Following the global COVID-19 pandemic, digital learning environments have gained in importance in education all around the world. A new digital learning format called a "Digital Learning Module" (DLM) was developed within the Physics Education Research (PER) group at CERN, consisting of a series of prerecorded explanatory and experiment videos, interactive elements, an interactive simulation experiment, and expert interviews. So far, one DLM has been created on the topic of positron emission tomography (PET). Aiming to establish a set of effective design principles that can be employed in the design of future DLMs or similar digital learning environments, the goal of this research is to evaluate the already existing PET DLM. In doing so, this research brings together different theoretical and empirical studies, in order to identify design principles fitting the format of the DLM. It also reviews key physics literature on PET to identify its disciplinary-relevant aspects. First, an analysis of the PET DLM was performed in terms of the identified design principles and disciplinary-relevant aspects. Then, nine semistructured interviews were conducted with high-school students from Greece and Austria as they worked through the PET DLM. A qualitative content analysis of the interview transcripts has determined the set of effective design principles that will constitute the foundation of future DLMs developed by the CERN PER group and can serve as a starting point and reference for researchers developing similar digital learning environments elsewhere.