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Fluence measures the number of particles or energy in a square area and is used in fields such as nuclear medicine and space travel for radiation dosimetry. Proton and electron fluence rates are measured in particles or joules per square meter multiplied by time. HZE particles in cosmic rays can cause carcinogenic effects, and neutron fluence is important in science for calibrating equipment and colliders. Fluence rates also affect air travel and non-ionizing radiation exposure.
Fluence is a measure of flux in physics to calculate the number of elementary particles in a square area, or the amount of energy in that area in values of particles per square meter or joules of energy per square meter. It is often used in the fields of nuclear medicine and space travel to determine the amount of ionizing radiation delivered to humans or equipment, known as radiation dosimetry. When it comes to fluence, the positively charged particles that make up atomic nuclei, known as protons, and the negatively charged particle energy of orbiting electrons are considered to be the most important.
To be accurate in three dimensions, fluence calculations are actually based on the number of particles or the amount of energy that falls on the surface of a predefined sphere within a specific time. Proton fluence is another term for particle fluence, and is often called particle flux density, which is measured in particles per square meter multiplied by a time value. The energetic fluence of electron-based ionizing radiation is measured in joules per square meter multiplied by a time value.
In space research, the fluence rate is calculated to determine the amount of ionizing radiation contained in heavy energetic nuclei, known as HZE particles. These particles are the main component of cosmic rays which can lead to carcinogenic effects in human tissue with significant exposure levels. While the Earth’s magnetic field protects spacecraft orbiting within it from some of this cosmic ray damage, in deep space or on extended missions farther from Earth, exposure levels will be much higher. Fluence rates per square area for radiation exposure are also considered to be more accurate than typical radiation dose level calculations, as research on laboratory animals has shown that heavy ions, such as those in iron, promote the expression of the tumor as much as the lighter ones produced by lanthanum or niobium.
Neutron fluence is also an important measurement in science, which is used to calibrate equipment for their level of radiation protection and to calibrate radiation meters. The velocity of the neutron flux is an important element of nuclear particle colliders in high energy physics research. Research facilities such as the Large Hadron Collider (LHC) near Geneva, Switzerland can have neutron fluence energy levels as high as 1,000,000 electron volts. Neutron energy levels are also an important component of nuclear weapons research and production, nuclear fission power generation, and astronomical research of supernova stars.
However, routine exposure to ionizing radiation does not occur only in narrow research areas of physics or space travel. Fluence rates are also a significant factor in air travel, as passengers and crew of commercial aircraft are exposed to more radiation from space than they would be on the ground. Photon fluence is another more routine exposure method for non-ionizing radiation that poses less risk to human health and is calculated for fields such as laser dentistry.
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