Answer to Question #2064 Submitted to "Ask the Experts"

Q

My question is about a radiotherapy shielding door. The radiation source is a linac, and I'd like to know if the gamma capture radiation in the neutron shielding layer (that I suppose is the inner one) is negligible in comparison with the gamma radiation inside the room near the door for calculating the lead thickness layer. On the other hand, I have read lithium polyethylene doesn't produce capture gamma rays. Is it a good choice for this door?

A

You are perfectly correct in that the neutron/alpha reaction on ¹⁰B also yields a gamma a portion of the time and the neutron/alpha reaction with ⁶Li does not. This source of gamma radiation is a concern only if you are designing a "plug door" and even then it is a minimal concern. Two other sources of gammas tend to be the dominant problem in neutron shielding. Neutron doors typically have several inches of "pure" polyethylene or other hydrogenous material to thermalize the neutrons prior to their passing through the "borated" polyethylene. Both ⁶Li and ¹⁰B have very high cross sections for thermal neutrons and the use of a thermalizing medium prior to the neutron shield makes the neutron shield more efficient. The thermal neutron capture cross section of ⁶Li is significantly smaller than that of ¹⁰B and it is present to a smaller percentage in natural lithium than ¹⁰B is in natural boron. The other factor is that the Compton and capture gammas of the accelerator room that are incident on the door tend to be the overwhelming source of photons that the inner door, the maze, and the outer door have to attenuate. The capture gammas of the ¹⁰B reaction add to this some but not enough to eliminate boron as a choice of neutron-absorbing material. All of the above factors, in addition to the cost difference for natural boron compared with natural lithium, make the use of boron the preferred agent. One supplier of both materials is Thermal Electron Corp. This company bought the old Reactor Experiments company of the San Francisco Bay area, who was a major supplier of these shielding materials, and can give you some detailed technical information. A shielding code that allows you to iterate the thickness of the polyethylene compared with borated polyethylene compared with lead/steel can be obtained from the Attainia Corp. Its code uses measured data in the calculations and the capture gammas you are concerned about are properly treated in the calculations. Using Attainia Corp's Web site, you can download a free version of the code for the accelerators which Siemens manufactures. If you want the data for other vendors, a license fee has to be paid. I think Siemens pays a fee to make its data available free. A simple way to get the code is:

1. Go to the Attainia Web site
2. Click on "How Does Attainia Do It"
3. Click on "Attainia Design"
4. Click on "Design Linac" for a free download for Siemens accelerators.

Contact the company at 408-468-4710 to purchase a license to download the data for all linacs manufactured. Hope this is of some assistance to you. James B. Smathers, PhD, CHP Professor Emeritus Department of Radiation Oncology UCLA