1 Preliminary results of a dosimetric system to be applied in microdosimetry as a support instrument in operational routines in nuclear medicine and radiotherapy José Patrício N. Cárdenas *1a , Tufic Madi Filho a , Letícia L, Campos a a Instituto de Pesquisas Energéticas e Nucleares, Av. Prof. Lineu Prestes 2242, CEP 05508-000, São Paulo, SP, Brasil. Abstract: Most of the radiation effects (in particular biological effects) depend on the microscopic pattern of energy deposition. This fact become apparent if one observes that, although the average energy expended to produce el 1 ementary units of physical damage (ionizations) is fairly independent of particle type and energy, the biological effectiveness of otherwise equal doses of different radiation types may be quite dissimilar. Microdosimetry, the study of the fluctuations of energy deposition and the associated stochastic quantities, was developed to provide a comprehensive description of the spatial and temporal distribution of absorbed energy in irradiated matter. An important step in understanding the radiobiology quality of therapeutic beams is the development of a microdosimeter based on the measurement of deposited energies at a cellular level. Microdosimetry deals with the problem of identifying radiosensitive targets and obtaining the probability of energy deposition therein. Models of radiation action, biological or otherwise, may then be used to convert this information in observable quantities. The aim of this work is the development of a dosimetric prototype system using semiconductors as sensitive material for microdosimetric measurements to determine equivalent doses and energies of incident beams in order to be applied as a support tool in operational routines in radiobiology, radiotherapy, microelectronic and radiation protection. The radiation response of silicon components to neutron fields from nuclear research reactors, IEA-R1 and IPEN- MB1 (thermal, epithermal and fast neutrons), from beam holes, experiments halls, AmBe neutrons source and in the BNCT (Boron Neutron Capture Therapy) Research facility at the IEA-R1 reactor of IPEN/CNEN-SP was investigated. KEYWORDS: Dosimetry, Radiotherapy, Nuclear Reactors Facilities 1. Introduction Microdosimetry When the first papers about microdosimetry were published in the fifties or sixties by Rossi and others [1-3] the relevance of this new approach was immediately apparent: in fundamental radiobiology, for the better understanding of primary mechanism of radiation action and in radiation protection, where one deals with low doses, a small number of events and with different types of radiation. In radiation therapy where the doses are relatively high, the relevance of microdosimetry appeared, at first, to be limited. However, with the development of neutron therapy (and high LET therapy), the possible application of microdosimetry became more evident [4]. One of the ways in which microdosimetry has evolved in the past decade has been its increasing application in practical fields such as health physics and medical physics. With this increased dissemination of microdosimetry to other fields, many new practitioners of microdosimetry are being engendered who may not always be familiar with the experimental techniques [5]. Microdosimetry is the area that deals with the distributions of energy deposition events at the microscopic level and their correlation to the effects of radiation on biological targets. Microdosimetry is formally defined by Rossi and Zaider [6] as “the systematic study and quantification of the spatial and temporal distribution of absorbed energy in irradiated matter” [7]. Microdosimetry, as stated by the ICRU Report 36 [8], is a conceptual framework (with corresponding experimental methods) for the systematic analysis of the microscopic distribution of energy deposition in irradiated matter, being its objective to develop concepts which relate some of the principal features 1 * Presenting author, E-mail: ahiru@ipen.br