The term escape fraction is used for the faction of photons capable of ionizing atoms that escape from a source that generates them, e.g., stars. A galaxy has an escape fraction of such photons which can ionize surrounding neutral hydrogen. Early stars produce much of such EMR in galaxies and current modeling suggests such fractions can be as low as a few percent and can be more than a half.
The escape fraction of early galaxies is of interest in working out the history of reionization, e.g., whether galaxies provided the photons. Photons that ionize are typically in the ultraviolet range or even more energetic.