| Abstract: | We have identified the sources of neutron production in the beam transport system of the 720-MeV helium beam used for radiation therapy at the 184-in synchrocyclotron of the Lawrence Berkeley Laboratory, and determined their magnitude. Measurements with activation detectors of differing energy response were used to unfold secondary particle spectra at various locations on the patient table. The effect of charged particles was estimated using a calculation of neutron-flux densities derived from published cross sections. The absorbed dose, as a function of distance from the beam axis, was calculated using the unfolded spectra and evaluated fluence-to-dose conversion factors. The values of absorbed dose obtained from the unfolding of experimental data agree with the values obtained from the calculated spectra within the estimated uncertainty of +/- 25%. These values are approximately 5 X 10(-3) rad on the beam axis and approximately 1 X 10(-3) rad at distances greater than 20 cm, perpendicular to the beam axis, per rad deposited by the incident alpha-particle beam in the plateau. Estimates of upper limits of dose to two critical organs, the lens of the eye and red bone marrow, are approximately 25 rad and approximately 5 rad, respectively, for a typical treatment plan. |
