Calculating Lead Shielding Thickness for Radioactive Sample Transport

[habman_6]

Hello, i have to do a problem for a written problem set:

1. Shielding. The transport of radioactive samples must follow strict guidelines to ensure that the exposure is very low. A 100 mCi sample of 212Pb is placed at the centre of a cubical lead lined box, each side of which has a length of 30cm. Calculate the thickness of lead required to reduce the dose level at the surface of the box to 10mR per day at most. The half-life of 212Pb is 11 hours, and each gamma disintegration yields 0.24 MeV; the mass energy absorption coefficient for lead at this energy is 0.48 cm2g-1, and its density is 11.4 g.cm-3. (Make the simplest reasonable assumption you can, and provide a full explanation).

Am i right in the "simple assumption" that the air does not affect the dose level, so this way the dose at each point along the surface of the cube would be equal (since distance from the sample would not matter). Then using this surface area i could continue to solve for fluence, etc.

 

[Hootenanny]

Am i right in the "simple assumption" that the air does not affect the dose level, so this way the dose at each point along the surface of the cube would be equal (since distance from the sample would not matter). Then using this surface area i could continue to solve for fluence, etc.

You can make the assumtion that the air will not affect the intensity of the radiation. However, this does not mean that the dose at each point on the surface of the cube will be equal. The instenity of radiation varies with distance in accordance to the inverse square law, therefore that distance will still affect the intenisty of the radiation.

-Hoot

 

[habman_6]

This is what I did. Can someone confirm so i can settle my feelings?

Also, I scanned this before i caught a conversion error for the mass energy coefficient of air, it should be 0.0027 instead of 0.027, which changes the answer to 1.2cm.