SU‐F‐T‐88: Field's Metal for Electron Beam Inserts and Blocks

Abstract

Purpose:The aim of this work is to show that Field's metal can be used as a viable alternative material to lead‐based low‐melting temperature alloys (e.g., Cerrobend) for electron beam inserts and blocks. The goal is to eliminate exposure risks associated with lead and cadmium.Methods:Field's metal (51% Indium, 32.5% Bismuth, 16.5% Tin) is a low‐melting point eutectic alloy, and is proposed as a lead‐free replacement for Cadmium‐free alloy (52.5% Bismuth, 32% Lead, 15.5% Tin). The experiments were done using a Varian 21EX linac equipped with 6, 9, 12, and 16 MeV beams. The 10×10 cone was used with 1.8cm thick insert made of Fields metal, Cd‐Free alloy or open field. The transmitted radiation was measured using a diode at various depths in water. Block transmission factor (BTF) is determined as the ratio of maximum transmission radiation dose over the dose at dmax of the open field.Results:The data shows BTFs for both alloys are very similar, well below 5% for 6, 9, and 12 MeV beams. For 16 MeV, the BTF of the Fields metal is about 5.3%; the Cd‐free metal 4.2%. The peak dose of the transmitted radiation occurs less than 1 cm below the water surface, at a much shallower depth than the dmax of the electron beam in the open field. The beam profile shows the maximum block transmission is along the central axis of the beam.Conclusion:The present study shows that Fields metal can be used to make electron cutouts and blocks for 6, 9, 12, and 16 MeV beams. Because it has a similar density as other lead‐based alloys, the same thickness of Fields metal would be adequate for shielding the electron beams. This means that it can be used as a replacement without modification to the existing block holder.