Information About Radiation And Radioactivity
What is radiation?

Radiation is a process in which energetic particles or energetic waves travel through a vacuum, or through matter-containing media that are not required for their propagation. Two energies of radiation are commonly differentiated by the way they interact with normal chemical matter: ionizing and non-ionizing radiation. The word radiation is often colloquially used in reference to ionizing radiation (i.e., radiation having sufficient energy to ionize an atom), but the term radiation may correctly also refer to non-ionizing radiation (e.g.,radio waves, heat or visible light). The particles or waves radiate (i.e., travel outward in all directions) from a source. This aspect leads to a system of measurements and physical units that are applicable to all types of radiation. Because radiation radiates through space and its energy is conserved in vacuum, the power of all types of radiation follows an inverse of power with regard to distance from its source.

Both ionizing and non-ionizing radiation can be harmful to organisms and can result in changes to the natural environment.

In general, however, ionizing radiation is far more harmful to living organisms per unit of energy deposited than non-ionizing radiation, since the ions that are produced by ionizing radiation, even at low radiation powers, have the potential to cause DNA damage. By contrast, most non-ionizing radiation is harmful to organisms only in proportion to the thermal energy deposited, and is conventionally considered harmless at low powers which do not produce significant temperature rise. Ultraviolet radiation in some aspects occupies a middle ground, in having some features of both ionizing and non-ionizing radiation. Although nearly all of the ultraviolet spectrum of radiation is non-ionizing, at the same time ultraviolet radiation does far more damage to many molecules in biological systems than is accounted for by heating effects (an example is sunburn). These properties derive from ultraviolet’s power to alter chemical bonds, even without having quite enough energy to ionize atoms.

Natural Radiation Sources

What radiation levels are considered safe?

The first step is to determine what your estimated annual radiation dose level is. There are many sources of radiation, some natural, and some man-made.

Cosmic Radiation

This is radiation from outer space, from the Sun and other stars. It is partly blocked by the Earth’s atmosphere, so the higher your altitude, the less air is present to stop it, and the higher the levels. It varies from around 25 millirem (mR) a year at sea level, to around 50 millirem (mR) a year at an altitude of 1 mile. At two miles, it would be around 100 millirem (mR) a year.

This is the reason that you get a small radiation dose when you take an airplane flight. Planes fly at altitudes of several miles, though fortunately the length of a flight is only a few hours. A typical dose rate is 0.5 millirem (mR) per hour of flight.

Terrestrial Radiation

This is due to radiation from uranium, thorium, and other radioactive materials naturally found in the soil. An average value is around 30 millirem (mR) a year, though this can be much less along the coasts, around half as much.

Inhaled Radon is estimated at around 200 millirem (mR) a year.

Other Sources of Radiation

Nuclear weapons fallout is estimated to be less than 1 millirem (mR) a year.

Watching TV gives a dose of about 1 millirem (mR) a year.

Porcelain false teeth or crowns give around 0.1 millirem (mR) a year.

While it is true that there is a slight increase in radiation does due to living close to a nuclear power plant, typically on the order of 0.01 millirem (mR) a year (insignificant), the average dose from living near a coal fired power plant is three times as high! This is due to the release of uranium/etc naturally mixed in with the coal.

A plutonium powered pacemaker gives a yearly dose rate of about 100 millirem (mR).


The Ionising Radiations Regulations 1985 set an annual dose limit of 50 mSv/year for radiation workers. Any worker who exceeded 3/10 of this limit was designated a “classified” radiation worker.

ie, 50 mSv/yr = 1 mSv/week = 25 µSv/hr (for a 40 hour working week)

3/10 of this limit = 25 x 3/10 = 7.5 µSv/hour = the adequate shielding level

However, the Ionising Radiations 1999 have reduced the annual dose limit from 50 mSv/year to 20 mSv/year, but have retained the adequate shielding level at 7.5 µSv/hour. This makes little sense as following the above formula using 20 mSv as an annual dose limit reduces the adequate shielding level to 3 µSv/hour. As this figure is easily achievable at Glasgow University, this limit has been set on all systems of work on entrances to “controlled” radiation areas.

In translating the above, we can surmise that radiation levels above 3 µSv/hour are treated as hazardous.

Radiation in Food

Foods naturally contain Carbon-14 which is radioactive, as well as Potassium, of which a small amount is radioactive. This results in an average dose of around 20 millirem (mR) a year. Also, some plants and animals naturally accumulate radioactive materials, resulting in higher than background dose rates.

The Human Body

The human body naturally contains Potassium, Carbon-14, and other radio nuclides. This makes it radioactive, to the tune of around 40 millirem (mR) a year. People are also radioactive, so a person could get slight doses from being around other people as well.

Radiation from X-Rays and Medical Tests

According to the American Nuclear Society, the following are the typical dose levels from various medical tests:

  • Extremity (arm, leg, etc) Xray: 1 millirem (mR)
  • Dental Xray: 1 millirem (mR)
  • Chest Xray: 6 millirem (mR)
  • Nuclear Medicine (thyroid scan): 14 millirem (mR)
  • Neck/Skull Xray: 20 millirem (mR)
  • Pelvis/Huip Xray: 65 millirem (mR)
  • CAT Scan: 110 millirem (mR)
  • Upper GI Xray: 245 millirem (mR)
  • Barium Enema: 405 millirem (mR)

Totally all the expected exposure comes to around 300 millirem (mR) a year. The major source of the radiation dose rate is due to natural sources, radon, cosmic radiation, and terrestrial radiation. Man made sources of radiation are completely swamped by these natural sources in most cases.

