SUMMARY

The characterization and measurement of the spatial, temporal and energy emission of air- scattered photons and neutrons generated near accelerator-based bremsstrahlung photon sources is becoming important in many modern applications. The national and homeland security research community is interested in developing technologies which can detect illicit materials at greater standoff distances in outdoor environments. These systems are referred to as “ active ” interrogation systems and are defined as inspection systems that take advantage of an externally applied “ source ” to perform traditional imaging of, or to stimulate characteristic emissions from, an inspected object. A key concern in the development of these systems is the ability to effectively predict the dose equivalents at long standoff distances from these sources in order to ascertain the operational safety of said systems. Current computational radiation transport simulation tools have the ability to effectively model these systems, however, a paucity of experimental data exists in comparing the results of these simulations. A predictive methodology to assess the radiation dose surrounding a high-energy bremsstrahlung-based accelerator system for national defense applications was developed. Fluence-to-dose conversion coefficients for the International Commission on Radiation Units and Measurements operational quantity ambient dose equivalent were calculated for photons and electrons up to 25 MeV utilizing the Los Alamos National Laboratory Monte Carlo N-Particle code, MCNP5 Version 1.51. Special consideration was given to the treatment of secondary charged particle equilibrium in all simulations. The latest photon and electron cross section data from ENDF/B-VI.8 and the e103 libraries, respectively, were used to perform all calculations. An extensive set of system simulations was performed around a prototype high-energy bremsstrahlung-based accelerator system to obtain photon, electron and neutron fluence spectra. These fluence data were folded with the calculated ambient dose equivalent conversion coefficients as well as previously published effective dose conversion coefficients. A substantial set of integral air scatter measurements for accelerator-generated primary and secondary radiations (photon and neutron) were performed around the prototype system in order to provide a comparative data set from which to determine the total dose equivalent both in the beam (on-axis) and outside of the beam (off-axis). The results of both the simulations and measurements will be presented. Comparison with system simulation calculations indicate agreement to within 20% for the total dose equivalent.