Answer to Question #10036 Submitted to "Ask the Experts"
Category: Instrumentation and Measurements
The following question was answered by an expert in the appropriate field:
Yes, it is possible to use wood to simulate soft tissue in an ionization chamber, but the answer comes with some caveats. Different types of woods have different compositions, so you might have to do some research to find a type that is satisfactory to you. The major constituents of most woods are cellulose (C6H10O5), lignin (C10H12O3), and hemicellulose (C5H8O4). Lesser amounts of other materials are also present, including compounds containing nitrogen.
A nominal composition for an arbitrary untyped wood might contain, by weight, something like 50 percent carbon, 6 percent hydrogen, 42 percent oxygen, and 2 percent nitrogen, but these numbers will vary noticeably among different woods. In the International Commission on Radiation Units and Measurements Report 44, four-element composition of soft tissue is about 10.1 percent hydrogen, 11.1 percent carbon, 2.6 percent nitrogen, and 76.2 percent oxygen. Because the atomic constituents of both wood and soft tissue are light elements with low atomic numbers that are not greatly different, I would expect that a properly designed wood chamber would provide a response very close to that of a soft tissue-equivalent material when used for photon dose measurements. This would be especially true for higher-energy photons (from perhaps 100 keV to a few MeV). At very low photon energies, the response differences between the wood and soft tissue would tend to increase because of the strong dependence of the photoelectric effect on atomic number. I note a reference to a 2009 paper by Shakhreet and colleagues on the Internet that describes a particular wood (Rhizophora) for which the measured mass attenuation coefficients for low-energy photons (15.8 keV to 25.3 keV) were very close to those for some soft tissue.
Other considerations also apply in a decision to attempt to make and use an ionization chamber made of wood. Some woods are hard and others are soft, some are dense and others less so, and some are fine grained and others are open grained. I would judge that a fine-grained wood and rather hard wood would have some advantages in that machined surfaces would tend to be smooth, with fewer surface irregularities that might affect performance. Also, typical air-dried woods are not sufficiently conductive that they could be used without special treatment. A good material to provide an electrically conductive coating, required for the inner walls of the chamber, is colloidal graphite, which is available as an easy-to-use aerosol spray, which you can readily find on the Internet. If you are planning on fabricating the collecting electrode also from wood, you would similarly have to provide a conductive coating for it. Woods are also subject to uptake and release of water vapor and associated expansion and contraction as ambient humidity changes. You might be able to minimize such effects by coating the external surfaces of the chamber with a water-vapor-resistant coating (if you do this I would not recommend using any paints that contain metal-based pigments, especially if you intend to measure lower-energy photons).
While wood may not be the most desirable material to select for fabrication of an ionization chamber, it is a material that is easily available and that is amenable to machine- and hand-shaping, although fabricating a cylindrical-walled ionization chamber can be challenging. I do not know what your professional level of development is, but I know that it can be an intellectually rewarding experience to design, construct, and use an ionization chamber as you seem to be contemplating, and I wish you success in your endeavors.George Chabot, PhD