SU-E-T-267: Construction and Evaluation of a Neutron Wall to Shield a 15 MV Linac in a Low-Energy Vault.

PURPOSE

To design and quantify the shielding efficacy of an inner Borated Polyethylene (BPE)wall for a 15 MV linac in a low energy vault.

METHODS

A Varian TrueBeam linac with a maximum photon energy of 15 MV was installed in asmaller, preexisting vault. This vault originally housed a low-energy machine and did not havesufficient maze length recommended for neutron attenuation. Effective dose rate calculationswere performed using the Modified Kersey's Method as detailed in NCRP Report No. 151 andfound to be unacceptably high. An initial survey following the machine installation confirmedthese calculations. Rather than restrict the linac beam energy to 10 MV, BPE was investigatedas a neutron moderating addition. An inner wall and door were planned and constructed using4'×8'×1″ thick 5% BPE sheets. The resulting door and wall had 2″ of BPE; conduits and ductwork were also redesigned and shielded. A survey was conducted following construction of thewall.

RESULTS

The vault modification reduced the expected effective dose at the vault door from 36.23to 0.010 mSv/week.

CONCLUSION

As specific guidelines for vault modification are lacking, this project quantitativelydemonstrates the potential use of BPE for vault modification. Such modifications may provide alow-cost shielding solution to allow for the use of high energy modes in smaller treatment vaults.