Purpose: He4 fast neutron scintillation detector can measure a wide range of neutron energies up to 10MeV with low gamma-sensitivity. This provides faster and wider spectral analysis than the standard Bonner spheres method, which needs measurements at different activation foil thicknesses and is mostly within thermal range.
Methods: He4 fast neutron detector (Arktis Detection Systems, USA) was used to measure neutron spectrum from 18MV x-ray linac and IBA double-scattering proton gantry. Measurements were done with 20cm solid water (located at isocenter) irradiated with 18MV X-ray beam and 20x20 cm² collimation; and 180MeV proton beam with 15.1cm SOBP and 20cm² field size. A pulse-shape-discrimination algorithm was used to separate gamma-ray and neutron signals based on charge-integration method that calculates the integral of slow and fast components of each pulse; and spectrum unfolding algorithm, separately validated with a spectrum (up to 10MeV) using time-of-flight techniques calibrated by a deuteron-beryllium neutron source. Detector was placed near gantry room wall at 220cm from isocenter at distances of 30cm and 60cm lateral to edge of solid water. Measurements were done for 15cm, 20cm, 25cm, and 30cm water thicknesses.
Results: Secondary neutrons are dominant in thermal range for the 18MV X-ray beam. He4 detector could not be used to measure neutron spectrum due to counting inefficiency. For <1MeV neutrons, other detectors (such as organic liquid scintillator EJ-309 and Cs₂LiYCl⁶:Ce³⁺ (CLYC) scintillator) with higher detection efficiency were used. Neutron spectra at >1Mev were successfully measured with He4 for 180MeV proton beam. Differences in measured exiting neutron spectra from various solid water thicknesses (15cm, 20cm, 25cm, and 30cm) and detector-solid water distance (30cm versus 60cm) were observed.
Conclusion: The secondary neutron spectrum and dose from 18MV x-rays and 180MeV protons were measured directly with He4 fast neutron detector. Measured results are comparable to simulations conducted by previous researchers.