The neutron target comprises metal atoms on which the high-energy neutrons impinge. If the incident neutrons strike a metal atom with numerous nucleons, the result is spallation, meaning evaporation of protons and neutrons. An intranuclear cascade is created in the nucleus of the metal atom in which protons are emitted. The neutrons impinging on the metal atoms cause the nuclei of the metal atoms to break up, there is a fragmentation of the metal atom into several nuclei and then vaporization of charged particles. The protons generated by cascade processes have higher energy, corresponding to the higher energy of the impinging neutrons.

Actinium-225 production is dependent on the proton flux and energy. In some embodiments, the neutron target comprises nickel and has a thickness of 0.01-1 mm, e.g. 0.01-0.06 mm or about 0.5 mm. In some embodiments, the neutron target comprises iron or manfanese and has a thickness of 0.01-1 mm, e.g. 0.2-0.3 mm or about 0.25 mm Other suitable neutron targets include, but are not limited to, aluminum, silicon, potassium, copper, zinc, germanium, and various types of steel such as stainless steel.

In some embodiments, the proton target comprises radium chloride (RaCl₂) and has a thickness of 4.5-5.5 mm, e g about 5 mm RaCl₂ may be obtained from precipitation with concentrated HCl, or radium carbonate RaCO₃.