A novel gas system for low pressure tissue equivalent proportional counters

This work of thesis has two principal motivations: the construction of a new vacuum and gas circulation system and the research of a photodiode, which can function as trigger for the acquisition of a spectrum, for the new TEPC projected for MITRA. The thesis has been developed subdividing the theoretical part from the experimental section. The first three chapters introduce the concept of microdosimetry and all the necessary quantities to develop completely this science. The history of the TEPC the role played by this type of detector in radiation protection and biology has been subsequently described. An entire chapter is dedicated to gas gain and electronic avalanche is highlighted. This process guarantees a high gain and a high resolution of the TEPC. The last chapters focus their attention on the two scopes of my thesis. In chapter 4 there is the complete description of the vacuum and gas circulation system built at Politecnico di Milano for the new TEPC. This system has been tested at LNL and it demonstrates a good operation showing no vacuum leaks. The system displays a good response to the demands of pressure required in the sensitive volume of the detector to simulate nanometric volume. Chapter 5 describes all the studies made to find a photodiode, which can be correctly used as trigger for TEPC. The dimension represented the major problem. The new project requires to lay the photodiode in a cavity obtained in the case of the TEPC. A photodiode correct size and with good resolution has been found, if compared to the old SSD used in TEPC, which had a bigger sensitive area as it was placed in a metal cap outside the TEPC.