Surrogate Distributed Radiological Sources-Part I: Point-Source Array Design Methods

In this first part of a multipaper series, we demonstrate a method for using arrays of point sources to emulate continuously distributed gamma-ray sources when measured from a standoff of at least several meters. The method relies on the Poisson deviance statistic to test whether the array source "looks like" its continuous analog when measured by a particular gamma-ray detector moving through 3-D space on a particular trajectory. This point-source method offers significant advantages over truly distributed sources such as powders, solutions, or aerosols; notably, arrays of sealed point sources are safer to both personnel and the environment, and are more easily deployed, reconfigured, ground-truthed, and removed. We use this Poisson deviance metric to design eight different mock distributed sources, ranging in complexity from a 3 x 36 m uniform square grid of 5 mCi Cu-64 sources to a configuration where regions of higher and zero activity are superimposed on a uniform baseline. We then present several example calculations for various detector systems, altitudes, array source spacings, and source patterns, and examine under what parameters it is possible to design a surrogate array source that is nearly indistinguishable from a truly continuous distributed source. In Part II, we will detail the design, manufacture, and testing of Cu-64 sealed sources at the Washington State University research reactor, discuss their deployment during the aerial measurement campaign, and present results from several measurements.