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Field's Metal for Electron Beam Inserts and Blocks

J Ha

J Ha*, A Gallogly , J Cui , R Jennelle , University Southern California, Los Angeles, AA


SU-F-T-88 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

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 10x10 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.

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