Search

THE detection of ultra-violet light and X-rays in the human eye, through a fluorescence mechanism, is well known1, and it would be expected that a high flux of ionizing particles would be similarly detectable. There is apparently no record in the literature of the detection of single minimum ionizing particles, though there are several mechanisms by which a relativistic particle may release a detectable amount of energy in traversing the eye. These include the emission of Čerenkov radiation2in the vitreous humour. An ultra-relativistic singly-charged particle, in median passage through a normal eye, from lens to retina, generates approximately 600 photons in the visible, which will be collected over a large part of the retinal area. The visual signal would be equivalent to that from an illuminated disk, with intensity increasing from about 0.5 photon/mm2at the periphery to about 10 photons/mm2at the centre. This light is emitted essentially instantaneously (within 10−10sec) and has an intensity comparable with that at the threshold of vision3. In addition to Čerenkov radiation, light may be released by scintillation in the crystalline lens or vitreous humour, or direct excitation of the retina may occur.