The average total dose rate for the USA is 360 mrem a year. It has been estimated that your chance of dying from cancer increases 10% if you accumulate a total of 250,000 mrem. This would be over 3,000 mrem a year over 80 years, for example. This estimates presumably assume a linear risk factor between dose and the chance of getting cancer, and there are those who now dispute such assumptions, which means the risks from low levels of radiation may be overstated.

A single dose of around 450 R (450,000 mR) is usually considered produce death in 50% of the cases.

What is radioactivity?

Radioactive decay, also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy by emitting particles of ionizing radiation. A material that spontaneously emits this kind of radiation – which includes the emission of energetic alpha particles, beta particles, and gamma rays - is considered radioactive.

There are many different types of radioactive decay. A decay, or loss of energy, results when an atom with one type of nucleus, called the parent radionuclide, transforms to an atom with a nucleus in a different state, or to a different nucleus containing different numbers of protons and neutrons. Either of these products is named the daughter nuclide. In some decays the parent and daughter are different chemical elements, and thus the decay process results in nuclear transmutation (creation of an atom of a new element).

The first decay processes to be discovered were alpha decay, beta decay, and gamma decay. Alpha decay occurs when the nucleus ejects an alpha particle (helium nucleus). This is the most common process of emitting nucleons, but in rarer types of decays, nuclei can eject protons, or specific nuclei of other elements (in the process called cluster decay). Beta decay occurs when the nucleus emits an electron or positron and a type of neutrino, in a process that changes a proton to a neutron or the other way around. The nucleus may capture an orbiting electron, converting a proton into a neutron (electron capture). All of these processes result in nuclear transmutation.

The trefoil symbol is used to indicate radioactive material.

The new radioactive danger symbol is used to indicate dangerous ionizing radioactive material.


ionizing radiation

Illustration of the relative abilities of three different types of ionizing radiation to penetrate solid matter. Alpha particles (α) are stopped by a sheet of paper while beta particles (β) are stopped by an aluminium plate. Gamma radiation (γ) is dampened when it penetrates lead.

Radioactive Elements

This is a list or table of elements that are radioactive. All elements have radioactive isotopes. This table contains the elements that have no stable isotopes. Each element is followed by the most stable known isotope and its half-life.

This list is sorted by increasing atomic number.

Reference: International Atomic Energy Agency ENSDF database (Oct 2010)

radioactive elements


Most Stable Isotope

of Most Stable Istope



4.21 x 106 years



17.4 years



102 years



8.1 hours



3.82 days



22 minutes



1600 years



21.77 years



7.54 x 104 years



3.28 x 104 years



2.34 x 107 years



2.14 x 106 years



8.00 x 107 years



7370 years



1.56 x 107 years



1380 years



898 years



471.7 days



100.5 days



51.5 days



58 minutes



4 hours



13 hours



32 hours



2.4 minutes



17 seconds



9.7 seconds



0.72 seconds



11.1 seconds



26 seconds



29 seconds



0.48 seconds



2.65 seconds



87 milliseconds



61 milliseconds






1.8 milliseconds

How is radioactivity measured?

There are four different but interrelated units for measuring radioactivity, exposure, absorbed dose, and dose equivalent. These can be remembered by the mnemonic R-E-A-D, as follows, with both common (British, e.g., Ci) and international (metric, e.g., Bq) units in use:

Radioactivity refers to the amount of ionizing radiation released by a material. Whether it emits alpha or beta particles, gamma rays, x-rays, or neutrons, a quantity of radioactive material is expressed in terms of its radioactivity (or simply its activity), which represents how many atoms in the material decay in a given time period. The units of measure for radioactivity are the curie (Ci) and becquerel (BP).

Exposure describes the amount of radiation traveling through the air. Many radiation monitors measure exposure. The units for exposure are the roentgen (R) and coulomb/kilogram (C/kg).

Absorbed dose describes the amount of radiation absorbed by an object or person (that is, the amount of energy that radioactive sources deposit in materials through which they pass). The units for absorbed dose are the radiation absorbed dose (rad) and gray (Gy).

Dose equivalent (or effective dose) combines the amount of radiation absorbed and the medical effects of that type of radiation. For beta and gamma radiation, the dose equivalent is the same as the absorbed dose. By contrast, the dose equivalent is larger than the absorbed dose for alpha and neutron radiation, because these types of radiation are more damaging to the human body. Units for dose equivalent are the roentgen equivalent man (rem) and sievert (Sv), and biological dose equivalents are commonly measured in 1/1000th of a rem (known as a millirem or mrem mR).

For practical purposes, 1 R (exposure) = 1 rad (absorbed dose) = 1 rem or 1000 mrem (dose equivalent).

People are exposed on average to around 2 mSv of radiation a year from the natural environment, although there is considerable variation in this dose between individuals. The single-year limit for occupational exposure of workers is 50 mSv.

For the purposes of checking radioactivity levels on scrap, beta and gamma radiation is measured using a wide range of radioactivity meters, Geiger counters and Spectrometers. Reading is measured and recorded in terms of Microsieverts per Hour (uSv/hr) for the purposes of issuing Pre- Shipment Inspection Certificates.

Conversions or millirem (mR) To Millisieverts per Hour (mSv/hr) and Microsieverts per Hour (uSv/hr)

mR/hr mSv/hr uSv/hr
1 mR/hr 01 mSv/hr 10 uSv/hr
10 mR/hr .1 mSv/hr 100 uSv/hr
100 mR/hr 1 mSv/hr 1,000 uSv/hr
1,000 mR/hr 10 mSv/hr 10,000 uSv/hr
10,000 mR/hr 100 mSv/hr 100,000 uSv/hr

Some of the meters being used to measure radio activity are shown below:

MEDSpec Hand Held Gamma Spectrometer

measured radioactivity
Radioactivity Meter Gamma EASY

 RDX Radioactivity Meter

         Radioactivity Meter GS2