Charged particles have electric field around them and the energy is carried by photons. This makes the field only able to travel at the speed of light. If the particles are made to travel at a speed higher than that in a liquid medium such as water, the electric field is left behind causing a shock front that appears in terms of light energy. This is because of the exited atoms present in the medium during the process(Leontic, 1967). The liquid atoms are excited then de-excited by the shock wave of the particles travelling faster than the speed of light and emit a blue light. It should be noted that the quantity photons emitted by a charged particle at that high speed is varies indirectly as the wavelength. This implies that the wavelengths will be shorter hence the light spectrum will be geared towards the blue side.
In simpler terms, the randomly the randomly positioned water molecules will be made to align themselves with the high speed moving particle thus acquiring energy which is then released in form of light to enable them regain stability.
It is also important to note that different isotopes that are unstable experience radioactive decay. Some produce a- particles that require a relatively higher threshold energy hence are more difficult to experience Cerenkovs luminescence while others may release b-particles that are relatively easier to experience the Cerenkov process due to their light nature hence low threshold energy requirements(Ma, Wang & Cheng, 2014). Various scientists and researchers have published several articles about Cerenkov luminescence. The basic scientific theory is, however, constant. When charged particles travel more than the speed of light in a specific medium, in which they are moving, light is produced which appears as a faint blue glow.
It is also important to note that different isotopes that are unstable experience radioactive decay. Some produce a- particles that require a relatively higher threshold energy hence are more difficult to experience Cerenkov's luminescence while others may release b-particles that are relatively easier to experience the Cerenkov process due to their light nature hence low threshold energy requirements(Ma, Wang & Cheng, 2014).
Cerenkov light and refractive index of the medium
The light emitted during the Cerenkov process is largely determined by the refractive index of the medium. The minimum energy required for the Cerenkov light to be produced on refractive index. It is also clear that physical phenomena cannot be responsible for the production of the glow since it has been scientifically proven that generation of light depends on the refractive index of the medium.
The number of photons produced varies directly as the refractive index. The threshold energy has an inverse proportionality relationship with a refractive index of materials with b-particles(Leslie & Greenberg, 2003).
To explain this, the minimum energy required for the Cerenkov process is much lower in dense liquids with higher refractive index making it harder for the light to be produced compared to less dense media with a relatively lower index of refraction. It is, therefore, important to manufacture Cerenkov detectors for low energy radiations with higher refractive index materials.
Production of Cerenkov luminescence from a- particles
The threshold energy for production of Cerenkov light from a- particles is significantly higher compared to b particles. This is due to the heavy nature of the a particles which are about 104 times the mass of b particles.
While it is scientifically possible to produce Cerenkov light from a particles, it is much more difficult to produce the same in a practical scenario. It has been discovered that the light emitted in these experiments is from the b particles that are produced in the secondary part of the radioactive decay process as daughter radionuclides. Most experiments have noted a significant delay time before the presence of lamination from a particles. This is due to the formation of secondary b particles from the a parent particles. Experiments involving pure a particles have been unsuccessful due to high levels of toxicity.
Conical wave properties of Cerenkov wave-front
The Huygens wave construction for the Cerenkov wave behavior can be used to compute the angle between the generated light wave and the direction of travel of the charged particle. This angle most significantly relates to the amount of energy released by the particle. At threshold energy, the photons are in the direction of the high-speed particle. However, as the energy increases, the angular difference of direction starts to increase up to a maximum of 41.3o in water. The photons can be polarized at a different angle but with a negligible significance in an application today. The wave is shown below("Current Readings in Nuclear Medicine", 2007).
The characteristics of Cerenkov spectrum
The magnitude of photon produced from the Cerenkov process can be described mathematically as:
dN/dl = 2paz2 (1/ l1 1/ l2) sin2 th
Where th is the Cerenkov's angle, l1 and l2 are the upper and lower values of the relevant wavelengths; l is the length, z the speed and a, the structure constant equal to e2/h.c=1/137.
It is thus obvious that greatest number of photons is emitted in the ultraviolet area, blue region since dN2/dl is directly proportional to 1/ l2 and is maximum at 330nm. Experiments have shown continuous spectrum at threshold energy and a further increase with an increase in wavelength. Changing the radionuclides only changes the magnitude of photons and subsequent light intensity but not the spectrum (Caglioti, Cervellati & Mezzetti, 1959).
