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Answer to Question #5721 Submitted to "Ask the Experts"Category: Radiation Basics — Interaction Coefficients The following question was answered by an expert in the appropriate field: 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
Answer posted on 22 August 2006. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of 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 specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
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