Neutron measurements with a CdTe spectrometer on a proton therapy unit

The purpose of this work was to investigate the use of a commercial cadmium-telluride detector as an absolute thermal neutron measurement device on a proton therapy unit. Experimental measurements were performed on water phantoms using a 25 mm2, 1 mm thick uncollimated CdTe spectrometer. The detector was shielded using sheets of borated polyethylene minimizing the measured direct neutron spectrum from the treatment nozzle. For each measurement a significant 95.9 keV prompt gamma-ray peak resulting from 113Cd thermal neutron capture within the CdTe detector was observed. For comparison, and to identify relevant trends, all setups were simulated within the Monte Carlo code MCNP6. For a dose of 21.0 Gy to a 100 mL water phantom, 0–5 cm spread out Bragg peak (SOBP), 7760 ± 299 counts (corrected for dead-time) were measured at the detector location 10 cm from the water surface and perpendicular to the proton beam, which corresponds to an absolute fluence of 1.5 × 106 thermal neutrons/cm2. The delivery of 8.3 Gy, 1.5 to 6.5 SOBP within a water tank phantom resulted in 2522 ± 233 counts or 4.9 × 105 thermal neutrons/cm2 with the detector located 5 cm away from the surface of the water tank phantom. Using MCNP6, the simulated thermal neutron fluence was determined to be within the same order of magnitude and within a factor of 3 for all configurations. This research demonstrates the first use of cadmium-telluride (CdTe) as an absolute slow neutron detector for the application of proton therapy and neutron dosimetry.