Infrared‐sensitive focal plane arrays based on extrinsic silicon, which integrate the detection and signal‐processing functions onto a single chip, are currently being developed at several laboratories. For imaging in the 3–5‐&mgr;m atmospheric window, highly doped Si : In is a leading candidate due to its spectral range, quantum efficiency, and moderate cooling requirements (50–60 K). The effects of residual boron impurities in the Si : In detector must, however, be compensated by donor concentrations to achieve these operational temperatures, so that precision compensation is a key factor for the production of uniform high‐responsivity detector material. We report here the successful use of thermal‐neutron irradiation for transmuting a small fraction of the silicon atoms into a known concentration of phosphorus donors in order to compensate Si : In detector material. Czochralski‐grown Si : In starting material of ⟨100⟩ orientation was evaluated by variable‐temperature Hall effect studies to containNIn=2.5×1017cm−3,NB=1.6×1014cm−3, andNP=0.6×1014cm−3. Samples were irradiated in a light‐water reactor and then thermally annealed for lattice damage recovery. Detector measurements indicate detectivitiesD* which approach background‐limited performance at temperatures of 55 K at a photon flux of 1015cm−2 sec−1. Maximum photoconductive lifetimes of 3 nsec were determined from responsivity measurements and are consistent with the known phosphorus concentration (2.2×1014cm−3) introduced by the transmutation of silicon and the measured impurity concentrations in Czochralski starting material. No attempts were made to optimize the compensation density to produce improved responsivity material, but the absence of any anomalous behavior is encouraging for future efforts at more precise compensation. Measurements of thermal‐carrier concentrations in both conventionally compensated and neutron‐compensated material also reveal an additional deep‐lying acceptor level at 0.11 eV above the valence band edge in Si : In. Concentrations of (3–6) ×1014cm−3were observed. A spectral dependence beyond the indium cutoff wavelength which can be attributed to a 0.11‐eV level in the Si : In material is observed.