Answer to Question #10394 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

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

According to National Council on Radiation Protection and Measurements (NCRP) Report No. 79 the lead TVL (Tenth Value Layer) for neutron capture gamma rays (NCG) is 6.1 cm. This is the usual parameter for high-energy linear accelerator door shielding design.

Nonetheless the International Atomic Energy Agency (IAEA) Safety Reports Series Number 47 states on page 42 that, "For rooms with a maze length greater than 5 m, the energy of the gamma rays is much lower, requiring a TVL of about 6 mm lead.”

This last statement, supported solely on one referenced paper, differs substantially on the value recommended for that parameter in all of the rest of the literature, and has important practical as well as cost implications. Can this recipe be safely applied? Are there any methods for estimating the effective lead TVL for neutron capture gamma rays for a specific maze design other than complex Monte Carlo simulations?

Per NCRP Report No. 79, Neutron Contamination from Medical Electron Accelerators, in a room with a poorly designed maze, the neutron capture gamma ray component may be 1/5 or more of the total dose equivalent at the entrance to the maze. The neutron capture gamma rays are of very high energy ranging up to 10 MeV. The average value is 3.6 MeV. Measurements made by one researcher on about six linear accelerators yielded TVLs of 5 to 7 cm of lead for the capture gamma rays. More careful measurements made by another researcher resulted in a TVL of 6.1 cm. This was the basis for the recommended value of 6.1 cm of lead in NCRP Report No. 79.

The only other reported measurements at that time were those made by Kersey (referenced in IAEA Safety Reports Series Number 47), for which a TVL of 6.1 mm was obtained. These measurements were made for rooms with much better mazes where the scattered photons were expected to be the dominant component at the maze entrance, and not the capture gamma rays. Therefore, your shielding expert should exercise his or her own judgment in using the appropriate capture gamma ray TVL.

Nisy Elizabeth Ipe, PhD, CHP
Answer posted on 29 August 2012. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.