The fast neutron component in treatment irradiations with 12C beam.

Using 12C beams of 200 AMeV kinetic energy the production of secondary fragments from nuclear reactions in a thick water absorber (12.78 cm) was investigated. Fast neutrons and energetic charged particles (p-, d-, t-, a-particles) emitted in the forward hemisphere were identified by a BaF2/plastic-scintillation detector telescope. Neutron energy spectra were recorded at various angles using time-of-flight techniques. The neutron emission is forward peaked and the energy spectrum shows a broad maximum about half the energy per nucleon of the primary 12C ions. The total yield of fast neutrons emitted into the forward hemisphere integrated over the energy range of 25 to 500 MeV was found to be 0.43 +/- 0.1 per primary ion. The dose contribution of fast neutrons in patient treatments with carbon ions is estimated to be less than 1% of the total treatment dose.