The present work is the first part of a series of Technical Reports, summarizing the various analyses carried out using Monte Carlo computational methods for the characterization of reference photon radiation fields for the calibration of personal dosemeters. For this purpose the tissue equivalent ICRU (International Commission on Radiation Units) sphere was adopted as a suitable phantom. In this first Report, which will be followed by other three Reports, a complete analysis of Backscatter Factors and their angular dependence have been carried out for five X-ray reference series as stated by the ISO (International Standards Organization) e B. I. P. M. (Bureau International de Poids et Measurements), besides Cesium and Cobalt. The results so obtained have been compared with values computed in similar conditions at other European Institutions, with satisfactory agreement. The work has been carried out under the CEC contract n 160347 and connected to the activities of EURADOS WG-4 (Computational Dosimetry).