Development of low level counting systems for high sensitivity measurements

Low level counting techniques are a powerful tools in many different fields including biological and chemical tracer studies, archaeological and geological dating, investigation of natural and induced radioactivities and the study of fundamental particles physics. Since they play a crucial role in many applications, developments are continuously pursue to improve the extreme sensitivity typical of these techniques. The fundamental request of low level counting techniques is the capability to detect feeble signals, characterized by low counting rates, above the background composed of spurious counts; they thus have the capability to measure very low activities of natural and artificial radionuclides. During my PhD work I have focused on two main applications using low level counting systems: the search of rare physics events and the environmental monitoring. Both the studied systems use γ spectroscopy with High Purity Germanium detectors (HPGe). This technique is one of the most sensitive, it exploits the excellent energy resolution typical of such detectors and their low intrinsic background. The first system is composed of two n-type HPGe GMX 100-95 in Low Background configuration, they have been designed with radiopure selected materials to reach an intrinsic background as low as possible. This work has brought to a unique configuration associated to an electrical cooling system, different from the one that was available on market. Since the two GMX detectors have been conceived to work in coincidence I have optimized the measuring system studying different radionuclide decay schemes to consider the most probable coincidences between the γ -rays emitted during the decay of the isotope under study. A dedicated data acquisition has been developed with particular attention to the co- incidence detection efficiency. Furthermore I have developed the analysis software to For some rare physics events experiments the only way to enhance the sensitivity is the background reduction; for this purpose all the materials of the experimental facility should be selected as radiopure as possible. The development and optimization of the low background system composed of the two GMX detectors, working in coincidence, can select suitable materials through the measurement of very low radionuclide concentrations. The other system I have worked on is a Broad Energy Germanium (BEGe5030) in low background configuration. This detector can register with excellent energy resolution and high efficiency a wide energy spectrum, from 3 keV up to 3 MeV, thanks to some of its peculiar features. The thin dead layers surrounding the active volume and the thin entrance window on its top are responsible for its capability to detect very low energy radiations. I have optimized this system using Monte Carlo simulations to detect low contamina- tions of radionuclides in several samples. In particular in the last century, the environmental monitoring has became an impor- tant aspect of radio-protection; nuclear tests, nuclear accidents, wastes and fall-out in general can release a large quantity of radionuclides. Since toxicity and radioactivity of these contaminations are dangerous it is mandatory an environmental monitoring at the area of interest. Plutonium isotopes significantly contribute to the contamination due to nuclear fall- out events in environment; since they are very toxic it is important to quickly monitor a large number of samples in the area of interest. Using the BEGe I have developed a quick and sensitive method to detect Plutonium isotopes concentration in environmental samples through the detection of the X-rays emitted during their decays. This result is a very important improvement in Plutonium detection for monitoring measurements since the counting methods commonly used are characterized by long measuring and sample treatments time.