Screening di materiali tramite ICP-MS per caratterizzazione di background in esperimenti di bassa radioattività

Search for very rare astroparticle physics events, such as Neutrinoless Double Beta Decay or direct Dark Matter interactions demands for extremely low background environments and materials for the experiment construction. The former requirement is fulfilled by underground laboratories, such as the Laboratori Nazionali del Gran Sasso of INFN, where the overburden rock reduces the cosmic ray flux by six orders of magnitude with respect to the sea level. The latter requirement, i.e., the material selection and cleaning for low-background experiment, increasingly challenges the international scientific community to develop new purification techniques and push the sensitivity to the edge of instrumental limits. Indeed, given the variety of materials used for the construction of experiments (i.e., metals, plastics, electronics compounds, loaded liquids, pressurised gases) and the very low levels of radiocontamination required, versatile and cutting-edge purification and characterisation techniques are needed. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is one of the most suitable technique for material screening because of its versatility and additional features such as high sensitivity (ppt level), short analysis time (days), small mass of samples (mg), possibility to acquire many elements simultaneously, and development of efficient analysis methods. Furthermore, several processes and chemical treatments are involved in the production of the raw materials, thus the effect of each step on the final product must be known in detail in order to be able to optimize the production protocol from the radiopurity point of view. ICP-MS is the only screening technique capable of investigating contaminations in all stages of production, for example separating bulk contaminations from surface ones by means of progressive chemical attacks. In order to reach excellent results with ICP-MS, the workspace is crucial, because environmental contaminations must be avoided. For this reason, clean-rooms are ideal environment to perform both sample’s treatment and characterization through ICP-MS. My thesis work was developed at the Chemistry and Chemical Plants Service of the Gran Sasso National Laboratories. I was involved in the development of innovative techniques for the cleaning and characterization of materials. Especially of the X and Y experiments. In the framework of the LEGEND experiment, highly enriched germanium crystals have been purified exploiting a chemical plant designed for this specific purpose. A sequence of chemical reactions was tuned to convert metal germanium crystals into high pure GeO2. The purification process was continuously monitored online by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), in order to verify the purification efficiency and the conversion rate from starting material to final products. Moreover, another material widely used to build inner structures and external shields of astroparticle physics experiments is copper, which is one of the most radiopure metal, easy to machine and suitable also for cryogenic applications. Given the extremely low contaminations in thorium and uranium (10−12 – 10−15 g/g) in copper, different approaches must be developed in order to reach low detection limits at a level of 0.4 - 1 × 10−12 g/g). In this thesis, pre-concentration methods based on matrix extractions were developed to maximize quantification of Th and U in several samples of copper. On the other hand, also different materials (plastic, metals, organic) have been characterized through ICP-MS analysis, developing different treatments to solubilize them.