Oerenkov radiation, or the Cerenkov effect, is the visible bluish-white light surrounding a radioactive source observed in a pool of water. The phenomenon was first reported by the Russian physicist P. A. Oerenkov in 1934 (1), who was one of several scientists who observed the glow of light in close proximity to intense gamma-radiation sources. This effect is the reason why radiation sources are said to "glow in the dark." Its importance for molecular biology is that it is routinely used to measure the amount of the isotope P in a sample (see Radioactivity and Phosphorous Isotopes).
The Cerenkov effect can be explained by classic electromagnetic theory and principles of optic science. Analysis of uerenkov radiation has shown fundamental relationships between the velocity of charged particles, light, the intensity of the light, and its wavelength spectrum (2). Only a small fraction (<0.1%) of the charged-particle radiation is emitted by the absorbing medium as coherent light. According to Jelley (3), who provided the first scientific explanation for the fcerenkov effect, the observed light results from charged particles traversing a transparent dielectric medium, one that does not conduct electricity. Charged particles are produced in the absorbing medium (water) when gamma radiation from the radioactive source interacts with the absorber. These charged particles produce local polarization along their path in the dielectric. Light in the visible spectrum is emitted when the polarized molecules in the medium return to their rest state soon after passage of the charged particle. If the velocity of the charged particles is less than that of light in the same medium, the light emitted from molecules in the dielectric is overridden and not observed. However, if the velocity of the charged particles is greater than that of light in the dielectric, a wavefront of light is produced from individual molecules in the dielectric, and the emission is reinforced by constructive interference. The uerenkov effect is analogous to the bow wave from a ship that travels faster than the velocity of the surface waves, or to the shock wave trailing a supersonic aircraft passing though air. In other words, if the velocity v of a charged particle traversing a transparent dielectric material of refractive index n (= be) exceeds the velocity of light (chi) in the medium, or v > dn and b > 1 hi, then Oerenkov radiation is emitted at an angle x relative to the particle direction, where x = arccos (1/bc). The velocity c of light in a vacuum is 2.997 x 1010 cm/s, and b = vie. The Cerenkov photons form a conical wavefront of half-angle (90° – x) behind the particle.
The polarization effect actually decreases the energy lost by a charged particle that traverses a condensed medium, whereas the production of fcerenkov radiation increases the loss of energy by the particle. The visible spectrum is produced at a frequency interval of about 3 x 1014 Hz. Applications of Oerenkov radiation theory have been developed for detecting single high energy charged particles, measuring the energy of charged particles, and determining angles of incidence.
The light pulses emitted by charged particles traveling in a transparent medium can be collected and counted by modern scintillation counters. The amount of light produced in water is small compared to that produced in the presence of a scintillator, but it can be detected from beta-emitting radionuclides if their energy is greater than the threshold energy of 265 keV. The average energy of phosphorous-32 beta particles is 695 keV, so the majority of emitted beta particles can be detected.
