Parameter optimization of pulsed direct-current magnetron sputtering for niobium hydrogen diffusion layers in target systems of accelerator-based boron neutron capture therapy

Parameter optimization of Nb hydrogen-diffusion layers for accelerator-based boron neutron capture therapy neutron-producing targets was investigated by pulsed DC magnetron sputtering. Nb was selected as the protective layer material because of its favorable balance among hydrogen transport, thermal conductivity, and interfacial compatibility with Cu. Stopping and Range of Ions in Matter simulation showed that increasing Nb thickness progressively confined the proton stopping, energy deposition, and irradiation-damage regions within the protective layer, and a target thickness of 25 μm was determined for 1.88 MeV protons. The effects of sputtering power, argon flow rate, sputtering pressure, and duty cycle on deposition rate, morphology, and structural quality were systematically studied. The optimized deposition conditions were 300 W, 20 SCCM, 0.5 Pa, and 90%. Under these conditions, the Nb coating exhibited good surface continuity, structural integrity, and a stable Nb/Cu interface. XRD confirmed that the coating was mainly body-centered-cubic Nb, XPS revealed a naturally passivated oxide layer after air exposure, and cross-sectional TEM showed columnar growth without obvious reaction layer or elemental interdiffusion. These results provide an experimental basis for the preparation of Nb hydrogen-diffusion layers for AB-BNCT neutron-producing targets.