Understanding Radiation

What are radioactive materials, and how do they cause damage? A substance such as uranium, for example, is associated with harm to the human body based on its chemical structure as a heavy metal and its radioactive properties. As an example of chemical harm, exposure by the developing human body to heavy metals (rather than radiation) is associated with delayed brain development. Researchers often study either chemical or radiation harms separately but not as mixtures. Hazardous materials are commonly found in the environment as a mixture with other hazardous materials, but research doesn’t take these mixtures into consideration. Here we focus on radiation only.

Sources and types of radiation

Radioactive materials—also called radioisotopes or radioactive nuclides—are elements that spontaneously break apart, or “decay,” into lighter elements over time, and in doing so, release radiation, which is energy in the form of particles or waves. Some radioactive materials, such as uranium and radon, occur naturally and others, such as plutonium, are produced by human activity only (anthropogenic origin).

Elements on the periodic table are defined by the number of protons their atoms have. For example, uranium’s atomic number is 92 because it has 92 protons. Elements with the same number of protons but different numbers of neutrons are called isotopes of that element and are denoted by the number of protons plus the number of neutrons. So, when uranium has 146 neutrons, it is called uranium-238.

A radioisotope of an element is an isotope that is radioactive. Radioisotopes generally behave the same way chemically but have different radioactive properties.

Some take thousands or millions of years to decay and others do so relatively quickly. The half-life of an isotope is the time it takes for half of the atoms in a sample to decay.

When a radioisotope decays, it may emit energy (radiation) in the form of alpha or beta particles or gamma rays. If the energy of these particles or rays is high enough, that radiation can damage living tissue by ionizing, or stripping electrons from, atoms or molecules. Radiation can also be non-ionizing (in the form of visible light or heat), but for this explainer we are focused on the following types of ionizing radiation, which are often placed in the environment over the nuclear weapon life cycle.

Alpha particles are positively charged decay products of the heaviest radioactive elements (e.g., plutonium, radium, uranium) and can be blocked by a sheet of paper or a few inches of air. They cannot penetrate through skin, but can be very harmful if ingested or inhaled, or if they enter the body through a cut. They do not travel far in the body due to their size and low energy (although they can travel deeply into the lungs), and instead deposit their energy in a localized area and release large amounts of radiation on a few neighboring cells.

Beta particles are negatively or positively charged particles ejected from the atom during the decay of radioisotopes such as tritium (hydrogen-3) and strontium-90. They are about 1/8,000th the mass of an alpha particle, which allows them to travel further in the air—a few feet—before being absorbed. Though they are generally blocked by clothing, beta particles are dangerous when they are absorbed into the human body.

Gamma rays are not particles but electromagnetic waves, or photons like visible light, but with much higher energy, so they are ionizing. Because they have no mass, gamma rays can penetrate clothing and skin, then damage the body by destroying cells or disrupting DNA. Thus, gamma-emitting radioactive elements are dangerous even if not ingested or inhaled, and significant precautions such as lead (Pb) shielding are needed to block this type of radiation.

How radiation is measured

Radiation is measured in different ways depending on what kind of information is relevant. The release of radiation to the environment from a radioactive substance is measured using a becquerel (Bq), the decay of one atom per second, or a curie (Ci), the approximate number of atoms that decay per second in a gram of radium. The important quantity for absorbed radiation in the human body is energy per mass, which is measured in units of gray (Gy)—the energy (in joules) absorbed per kilogram of tissue—or rad (1 Gy = 100 rad).

Because the type of radiation affects absorption, as does the sensitivity of the organ absorbing the radiation, another unit called the effective dose adjusts for those considerations. Effective dose is used to set protective radiation exposure limits, measured in sieverts (Sv) or rem. These values are based on population statistics and may not represent differences and risks in individuals. Rem, rad, and curies are all US units of measurement, and becquerels, grays, and sieverts are international units.

Naturally occurring radiation at sea level is usually less than 1 milligray (mGy) per year, primarily from cosmic rays and inhaled radon. People are exposed to more cosmic rays at higher altitudes, so the cosmic rays absorbed during a transatlantic flight is 0.02 mGy, whereas a computed tomography (CT) scan is about 15 to 20 mGy.

Radiation from a nuclear detonation can be carried far from the blast site. In the example of the 1954 Castle Bravo test in the Marshall Islands—at 15 megatons, the most powerful US nuclear weapon ever detonated—the fallout and debris cloud spread over a 110-mile radius. Radiation absorbed into the thyroid gland ranged between 10 Gy for adults and 50 Gy for a one-year-old in Rongelap atoll (average external whole-body dose was 810 mGy).

The detonation of a nuclear weapon will produce gamma rays initially which quickly tapers off, and can leave beta and alpha particles in the area for some time.