A detailed electron spin resonance study has been performed on the Si-B5 paramagnetic defect (C-3v symmetry) in neutron-irradiated c-Si subjected to annealing in the range 200-350 degrees C. A rich and detailed Si-29 hyperfine structure is newly resolved and fully angularly mapped. Computer simulations point toward the defect's unpaired electron as having its strongest interaction with one Si site, followed by interaction with two shells of two equivalent Si sites each, and two more shells of both three equivalent Si sites. Two previously proposed tri-interstitial models are discussed as possible candidates for the Si-B5. However in lack of extensive theoretical calculations, assignment of the Si-B5 to one of the tri-interstitial models remains undecided. In addition we also report on an unusual temperature behavior of the hf splittings as well as the principal g matrix values. (C) 2008 Elsevier B.V. All rights reserved.