Radioactivity

Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Because the nucleus experiences the intense conflict between the two strongest forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation. The most common types of radiation are called alpha, beta, and gamma radiation, but there are several other varieties of radioactive decay.

Radioactive decay rates are normally stated in terms of their half-lives, and the half-life of a given nuclear species is related to its radiation risk. The different types of radioactivity lead to different decay paths which transmute the nuclei into other chemical elements. Examining the amounts of decay products makes possible radioactive dating.

Radiation from nuclear sources is distributed equally in all directions, obeying the inverse square law.

Common sources of radiation

“Man-made” radiation

Smoke detectors

Smoke detectors make use of the isotope Americium-241. This isotope emits alpha-particles at energies up to 5.4 MeV. The energetic alpha particles are used to ionize air. Once the air is ionized, a small current runs through it. When smoke enters the chamber, the current experiences an increase in resistance and a circuit sounds the alarm.

Coal-burning power plants

Coal is an impure fuel, and it usually contains 1.3 ppm of uranium and 3.4 ppm of thorium (not to mention arsenic, mercury, and sulfur). When coal burns, these isotopes are emitted into the atmosphere, where they enter our ecosystem. This leads to the astounding fact that the population effective dose equivalent from coal plants is 100 times that from nuclear plants.

Nuclear weapon detonations

The hundreds of atmospheric nuclear weapons tests that occurred before they were banned by the 1963 Limited Test Ban Treaty left long-lived radioisotopes in the atmosphere. Some of these are still in the atmosphere and account for some of our daily doses.

Natural radiation

Radon gas This natural occurring gas comes from the soil and is found throughout the world. It emits alpha particles, and can, therefore, damage DNA and lead to cancer if inhaled. The EPA recommends you check your house for radon gas.

Cosmic rays

Cosmic rays are energetic particles that originate outside of earth, in the sun, distant stars, galaxies, and supernovae. Most of these are protons. The atmosphere shields us from most cosmic rays, but during air travel, one will accumulate much higher dose. Don’t believe it? Check out our Radiation on Flights page.

Video of radiation detection

Background readings in Ann Arbor, MI

For a class in 2005, with no radioactive sources within range, we measured a long (30 minutes +) reading with a high-purity germanium (HPGe) gamma-ray detector system. We then identified the source of each peak. The spectrum is shown in the figure. Click it for the identifications. HPGe detectors are known for excellent resolutions, and as you can see, many peaks are clearly visible. Each one represents a specific nuclear reaction. Some major gamma-rays are highlighted on the figure. Thallium-208 is a decay-product of Thorium-232, which is naturally present in the soil. Protactinium-234 results from the natural alpha-decay of Uranium-238. Potassium-40 is found all around, including in bananas and in salt-substitutes at the grocery store.

The effects on human health

There are two main health effects caused by radiation, which act over the short- and long-term and also at shorter and greater distances.

• Radiation causes health problems by killing cells in the body, and the amount and type of damage done depends on the dose of radiation received and the time over which the dose is spread out.
• The dose limits for emergency workers in the event of a nuclear accident are 100 mSv if protecting the property or 250 mSv in a life-saving operation.
• Between that upper limit and 1 Sv received within a single day, exposure is likely to cause some symptoms of radiation poisoning, such as nausea and damage to organs including bone marrow and the lymph nodes. Up to 3 Sv, these same effects are more serious with a likelihood of acquiring infections due to a reduced number of white blood cells in the body – with treatment, survival is probable but not guaranteed.
• Larger doses will, in addition to those symptoms above, cause haemorrhaging, sterility and skin to peel off; an untreated dose of more than 3.5 Sv will be fatal, and death is expected even with treatment for doses of more than 6 Sv.
• The radiation level decreases with the square of the distance from its source, so someone twice as far away from an external source will receive a quarter of the radiation.
• Receiving a high dose in a shorter time usually causes more acute damage, as greater doses kill more cells, while the body can have had time to repair some damage with more time having elapsed between doses.

However radioactive material that is spread to a wider area can cause longer-term health effects via prolonged exposure, particularly if they enter the food chain or are inhaled or ingested directly.

Taking radioactive materials into the body also presents the greatest danger from atoms that undergo alpha-decay, as alpha particles are not very penetrative and are easily absorbed by a few centimetres of air. It was alpha-emitting polonium-210 that was used to murder Alexander Litvinenko in 2006.

Radioactive isotopes of iodine, which undergo beta-decay, can build up in the thyroid gland and can cause thyroid cancer. Attempts to prevent this involve distributing pills that include nonradioactive iodine-127 and which flood the thyroid, preventing uptake of radioactive iodine.

For one-off doses, such as those from medical scans, the risk of later developing cancer is estimated at around 1 in 20 000 per mSv received.

Absorbing an accumulated dose of 1 Sv over a longer period of time is estimated to eventually cause cancer in 5% of people.

However, there is disagreement over whether very small doses comparable to the level of background radiation actually contribute to health effects.