Development of new materials for next-generation astroparticle physics experiments

Materials science is a multidisciplinary approach to various fields of physics and chemistry. The choice of the proper materials is crucial during design, R&D, and construction of an experiment where construction materials are the main source of background during data taking. Keeping materials-induced background under control is a key point for rare events physics searches. In this field, the background is due to particles whose interactions with the materials of the experimental apparatus can mimic a signal similar to the events that the experiment is looking for. Other unwanted particles can be detected and distinguished from the desired signal, but their detection affects the so-called dead time, in which the detector is blind for the tracing of the desired events. To avoid these drawbacks, detection strategies of unwanted particles are adopted through the construction of specific detectors, or vetoes, which have the objective of shielding the active volume of the experiment (i.e. the part used for the detection of the rare event). This thesis work is focused on these devices: on one hand, I worked on the construction of a neutron veto for the DarkSide- 20k experiment, on the other I tested some commercial organic scintillators in cryogenic environments. The latter project may pave the way for the integration of the aforementioned devices into rare-event physics experiments. Firstly I introduce the DarkSide-20k experiment, whose aim is the direct search for dark matter, based on the Weakly Interacting Massive Particle (WIMP) model. In this context, one of the main sources of background is represented by neutrons, whose interaction with the liquid argon, which is the active target material, could simulate a signal due to a WIMP. In order to overcome this issue, a neutron detector used as veto apparatus was designed, consisting of a 15 cm thick wall of polymethyl methacrylate (PMMA) loaded with gadolinium. The use of gadolinium is due to its high cross-section (which is a physical parameter that expresses the probability of a process) for neutron capture. All the materials involved in the veto construction must meet severe radiopurity criteria: all the compounds are subjected to screening, in order to measure the presence of isotopes such as 40K, 238U, and 232Th. Therefore, an R&D project was conducted for the development of an innovative and radiopure hybrid material consisting of PMMA containing a gadolinium-based compound. For our purposes, it was chosen gadolinium oxide (Gd2O3) in the form of nanoparticles (indicated as NPs in this work). Since this compound is not miscible in methyl methacrylate (MMA, the starting monomer), it is necessary to functionalize the NPs in order to overcome the sedimentation and to obtain a quite homogeneous sample, still maintaining all the additives in a small concentration in order to avoid possible background sources. During the research activity, I worked on the gadolinium oxide R&D, which proposes a possible strategy to produce radiopure and homogeneous samples that meet the experiment requirements. This process is also scalable to industrial productions. Also, several characterizations were performed in order to test the thermomechanical properties of the composite material. In chapter 1, I will introduce the state of the art in the dark matter (referred to in this work as DM) research field, where the DarkSide-20k experiment is inserted. In chapter 2, I will illustrate the DarkSide purpose and the detector (whose construction started in 2022). Mainly, I will focus on the veto apparatus and I will briefly discuss the background control requirements. In chapter 3 I will focus on the R&D project on the gadolinium-loaded PMMA, so the mixing and the polymerization procedures that have been developed. Also, I will show some results of the analyses conducted on the treated NPs, in order to evaluate the effectiveness of the functionalization and the stability of these grains in a colloidal solution. Chapter 4 is fully devoted to the characterizations of the polymeric samples, investigating the effect of the NPs doping on the thermomechanical properties of the polymeric matrix and testing the prototypes in a cryogenic environment (reproducing the conditions that will be encountered in the experiment). Chapter 5 is devoted to the description of the purification strategy which was adopted for a commercial surfactant involved in the synthesis process. In chapter 6, I will illustrate the work that has been done for the scaling of our production procedure to a partner industry. Also, in this chapter are reported some tests that have been done on the industrial samples. In chapter 7, finally, I will illustrate what has been done for the PESCE project, which is a grant funded by INFN for the past 3 years, whose purpose was the characterization of some organic scintillators in cryogenic environments. In the appendix, I briefly report additional projects that I followed during my Ph.D. years, related to astrophysics and space science, to which I contributed with my knowledge of materials science for the choice and characterization of the experimental apparata.