  The secondary neutron fields at the deep tumor therapy terminal at HIRFL
(Heavy Ion Research Facility in Lanzhou) were investigated. The distributions
of neutron ambient dose equivalent were measured with a FHT762 Wendi-II neutron
ambient dose equivalent meter as the 12C ions with energies of 165, 207, 270,
and 350 MeV/u bombarded on thick tissue-like targets. The thickness of targets
used in experiments is larger than the range of the carbon ions. The neutron
spectra and dose equivalent is simulated by using FLUKA code, and the results
agree well with the experimental data. The experiment results showed that the
neutron dose produced by fragmentation reactions in tissue can be neglected in
carbon-ion therapy, even considering their enhanced biological effectiveness.
These results are also valuable for radiation protection, especially in the
shielding design of high energy heavy ion medical machines.
