferable for almost anybody to proceed with a Monte Carlo simulation of doses.
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