Evaluation of the effectiveness of steel for shielding photoneutrons produced in medical linear accelerators: A Monte Carlo particle transport study.

Abstract

To confirm the effectiveness of steel as a material for shielding photoneutrons produced in medical linear accelerators and report values for neutron first and second tenth value layer to be used as radiation protection quantities. Monte Carlo code Geant4 was used to simulate transport of photoneutrons through primary barriers containing layers of concrete and steel. The photoneutron spectrum generated in high energy modern accelerator head components was simulated and projected to barriers of different thicknesses of steel including a control case of no barrier. To derive the neutron tenth value layer (TVLn), absorbed dose was evaluated at 1 cm depth in a water phantom outside the barrier. The fluence and energy spectrum of neutrons leaking outside the barrier were also calculated. The neutron source model was used to quantitatively assess the neutron shielding for a newly designed bunker. For an 18 MV photon beam produced by a linear accelerator, the first and second TVLn, for steel as a neutron shielding material were determined to be 15.7-16.1 cm and 28.5-32.4 cm, respectively, depending on the vendor. The lateral distribution of neutrons outside steel barriers is investigated. Also, we show the relative efficiency of steel for fast and epithermal neutrons and determine the neutron energy spectrum outside a steel barrier. This paper agrees with previous studies for Pu-Be sources, and we would advocate using these values for the purposes of high energy linear accelerator bunker design.