Chapter 1．Radiation is Everywhere

Section 1.1．What is radiation?

Radiation is all around us and present on the Earth at all times

$The majority of the radiation comes from natural sources and a small fraction comes from artificial sources$

Natural radiation levels may vary from place to place.

Section 1.1．What is radiation?

Radiation is energy that moves from one place to another in the form of waves or particles. Most radiation:

Cannot be heard

Cannot be seen

Cannot be smelled

Cannot be tasted

Cannot be felt

However, we can use instruments to detect or measure it.

Section 1.2．Atomic tidbits

Atom

All materials are made up of tiny particles called atoms. Each atom has a nucleus and a surrounding cloud of electrons. Uncharged neutrons and positively charged protons are confined inside a nucleus, while negatively charged electrons revolve around the nucleus in orbits.

Decay

Most atomic nuclei are stable and will maintain their original states for a prolonged period. However, some nuclei, in particular those bigger in size, are indeed unstable. An unstable (radioactive) nucleus can become more stable after emitting particles and energy. This process is called decay.

Section 1.2．Atomic tidbits

Half-life

During a decay process, the number of nuclei of radionuclides gradually decreases. The time required for the number to decrease to half of its original amount is called half-life of the nuclide.

Each radionuclide has a characteristic half-life. The half-lives of radionuclides can vary from millionths of a second to over million years.

Section 1.2．Atomic tidbits

Nuclear Energy Nuclear energy is the energy released when the structure of an atomic nucleus changes, either under nuclear fission or nuclear fusion.

Nuclear fission Nuclear fission is a process in which the nucleus of an atom (e.g. uranium-235) splits into lighter nuclei (daughter nuclei), accompanied with the release of energy. The process is usually triggered by a neutron.

Nuclear fusion Nuclear fusion is the fusion of two or more atomic nuclei to form a heavier nucleus and other particles. Like nuclear fission, nuclear fusion produces energy. Deuterium and tritium, both heavy forms of hydrogen, are common materials for nuclear fusion.

Section 1.2．Atomic tidbits

Chain reaction The neutrons produced by the fission reactions may strike other uranium nuclei and produce more neutrons. This multiplication process (i.e. chain reaction) happens in a split second and results in the release of a large amount of heat. Nuclear power plants utilise the heat generated from controllable chain reactions to produce electricity.

Section 1.3．Ionising and non-ionising radiation

Radiation can be classified as non-ionising and ionising.

Non-ionising radiation

Non-ionising radiation contains low energy electromagnetic waves. It does not have sufficient energy to remove electrons from atoms.

Section 1.3．Ionising and non-ionising radiation

Common examples of non-ionising radiation are:

Ultraviolet

Visible light

Infrared

Microwave

Radiowave

Although the energy of non-ionising radiation is lower, too much of it can still affect our health. For instance, prolonged exposure to UV radiation can cause sunburn.

Section 1.3．Ionising and non-ionising radiation

Ionising radiation

Ionising radiation includes high speed particles and high energy electromagnetic waves. Their high energy is able to knock out orbital electrons from atoms, thus generating positively charged ions and negatively charged electrons.

This ionisation process may often result in chemical changes in biological tissues, causing harm to living organisms.
The radiation discussed in the following chapters of this e-book refers to ionising radiation.

Section 1.4．Units of radiation

The amount of radiation exposures is quantified by radiation dose. The corresponding units are Sievert (Sv) and Gray (Gy). Radioactivity is the number of decays per second of the radioactive substance or isotope. The units are Becquerel (Bq) and Curie (Ci).

Radiation dose units

Radioactivity units

Know More:

Section 1.5．Radiation dose received

We are constantly exposed to different kinds of radiation, especially natural radiation. The average radiation dose received from natural background by an individual in Hong Kong is about 2.4 mSv per year. Over the world, the radiation dose received by an individual ranges from 2 mSv to over 10 mSv per year.

Radiation may cause damages to body cells and tissues. However, health effects are insignificant unless the absorbed dose is high. In the long-term, radiation can increase the risk of cancer. For every additional mSv of radiation exposure, the risk may increase by about 1 in 20,000.

Know More:

Section 1.6．Natural radiation

Natural radiation sources include cosmic rays, radon, radioactive materials existing in the rocks and soil of the Earth's crust, as well as radioactive materials in food and drinks.

Cosmic radiation
Cosmic radiation arises from cosmic rays. Cosmic rays mainly come from the streams of high-energy charged particles that originate from interstellar space and the Sun.

Interstellar space

High-energy charged particles that originate from the Sun

The cosmic radiation exposure is generally higher in polar regions and lower near the equator. Locations in higher altitude will also have higher level of cosmic radiation.

Section 1.6．Natural radiation

Rocks and soil

The Earth itself is a source of terrestrial radiation. Radioactive materials such as uranium, thorium, potassium and the radionuclides from their decay products exist naturally in the rocks and soil of the Earth's crust. The terrestrial radiation varies from place to place. Locations with higher concentrations of uranium and thorium generally have higher terrestrial radiation.

Radon

Radon is a colorless, tasteless, odorless radioactive gas mainly produced by the decay of natural radionuclide uranium-238 in soils, rocks and walls. Radon generated from the ground releases into the air, and can even enter indoor.

Section 1.6．Natural radiation

Food and drinks

Food and drinks are also naturally radioactive. Radioactive materials in soil and water can be transferred to plants and then to animals. For example, potassium-40 can be found in bananas and nuts.

Section 1.7．Artificial radiation

Artificial radiation sources include:

medical use of radiation;
nuclear power plants;
nuclear industrial incidents;
nuclear weapon tests; and
products which produce radiation.

The following are some products that contain trace amounts of radiation:

Cigarette

Ceramic ware containing radioactive uranium dye

Old style ionisation smoke detectors

Section 1.8．Worldwide distribution of radiation exposure

Radiation produced by human activities typically accounts for about 20 percent of public radiation exposure, mainly from medical applications (Reference: Radiation Effects and Sources, UNEP, 2016).
The other 80 percent comes from natural sources, mainly from terrestrial origin.

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