Barkla, Charles G. (1911). „XXXIX.The spectra of the fluorescent Röntgen radiations”. Philosophical Magazine. Series 6. 22 (129): 396—412. doi:10.1080/14786440908637137. „Previously denoted by letters B and A (...). The letters K and L are, however, preferable, as it is highly probable that series of radiations both more absorbable and more penetrating exist.”
