Scattered Dose Fractions From Electron Accelerators

In the design of shielding for megavoltage electron accelerators, analysts generally use the hand calculation techniques described in NCRP Report 151 and ANSI/HPS N43.3-2008. For accelerators used in medical treatments the low atomic number scatter fractions in NCRP 151 are relevant and they were developed using Monte Carlo computations of bremsstrahlung beams impinging on a spherical water phantom. In N43.3, the values for scattered radiation are based on similar scatter targets. That standard notes that the provided scatter fraction values (determined from tissue scatter objects) are conservative since they are based on scatter from low atomic number materials and high atomic number materials will scatter less and have greater absorption of the incident photons thereby decreasing the scatter ratio. With the increasing use of linear accelerators in industrial, radiographic, and other applications the scattering features for materials other than tissue are of interest. This work presents bremsstrahlung scatter fractions for higher-Z materials representative of other items used in these applications for linear accelerators with electron energies ranging from 2 MV to 24 MV. Scatter fractions were determined for different scatter targets having varying Z-numbers, namely four-element ICRU tissue substitute, aluminum, steel, and uranium. For each scatter target a 400 cm2 field size at 1 m from the accelerator source was modeled. The bremsstrahlung spectra used for the primary beam were generated using linac targets of tungsten, copper, and tantalum for comparison. For each electron beam energy and target the bremsstrahlung was directed to a 15-cm radius scatter target and the corresponding dose rates determined at scatter angles ranging from 5 to 170 degrees. While for most energies aluminum scatter fractions were similar to those of ICRU tissue substitute; as anticipated the steel and uranium scatter fractions were noticeably lower than those of tissue.