Answer to Question #5721 Submitted to "Ask the Experts"

Q

On a chart for HVLs (half-value layers) the graph for some shielding materials like uranium have a "switch" of slopes at certain energies. One of concern is uranium at approximately 120 keV. It goes from about 0.07 mm to 0.2 mm and continues on that slope. Why? What is this phenomena?

A

The reason for the sudden change in the value of the HVL (half-value layer) has to do with the manner in which the attenuation coefficient for uranium (and other materials) changes as a function of photon energy. The HVL is inversely related to the linear attenuation coefficient, the HVL being equal to the natural logarithm of 2 divided by the linear attenuation coefficient. As photon energy increases, the photoelectric cross section for a given material generally decreases; as energies approach the binding energy of electrons in a given shell, the photoelectric component of the attenuation coefficient will suddenly increase in a kind of resonance effect as soon as the photon energy barely exceeds the binding energy of the electron.

The binding energies of K-shell electrons in uranium are somewhat less than 120 keV, and the sudden change in the attenuation coefficient can be seen in a plot or table of values (see NIST); this sudden change is similarly reflected in the change in the value of the HVL. The changes so observed in the attenuation coefficients are said to be associated with absorption edges indicative of the binding energies of the electrons in the material. If you look at the NIST data for uranium you can observe additional absorption edges at lower energies associated with binding of L-, M-, and N-shell electrons. I hope this explanation is suitable.

George Chabot, PhD, CHP