Silicon Drift Detectors for high precision kaonic atom experiments

The work presented in this thesis is dedicated to precision experimental studies of kaonic atoms with a new radiation detector specially developed for this type of studies: Silicon Drift Detectors for high-precision X-ray spectroscopy. The experimental studies of light kaonic atoms o ers the unique opportunity to perform experiments equivalent to scattering at vanishing energies and to obtain a direct determination of the kaon-nucleus interaction at threshold, without the need of an extrapolation to zero relative energy, as in the case of scattering data. A kaonic atom is formed when a negatively charged kaon enters a target, is slowed down by losing its kinetic energy through the interaction with the medium and eventually is captured by an atom, replacing an electron. Since the kaon mass is almost thousand times higher than the electron one, the kaonic atom is formed in a highly excited state. Thus, the kaonic atom cascades down to a low n-state where the strong interaction between the kaon and the nucleus adds up to the electromagnetic one. It follows that the measurements of the X-ray emissions from de-excitations of kaonic atoms brings to the extraction of the shift ( ) and the width (Γ) of the atomic levels caused by the strong interaction. These observables are fundamental quantities for the investigations of KN interaction, and, consequently, for understanding of the non-perturbative QCD in the strangeness sector, with implications from particle and nuclear physics to astrophysics (the equation of state of neutron stars). Kaonic atoms are presently measured in two places in the world: at the DAΦNE collider at LNF-INFN and at J-PARC in Japan, the only kaon factories able to form in an e cient way these types of exotic atoms. In order to measure kaonic atoms de-excitations one needs to develop X-ray detectors able to work in high-background conditions, as the ones present in the experiments at DAΦNE and J-PARC. Silicon Detectors were used for many years in measurements of kaonic atoms. More recently, a new type of Silicon Detectors, the Silicon Drift Detectors (SDDs), were developed, with special geometries, eld con gurations and read-out electronics, which are able to work in the huge background environments of DAΦNE and J-PARC accelerators. These SDDs were characterized for the rst time in this work, and have excellent performances, as will be shown in this thesis, in terms of energy resolution, detection e ciency, stability, linearity and time resolution, such as to allow to implement a trigger to reduce the huge background. The newly developed Silicon Drift Detectors were used to measure kaonic lithium at J-PARC and the analyses of the data, presented in this thesis, produced the most accurate measurement of the kaonic lithium 3pÑ2d transition, fundamental for understanding low-energy QCD and kaonic atoms cascade processes. A more accurate characterization of the detectors' system was required for the more ambitious and di cult measurement of kaonic deuterium, planned in the coming years by SIDDHARTA-2 at DAΦNE. This characterization required dedicated studies, especially for optimizing the timing performance of the detectors, on which the background reduction by the SIDDHARTA-2 veto systems depend. The optimization work on the SDD and DAQ chain produced a timing response perfectly compatible with the SIDDHARTA-2 requirements. SIDDHARTA-2 will be installed at DAΦNE in 2019, and, thanks to the work presented in this thesis, all the conditions for a successful measurement of kaonic deuterium are ful lled. The thesis is organized as follow. Chapter 1, Kaonic atoms experiments, introduces the kaonic atoms scienti c case, together with a critical analysis of the Silicon Detectors used in for the kaonic atoms spectroscopy and a brief description of DAΦNE and J-PARC accelerators, where the most important activities in kaonic atoms physics is carried out. Chapter 2, Silicon Drift Detectors for X-ray spectroscopy, describes the working principle of the Silicon Drift Detectors, the best detectors for kaonic atoms measurements, with an outlook to its evolution during the years, which brought to the realization of the novel device characterized and optimized in this thesis. Chapter 3, Experimental setup for the characterization of the new Silicon Drift Detectors, presents the experimental apparatus and the acquisition systems for the characterization of the Silicon Drift Detectors in the SIDDHARTA-2 laboratory. Chapter 4, SDDs characterization results, presents the results obtained from a dedicated study of the Silicon Drift Detectors energy response as a function of applied voltage and temperature variation, which is fundamental to de ne the optimal working conditions. Chapter 5, Kaonic atoms at J-PARC: kaonic lithium measurement, presents the results of the data analysis of kaonic lithium measurement performed at J-PARC, which allowed to obtain the most precise measurements of the K Li Lα transition yield in the world (an article is in preparation). Chapter 6, Silicon Drift Detectors for precision kaonic atoms measurements, shows the results of the studies and optimization of the Silicon Drift Detectors system for SIDDHARTA-2, which put strong requirements to the system, in terms of linearity, stability and timing resolution. Chapter 7, Towards the kaonic deuterium measurement in SIDDHARTA-2, presents the SIDDHARTA-2 experimental setup, with a dedicated discussion to the role of the new Silicon Drift Detectors for the rst measurement ever of the kaonic deuterium transition to the fundamental level, which is going to be performed at DAΦNE in the coming years. The thesis ends with the Conclusions, Chapter 8, where the main results obtained during my Ph.D. activity and presented in this thesis are summarized.