Fluence is a flux measurement in physics for calculating 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 human beings or equipment, known as radiation dosimetry. When talking about fluence, the positively-charged particles that make up atomic nuclei, known as protons, and the energy of negatively-charged particles of orbiting electrons are considered most important.
To be accurate in three dimensions, fluence calculations are actually based on the number of particles or amount of energy that falls on the surface area 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. Energy fluence from 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 that energetic heavy nuclei contain, known as HZE particles. These particles are the primary component of cosmic rays that can lead to carcinogenic effects in human tissue with significant exposure levels. While the magnetic field of the Earth shields orbiting spacecraft within it from some of this cosmic ray damage, in deep space or for prolonged missions farther from Earth, exposure levels will be much higher. Fluence rates by square area for radiation exposure are also seen as more accurate than typical radiation dose level calculations, as research using lab animals has shown that heavy ions, such as those of iron, promote tumor expression as much as lighter ones made from lanthanum or niobium.
Neutron fluence is also an important measure in science, which is used to calibrate equipment for its level of radiological protection as well as to calibrate radiation dose meters. The rate for neutron flow 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 of up to 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 into supernova stars.
Routine exposure to ionizing radiation does not merely occur in narrow research areas of physics or space travel, however. Fluence rates are also a significant factor in air travel, as passengers and crew on 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 risks to human health, and is calculated for fields such as laser dentistry.