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Ionizing radiation

The invisible storm that has been raining down on Earth since the planet formed is composed of particles traveling at 99 percent of the speed of light. These cosmic rays, originating from distant supernova explosions and the heart of the Sun, bombard the atmosphere constantly, creating a cascade of secondary particles that reach the ground. This natural phenomenon, known as background radiation, contributes to the 3 millisieverts of exposure that the average human receives annually, with 80 percent of that dose coming from nature itself. The boundary between the visible and the invisible is not a line drawn by human eyes but a threshold of energy, starting at approximately 10 electronvolts, where photons and particles possess enough force to strip electrons from atoms and fundamentally alter the structure of matter. This energy level defines the difference between the light that allows us to see and the radiation that can rewrite the genetic code of living organisms.

Alpha And Beta Particles

In 1899, Ernest Rutherford ranked the known radioactive emissions in descending order of their ionizing effect, naming the most powerful of them alpha particles after the first letter of the Greek alphabet. These particles consist of two protons and two neutrons bound together into a helium-4 nucleus, moving at about 5 percent of the speed of light. Despite their strong ionizing power, alpha particles have low penetration ability and can be stopped by a single sheet of paper or the top layer of human skin. In contrast, beta particles are high-speed electrons or positrons emitted during radioactive decay, capable of traveling further than alpha particles but still halted by an aluminum plate. When these high-energy beta particles pass through matter, they can produce X-rays known as bremsstrahlung, or braking radiation, which complicates the process of shielding them effectively. The interaction of these particles with matter creates a complex dance of energy transfer, where the conservation of momentum sends secondary particles flying in all directions, causing cascading biological effects that can displace atoms and damage tissue.

The Neutron Mystery

Neutrons possess a neutral electrical charge, a property that often leads to the misunderstanding that they do not cause ionization, yet they are among the most dangerous forms of radiation when they strike matter. A free neutron has a mean lifetime of 14 minutes and 42 seconds before it decays into a proton, an electron, and an electron antineutrino. When fast neutrons interact with hydrogen nuclei in water or other materials, they transfer energy through linear energy transfer, scattering the nuclei and creating fast protons that are themselves ionizing. This process, known as neutron activation, can turn stable atoms like oxygen-16 into radioactive nitrogen-16, which then decays to emit powerful beta rays and gamma radiation. The reaction O-16 (n,p) N-16 is a major source of X-rays emitted from the cooling water of pressurized water reactors, contributing enormously to the radiation generated by a water-cooled nuclear reactor while operating. The best shielding for neutrons involves hydrocarbons that have an abundance of hydrogen, which slow and capture the neutrons to prevent them from causing further damage.

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Common questions

What is the average annual radiation exposure for humans from natural background radiation?

The average human receives 3 millisieverts of exposure annually, with 80 percent of that dose coming from nature itself. This natural phenomenon includes cosmic rays originating from distant supernova explosions and the heart of the Sun. These particles bombard the atmosphere constantly, creating a cascade of secondary particles that reach the ground.

How do alpha particles differ from beta particles in terms of composition and penetration?

Alpha particles consist of two protons and two neutrons bound together into a helium-4 nucleus and can be stopped by a single sheet of paper. Beta particles are high-speed electrons or positrons that travel further than alpha particles but are still halted by an aluminum plate. When beta particles pass through matter, they can produce X-rays known as bremsstrahlung, which complicates the process of shielding them effectively.

What is the linear no-threshold model for predicting cancer incidence from ionizing radiation?

The linear no-threshold model holds that the risk increases linearly with effective radiation dose at a rate of 5.5 percent per sievert. This model suggests that natural background radiation is the most hazardous source of radiation to general public health, followed closely by medical imaging. The mechanism involves the mutation of somatic cells, leading to stochastic effects that may not appear for years or decades after exposure.

What is the highest recorded background radiation level in an inhabited area and where is it located?

