[en] The work reported here was performed in order to investigate the usefulness of microdosimetry applied to these beams as a tool for characterizing the beam, and to supply corroborating evidence for independent measurements done with conventional ionization chambers. All spectra reported here were taken with a standard 2.54-cm spherical proportional counter with a 3.175-mm tissue-equivalent (TE) plastic wall. The counter was filled with propane-base TE gas to a pressure of 16.5 Torr, simulating a 1-μm diameter site in unit-density tissue. The counter was placed in front of the experimental beam port at the BMRR. Beam intensity was adjusted to obtain a counting rate between 1000 and 1500 cps. Electronic pile-up rejection was employed to provide further insurance against spectral distortion due to electronics. Calibration was done with a soft-X-ray source (aluminum characteristic X rays, 1.495 keV)