Answer to Question #2029 Submitted to "Ask the Experts"

Q

I am trying to estimate the relationship between the gamma dose rate measured on the top of a vehicle and that at 1 m above ground using the high-pressure ionization chamber. How can I find out the attenuation/attenuation factor of the vehicle as it is not uniform? I know that attenuation is energy dependent. Can I assume an average value in the estimation if the application of the measurement is for nuclear emergency?

A

You are correct that gamma attenuation through matter is energy dependent. In "good beam" geometry where there is little scatter contribution to the dose measurement point, it is exponential and depends on the thickness of material and its composition (that is, Z or atomic number), density, and mass attenuation coefficient. If one has the more-common scenario with scatter contributing to the dose, an additional "buildup factor" most be applied. If one has a complex geometry and nonhomogeneous attenuator (for example, a vehicle), then you could take an empirical approach like you've described. The geometry could also be simulated in a computer shielding code. We do not want to promote any commercial product, but a gamma-ray shielding program such as MicroShield (from Grove Engineering, 3416 Olandwood Court, Suite 211, Olney, MD 20832 USA; phone: 301-929-3028, fax: 301-929-3047) might be useful for this application. David J. Allard, CHP