The highest background radiation in an inhabited area has been recorded in the city of Ramsar, Iran, where some residents receive an average radiation dose of 10 millisieverts per year. This effective radiation dose due to external radiation was 135 millisieverts per year and the committed dose from radon was 640 millisieverts per year. The residents of Ramsar show no compelling evidence of experiencing a greater health risk despite these levels being over 200 times higher than the world average background radiation.

Why do airline flight crew workers receive more cosmic rays than other workers?

Airline flight crew workers receive more cosmic rays on average than any other worker because the Earth's magnetic field offers less protection at high altitudes and near the poles. A measured dose of 6 microsieverts per hour has been recorded on a polar route from London Heathrow to Tokyo Narita. The United States Federal Aviation Administration requires airlines to provide flight crew with information about cosmic radiation and has a recommended limit of 1 millisievert total for a pregnancy.

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The Stochastic Cancer Risk

The most widely accepted model for predicting the incidence of cancers due to ionizing radiation is the linear no-threshold model, which holds that the risk increases linearly with effective radiation dose at a rate of 5.5 percent per sievert. This model suggests that natural background radiation is the most hazardous source of radiation to general public health, followed closely by medical imaging. The mechanism by which radiation causes cancer involves the mutation of somatic cells, leading to stochastic effects that may not appear for years or decades after exposure. While deterministic effects, such as radiation burns and acute radiation syndrome, occur immediately following high doses, the stochastic effects of cancer and heritable diseases involve the mutation of reproductive cells that can be passed to offspring. The International Commission on Radiological Protection recommends limiting artificial irradiation to the public to an average of 1 millisievert per year, not including medical and occupational exposures, to mitigate these long-term risks.

The Ramsar Anomaly

In the city of Ramsar, Iran, the highest background radiation in an inhabited area has been recorded, with some residents receiving an average radiation dose of 10 millisieverts per year, which is ten times more than the International Commission on Radiological Protection recommended limit for exposure to the public from artificial sources. The highest level of purely natural radiation recorded on the Earth's surface is 90 micrograys per hour on a Brazilian black beach composed of monazite, yet the residents of Ramsar show no compelling evidence of experiencing a greater health risk. This unique case, where the effective radiation dose due to external radiation was 135 millisieverts per year and the committed dose from radon was 640 millisieverts per year, stands over 200 times higher than the world average background radiation. The ICRP recommendations are conservative limits and may represent an overrepresentation of the actual health risk, suggesting that radiation danger from background radiation may be a small overall risk compared to all other factors in the environment.

The Airline Hazard

Airline flight crew workers receive more cosmic rays on average than any other worker, including those in nuclear power plants, because the Earth's magnetic field offers less protection at high altitudes and near the poles. The cosmic-radiation dose rate on airplanes is so high that flight crews who routinely work flight routes that take them close to the North or South pole at high altitudes receive the maximum dose. A measured dose of 6 microsieverts per hour has been recorded on a polar route from London Heathrow to Tokyo Narita, and the United States Federal Aviation Administration requires airlines to provide flight crew with information about cosmic radiation. The FAA has a recommended limit of 1 millisievert total for a pregnancy, and no more than 0.5 millisieverts per month, leading many airlines to prohibit pregnant flightcrew members to comply with a European Directive. This exposure is a significant concern for the health of aircrews, who are constantly bombarded by relativistic particles from outside the Solar System.

The Shielding Paradox

The effectiveness of a material in shielding radiation is determined by its half-value thickness, the thickness of material that reduces the radiation by half, which is a function of the material itself and of the type and energy of ionizing radiation. While air or skin can be sufficient to substantially attenuate alpha radiation, and sheet metal or plastic is often sufficient to stop beta radiation, barriers of lead, concrete, or water are often used to give effective protection from more penetrating forms of ionizing radiation such as gamma rays and neutrons. The paradox of shielding arises because different types of radiation require different materials to stop them, and the interaction of radiation with matter can generate secondary radiation that is just as dangerous as the primary source. For example, when beta particles strike high atomic number materials, they produce bremsstrahlung, which is why material with low atomic numbers is used for beta source shielding. The design of radiation-hardened devices for the nuclear industry or outer space relies on careful material selection and fabrication methods to resist these damaging electrical and chemical effects.