Z{sub eff}, a parameter representing an 'effective atomic number' for a material, plays an important role in the Electronic Baggage Scanning Program (EBSP) to detect threats in dual-energy computed tomography (CT) baggage-scanning systems. We believe that Z{sub eff}, as defined and used on this program, does not provide the accurate representation of a material's x-ray absorption properties that is needed by the EBSP. We present the case for a new method that defines an effective atomic number for compounds and mixtures, which we refer to as Z{sub e}. Unlike Z{sub eff}, Z{sub e} is tied by definition to the x-ray absorption properties of each specific material. Use of this alternative will provide a more accurate scale for calibrating Micro-CT and EDS systems against standard reference materials and will provide a more accurate physical characterization of the x-ray properties of materials evaluated on those systems. This document: (1) Describes the current usage of the Z{sub eff} parameter; (2) Details problems entailed in the use of the Z{sub eff} parameter; (3) Proposes a well-defined alternative - Z{sub e}; (4) Proposes and demonstrates an algorithm for optimally associating Z{sub e} with any specified compound or mixture; (5) Discusses issues that can impact themore » usefulness of an effective-Z model; and (6) Recommends that, in order that the chosen effective-Z parameter not materially impact the accuracy of data produced by the EBSP program, the use of Z{sub eff} be replaced by Z{sub e}.« less