An approach to microdosimetry in a π- meson beam using nuclear emulsions

K5 emulsions 10 μm thick were exposed at various depths in a perspex phantom to a 70 MeV π- meson beam and counts taken of the tracks in emulsion volumes 7 x 7 x 10 μm3. Data are presented on the number of track events and also the total number of grains associated with each event in each of four categories spanning the LET range of the secondary particles. The number of heavy tracks (category 4) showed an increased incidence in the region of the stopping π- mesons (14.5 cm in perspex) while the number of single grains (category 1) decreased with depth. Categories 2 and 3 (grain clusters and light tracks) were approximately constant with depth. An estimate of the grain sensitivity was obtained by taking the proton as representative of the whole range of secondary particles. This procedure gave a value of 5 keV per grain in the π- peak. The LET of light tracks was therefore in the range 1 to 10 keV μm -1 in emulsion, scaling to 0.4 to 4 keV μm-1 in water. Heavy tracks had LET values in excess of 4 keV μm-1 in water. (author)