A radiotherapy apparatus comprises a linear accelerator 10 capable of producing a beam of charged particles and a neutron detector 26 capable of providing a signal to a control means 20 of the linear accelerator the control means being capable of varying the energy of the electrons generated by the linear accelerator. In a first embodiment a target 22 aligned with an electron beam produced by the linear accelerator of the apparatus, the target being capable of emitting photons when electrons are incident thereon and a material which is capable of producing neutrons when photons of sufficient energy are incident thereon. In a second embodiment, a method of calibrating the output of the apparatus includes the provision of a neutron detector placed near to a linear accelerator of such an apparatus but placed away from any electron or photon beam generated by the accelerator.
