[en] Mapping of relatively strong gamma radiation fields using computer modelling started already in the 60's. The Institute of Isotopes (Institute) was one of the developers and users of this technique in connection with high activity Co-60 source installation, for commissioning and dose planning. We have found this technique very useful, economical, fast, sometimes unique for the determination of the dose rate distribution for the operators and dosimetrists, and also for designers to design such gamma irradiators. Several institutions and individuals in Hungary and abroad were involved in the development work for example, Bulgaria, ex East Germany, Singapore, Iran, Turkey, Libya and Cuba. Computer programmes were distributed during the IAEA Radiation Engineering Courses in Budapest, IAEA Course for Health Physics Inspectors in Prague, and other National and Regional Courses organised by the IAEA. The continuous activities have resulted in several small programmes to calibrate and to readout the ECB dosimeters, to design shielding, transportation containers, irradiation units and large plants. Finally, we have developed a universal modelling programme to calculate the details of the operational parameters of irradiation units. Our concept was and is that, for process control, it is equally important to know the details of the irradiation unit as to have reliable dosimetry systems. (author)