Relationship between Cerenkov light intensity and spatial distribution
The b particle energy probability distribution is highly dependent on the resultant energy at the process end point. Particles with higher endpoint energies mostly correspond to the atomic number, have a higher number of particles capable of undergoing Cerenkov process compared to those with lesser end point energy values. For any isotope to undergo radioactive decay and produce Cerenkov luminescence, the area integrated into the probability function should be more than the threshold energy. An experimental example is provided by 90Y which has higher endpoint energy and the probability distribution of the particle energy falls above the threshold energy produces a lot of photons hence a significant Cerenkov light energy and compared to 18F, which undergoes non-significant Cerenkov process due low-end point energy.
Cerenkov luminescence in biological tissue
The light from Cerenkov process can pass through body tissue undetected, disperse or even undergo absorption depending on the wavelength. Photons appearing in the blue to the green region are absorbed by hemoglobin in the red blood cells. However, the photons located near the infrared region will seldom be absorbed. The light will undergo a scattering phenomenon if they move through biological fluids with different refractive indices. In an in vivo imaging, when a point source is placed under a tissue, the intensity reduces with increase in distance. This is because shorter wavelengths are easily attenuated compared to the longer wavelengths. However, image resolution depreciates faster with longer wavelengths compared to shorter wavelengths(Fruin & Jelley, 1968).
Significance of Cerenkov Light in Biology: CLI
For several decades, Cerenkov light was being used in the field of physics for several applications such as cosmic rays and the estimation of nuclear reactors fuel (Barone, 2008). It was only until the 1970's when the eyes of a rabbit generating CL lead to the measurement of 32P for radiotherapy. Thirty decades later in the year 2009, other scientists were able to describe the optimal imaging of 18 F-FDG in a mouse with a tumor as CLI. Initially, there were doubts of if it was CLI and was argued to be as a result of gamma rays with high-energy hitting on the CCD camera chip. The 18 F-FDG was covered with an opaque piece of paper. It was observed that the signal disappeared. This meant that the signal was a light which was visible and not high-energy photons (Georgescu, 2012).
Cerenkov Light from Medical Radiotracers
Researchers are keen on studying clinical isotopes such as 13N, 15O, 89Zr, 68Ga, 225Ac, 64Cu, 74As, 90Y and 124I. This is the realization that CLI can be used for preclinical molecular imaging. It has been observed that radioactivity has a correlation to radiance in both Vitro and Vivo. CLI has been used in cancer chemotherapy and radiotherapy and also in imaging gene expression. It is of great significance in radiotherapy since it can image radioisotopes that contain electrons or those with emissions of a-particles that may not be reasonably imaged any other way. These particles are suitable for CLI due to their high kinetic energy. Cerenkov is also used in radiochemistry quality control as it always for quantification of radioactivity of 18F for a microfluidic chip that is in a constant position. An activity of 0.19 uCi/mm2 was the minimum activity that was detected. Research has provided for an opportunity of utilizing the available clinical positron emission tomography (PET) by merging nuclear and optical imaging. Optical instruments are cheaper than PET and are also faster. Information on their use on other animals can be gathered before it is used on humans(Bolotovskii & Leikin, 1960).
Instrumentation of CLI
There is an average generation of 1.4 photons per decay from a standard tracer 18F. The amount of CL is therefore very low thus requires instruments that are very sensitive and also those that will take a shorter imaging time. There is however highly advanced camera that is available today for astronomy, bioluminescence, and chemiluminescence. The cameras available have high sensitivity for counting a single photon with minimal internal noise that is usually mounted inside a black box to shield it from ambient light and is used for preclinical imaging. For an effective signal to background ratio, the devices in use have to be very sensitive, and the acquisition must happen in complete darkness. This because light from the surroundings falls on the same window with a greater flux than CL. The CL spectrum is in UV(Hayata & Koshiba, 1989). Devices which are available today have built-in parameters for CLI. The devices use CCDs which can detect CL at the surface. These CCDs are normally cooled up...
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