X-rays produced by medical accelerator have been widely used in radiotherapy.During the process of therapy

a certain level of photonuclear neutrons are produced by the (γ

n) and (γ

2n) reactions of the X-rays and surrounding materials.The neutron dose distribution caused by the Prim μs-M linac at 15 MV in the standard exposure field of 10 cm×10 cm was M-C simulated

and measured at the treatment plane by CR-39 solid state nuclear track detector and neutron bubble detector (NBD).The results show that the measured neutron doses are lower than the simulated ones by up to 30%.The main reason for this difference is that the neutrons with energies below 100 keV produced by inelastic scattering of fast neutrons with surrounding materials are below the threshold energies of both detectors

and thus not detected.These results provide some basis for the optimal design of the facility shielding aiming at controllable and minimized contaminating neutron dose.