SU‐E‐T‐303: Neutron Measurements for the Monarch‐250 Proton Accelerator

Abstract

Purpose: The Monarch‐250 proton accelerator, a unit utilizing a superconducting magnet synchrocyclotron, is currently undergoing testing (Still River Systems, MA). The purpose of this study was to evaluate neutron exposure at various locations around the accelerator. Methods: Neutron dose equivalents per therapeutic dose (H/D) were investigated for a 14cm‐diameter small field (SF) and a 25cm‐diameter large field (LF). The effect of stopping the beam in solid brass apertures was also studied. Two types of neutron detectors were used: a SWENDI‐2 detector (Thermo Scientific) and BD‐PND™ dosimeters (Bubble Technology). Measurements were performed at three locations: 1) ∼1m from central axis along the patient plane (P1), 2) ∼1m below the beamline between the patient and accelerator (P2) and 3) ∼1m behind the accelerator (P3). An ionization chamber (IBA) was used to determine the delivered therapeutic dose in a water phantom. Results: Using the SF beam delivered to a water tank, the measured H/D using the SWENDI‐2 were 0.39, 0.36, 0.14mSv/Gy at P1–P3, respectively. Stopping the beam in brass increased neutron dose to 0.51, 0.54mSv/Gy at P1, P2 respectively. For the LF the H/D values were 1.93, 2.18, 0.56mSv/Gy for P1–P3, respectively. When the LF beam was stopped in brass, the H/D was 2.16mSv/Gy at P1. The measured H/D using BD‐PND™ at P2 was 0.31mSv/Gy with the SF beam stopping in a water tank, and 0.66 mSv/Gy at P2 with brass in place. Conclusions: The H/D values obtained using the BD‐PND™ were not significantly different from the SWENDI‐2 values. The greater H/D with brass in place was consistent with excess neutrons created in a high‐Z material. Preliminary results indicate that the measured neutron H/D from the Monarch‐250 are below the 1mSv/Gy guideline for radiation leakage (IEC 60601‐2‐1 average leakage for 10×10 field size). Supporting calculations are currently being performed using MCNPX2.6.