FCC NPs have become important for physics, medicine, chemistry, etc. However, their bulk properties have always been neglected and poorly explored. Vacancies-induced twinning is a new concept that describes the appearance of irregular NPs morphologies and HCP Au phase. Our preliminary synchrotron experiments at DESY showed that the X-ray beam causes the structural evolution of Au NPs. Analyzing SAXS+WAXS patterns, one can detect "fingerprints" of vacancies in NPs. The presence of "fingerprints" is predicted by our computational simulations and is the key element in the proof of vacancy-induced twinning. In addition, the current project will improve understanding of the X-ray photons-matter interactions, which heavily affect system structure, chemical reactions, and kinetics. These effects are underestimated and suffer from a lack of systematic research. Systematic studies of Au NPs evolution triggered by the EBS X-ray beam will allow a better understanding of beam-induced effects.