Answer to Question #9483 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:

Q

I'm having difficulty trying to find the tenth-value layer (TVL) for photons of 18 MeV in paraffin. Could you provide me some information about it and possibly a reference?

A

Your question took me on a roundabout path. All the direct information I was able to find for paraffin regarded its neutron attenuation characteristics. NCRP 151 (2005) says of paraffin and polyethylene, "These two materials will be considered together since they are very similar. Paraffin, sometimes called paraffin wax, has the same percentage of hydrogen (14.3 percent) as polyethylene and is less expensive. However, it has lower density and is flammable, so it is usually avoided in any permanent barriers." One might think twice about using paraffin.

Take the effective energy of the 18 MV beam as 6 MeV, which should be close. Attix (1986) gives the 6-MeV mass attenuation coefficients cm2 gm-1 for water and polyethylene as 0.0277 and 0.0276 respectively, but the density of paraffin is about 0.93 so the attenuation coefficients would be 0.0277 and 0.0257, respectively.

From NCRP 151, Figure A.1, at 6 MeV the TVL of water is 91 cm. Using µ to represent the attenuation coefficient, from TVL (paraffin) = TVL (water) x µ (water) / µ (paraffin), we obtain TVL (paraffin) = 98 cm.

That being derived, the question arises, where is this barrier to be used? If it is at the end of a maze, the photons reaching it will have a lower energy due to the interactions required to bounce into the maze and to the door. As noted in NCRP 151, at the door, capture gammas become an important, if not dominant, factor and the problem more complicated and dependent on the particular geometry involved.

Bruce Thomadsen, CHP

References

National Council on Radiation Protection and Measurements. Structural shielding design and evaluation for megavoltage x- and gamma-ray radiotherapy facilities. Bethesda, MD: National Council on Radiation Protection and Measurements. NCRP Report No. 151; 2005.

Attix FH. Introduction to radiological physics and radiation dosimetry. New York: John Wiley and Sons; 1986.

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