Experiments conducted in the 1970s also studied the phenomenon. These experiments revealed that although several explanations for why the LF were observed by astronauts have been proposed, there may be other causes as well. Charman "et al." (1971) asked whether the LF were the result of single cosmic-ray nuclei entering the eye and directly exciting the eyes of the astronauts, as opposed to the result of Cherenkov radiation within the retina. The researchers had observers view a neutron beam, composed of either 3 or 14 MeV monoenergetic neutrons, in several orientations, relative to their heads. The composition of these beams ensured that particles generated in the eye were below 500 MeV, which was considered the Cherenkov threshold, thereby allowing the researchers to separate one cause of the LF from the other. Observers viewed the neutron beam after being completely dark-adapted.