Purpose: Radiation therapy with high-energy photons poses a risk of secondary malignancies to patients due to photoneutrons generated in the linac head. The risk associated with these neutrons also raises safety concerns for the personnel involved. This study was performed to evaluate the neutron fluence spectra, generated during radiotherapy, using a passive Nested Neutron SpectrometerTM (NNS, Detec Inc, Gatineau, Quebec) with gold activation foils as thermal neutron detectors.
Methods: The measurement involved two phases; activation of gold foils through irradiation and activation analysis. The passive NNS with gold foil was irradiated with a 15 MV beam at 600 MU/min for 15 mins, and was repeated for all detector configurations. The radioactivity of the gold foil was then analysed using a HPGe spectrometer. The measured activities were then iteratively unfolded using the Maximum-Likelihood Expectation Maximization (MLEM) algorithm to obtain the neutron fluence spectra generated during the irradiation. For this purpose, the response functions for the NNS and gold foils were generated using Monte Carlo simulation.
Results: The response functions obtained for different detector configurations show that the sensitivity of the detector drifts towards higher neutron energies as the thickness of the moderator around the thermal neutron detector increases, as expected. The unfolded neutron fluence spectra at the point of measurement are obtained, and the fast neutron peak around 1 MeV can be observed in the spectrum.
Conclusions: The preliminary results shows that the neutrons were primarily of about 1 MeV at the location of measurement.