The first observation of CR is believed to be an account from Dr. Curie over a century ago [5
]. Pavel A. Cerenkov later characterized the phenomenon, which earned him a share of the 1958 Physics Nobel Prize together with Ilya Frank and Igor Tamm. The radiation is polarized and continuous with an intensity distribution that is inversely proportional to the square of the wavelength. Thus, the majority of the light is in the ultraviolet (UV) and blue end of the visible spectrum (). It is most commonly observed as the blue glow in the cooling ponds of spent nuclear fuel.
Figure 1 Cerenkov Light and Characteristics. A) Cerenkov radiation (CR) is produced by a charged particle traveling through a dialectric medium faster than the speed of light (in a vacuum) divided by the refractive index of that medium. Relaxation of the molecules (more ...)
The charged particles released upon radioactive decay may include electrons (such as β- particles, Auger electrons and conversion electrons), positrons (β+), and α-particles. As these particles travel, they lose energy through interactions with the surrounding matter. In the biological context this matter is almost always water. At speeds below the speed of light in water, the randomly oriented water molecules will align with the passing of the charged particle. After the particle passes, these aligned water molecules along this path will relax back to a lowest energy state. In cases when the particle is traveling at super-relativistic phase velocities (i.e. faster than the speed of light in that medium), these polarized molecules relax by releasing energy in the form of visible radiation luminescence.
The threshold speed for CR production is the phase velocity, or speed of light in that medium. The β-particle velocities (v) as a function of energy can be calculated from Equation 1
Here, c is the speed of light in a vacuum, E i the particle energy and Eo
is the mass of the β-particle at rest, in the same units as E. The number of Cerenkov photons produced along the β-particle’s path can be calculated using the Frank-Tamm formula over a specified region of the light spectrum as Equation 2
Here θ is the fine structure constant (1/137), the λ’s define the spectral range, n is the refractive index of the material and
is the velocity of the β-particle divided by c. The threshold energy to produce CR can be solved for a β-particle in water (with a refractive index of 1.33), to yield a minimum energy reqiurement of 263 keV. Many of the β-particles produced by 18
F decay possess greater energy than this threshold and therefore produce CR (the 18
F endpoint energy is 633 keV and mean is 250 keV). For radionuclides that are studied for biomedical applications (including 225
Ac), α-particles are generally not of sufficient energy to produce light directly. However, the CR emissions of daughter isotopes’ decay particles have been detected [7
]. It should be noted that as this process is refractive index dependent: the Emin
is reduced to ≥ 219 keV in tissue (using an approximate refractive index of 1.4) [8
]. A direct illustration of this dependence on refractive index is the measurement of light produced in varying media ().
At this time, several studies have theoretically and experimentally evaluated nearly all of radioisotopes of interest for CR production [7
]. The β-particle energy spectrum and branching ratios for a given radionuclide determines the amount of CR produced photons. Thus, for commonly used radionuclides the number of CR photons produced per disintergration follows the trend of 90
Y > 68
Ga > 15
O > 11
C > 124
I > 89
Zr > 18
F > 64
Cu. For a particular Cerenkov imaging application, different parameters must be weighed for the choice of radionuclide including the amount of light produced, radiolabeling strategy, half-life of the tracer, and biological implications of the higher energy particles.