For the first time the samples, cut from the same wafer of crystals of float-zone silicon, n-FZ-Si(P) and n-FZ-Si(Bi), were subjected to irradiation with 0.9-MeV electrons and 15-MeV protons at RT for studying them by low-temperature positron annihilation lifetime spectroscopy. Measurements of Hall effect have been used for the materials characterization. The discussion is focused on the open vacancy volume (Vop) of the thermally stable group-V-impurity-vacancy complexes comprising the phosphorus atoms; the bismuth-related vacancy complexes are briefly considered. The data of positron probing of PV pairs (E-centers), divacancies, and the thermally stable defects in the irradiated n-FZ-Si(P) materials are compared. Beyond a reliable detecting of the defect-related positron annihilation lifetime in the course of isochronal annealing at - 500-°C, the recovery of concentration of phosphorus-related shallow donor states continues up to -650-700-°C. The open vacancy volumes Vop to be characterized by long positron lifetimes 2 -271-289 ps in (gr.-V-atom)-Vop complexes are compared with theoretical data available for the vacancies, (V1), and divacancies, (V2). The extended semivacancies, 2Vs ext, and relaxed vacancies, 2Vinw, are proposed as the open volume Vop in (gr.-V-atom) Vop complexes. It is argued that at high annealing temperature the defect Ps Vop Ps is decomposed.