[en] The biomedical negative pion beam at the Los Alamos Scientific Laboratory has been studied with the use of lithium-drifted silicon detectors. A waterproof Lucite probe holds the detector and is immersed in a water phantom. The detector assembly is attached to a scanner which moves the probe in three dimensions. Detectors of 5000, 1000, and 10 μm thickness were used to obtain pulse-height spectra of the incoming beam and of the disintegrations produced by the pions. The mean momentum of the pions is 168 MeV/c, and the momentum spread is about 6 percent of the momentum (FWHM). The range is 18.8 cm in water. The beam components are approximately 4 percent electrons, 10 percent muons, and 86 percent pions. Pulse spectra were also taken under the collimator used with patients. The spectra show that the plateau region is essentially a low-LET region. Furthermore, in the peak region, the dose produced by the stars occurs predominantly on the downstream side of the peak due to the total dose. This structure of different doses must be included in treatment plans for small nodules