A γ-ray background model for Monte Carlo simulations

We present here a γ-ray background model for a typical urban environment. Such models are crucial for realistic evaluations of the sensitivity of nuclear detection systems. The model spectrum consists of a collection of lines and a continuum component. The details of the iterative approach used in the creation of the model are discussed. When the final model is used as the source spectrum emitted by the ground or the concrete walls surrounding a typical coaxial Ge detector, the simulated response of the detector matches the data observed in the field extremely well. The final emission spectrum is very precise, and contains features crucial in nuclear detection applications. The prominent 40K line (1460.8 keV) is clearly seen, as well as the 2615 keV peak due to Tl-208 (thorium natural decay series). The lines of 238U (1001.2 keV) and 235U (185.7 keV) are also present. We find this spectrum to be a useful tool for estimating sensitivities in our Monte Carlo modeling of several nuclear detector setups. We offer this model to the community for unlimited use, in the form of a Geant4 class or as a table of ‘intensity vs. energy’ for use with other Monte Carlo packages. We also note that the iterative procedure described here can be used for the creation of other background spectra.