The biophysical properties of 3.9 GeV nitrogen ions. II. Microdosimetry

The distributions in lineal energy of the events produced by 3.9 GeV nitrogen ions incident on a phantom have been measured at several depths with both walled and wall less proportional counters. Upstream from the Bragg peak, the distributions are narrow, reflecting the almost monoenergetic character of the radiation, while beyond the peak the distributions are broad. The dose mean lineal energy increases from about 20 keV/μm on entrance to almost 200 keV/μm at the peak, with most of the increase occurring in the last few millimeters before the peak. The distributions in LET calculated from the data are compared to corresponding distributions for 14.6 MeV neutrons and 90 MeV π^- mesons and show that at the Bragg peak a much higher proportion of the nitrogen ion dose is delivered at LET values above 100 keV/μm than by either of the other radiations. The wall effect is found to increase the measured dose mean lineal energy by about 25% in the upstream region but it drops to about 10% at 3 mm from the Bragg peak.