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Currently, source emission and detector responses are only rudimentarily characterized. For a given source, the dose rate at a nominal distance is determined, and this dose rate is scaled according to the source-detector distance. Efforts are underway to incorporate advanced modeling into the VIRTUS system, as described below.
With funding from DTRA and the Domestic Nuclear Detection Office, the Naval Research Laboratory developed SoftWare for the Optimization of Radiation Detectors (SWORD) to greatly simplify the process of radiation detector modeling. SWORD is a graphical front-end with a CAD-like interface, and it contains a library of standard detectors, vehicles, and sources. It can feed data into two different Monte Carlo radiation transport codes, Geant4 (CERN) and MCNP (LANL), and analyze the results from the transport codes. SWORD enables developers and technical evaluators to assess instruments and systems in relevant radiation environments, without actual hardware, using a single software package that does not have a steep learning curve. SWORD is available to users through the Radiation Safety Information Computational Center (RSICC) at ORNL.
In addition to point sources, a variety of complex radioactive sources were approved by the US Department of Energy to be modeled within VIRTUS, including medical patients, vehicle-mounted radioactive material, special nuclear material, and industrial instrumentation. The radiation propagation within the local source environment would be too computationally intensive to model on an Android device during runtime. Instead, a sphere was placed around each local source environment within SWORD, and the flux through this sphere was calculated. The flux through the sphere becomes the source emission profile used by the VIRTUS apps. Currently, an isotropic emission profile is used, but angle dependencies are planned for future versions.
In the radiation detection community, the intrinsic efficiency of a detector is defined as the number or radiation quanta recorded divided by the number of radiation quanta incident on the detector. There are many reasons why incident radiation quanta would not be recorded by the detector, including scattering in the detector housing and a lack of interaction with the detector element. SWORD accounts for these effects, so it was used to determine the intrinsic efficiency of the VIRTUS detectors.
Beams of radiation at varying energies were fired at the detectors from different angles, and the corresponding number of radiation quanta recorded were tallied. In general, a detector's efficiency can be highly angle dependent. Currently, each detector's efficiency was averaged over all angles. Angle-dependent detector efficiencies are a goal for future development.