General Overview of Ionizing Radiation Ionising radiation is defined as any radiation with sufficient energy to remove an electron from an atom. Interestingly, not all ionising radiation is
high-energy. In fact, visible light is considered ionising radiation for some impurity states in semi-conductors. Generally, however, it is rare for a molecule to be ionised by visible light. The
two elements that bookend the ionisation energies for the periodic table are cesium and helium, with ionisation energies of 3.89 and 24.6 eV, respectively. When compared to this energy range, however,
visible light falls rather short- with a maximum energy of 3.10 eV (violet) and a minimum energy of 1.77 eV (red). Of course, as we have seen with semiconductor impurity states, some molecules have
ionisation energies that fall above or below those of elements. However, because it falls short of ionising any pure elements, visible light is generally considered non-ionising radiation. As a
rule, the ionising range of the electromagnetic spectrum is considered to range from UV to Gamma rays. We experience the effects of ionisation due to ultraviolet radiation when we receive sunburn.
What is Ionising Radiation?
Ionising radiation is produced by a myriad of sources. Radioactive decays,
nuclear fission and fusion, high-temperature bodies, and accelerating charges all produce ionising radiation. The products of radioactive decay take one of three forms: Alpha (¥), Beta (¥), or Gamma (¥) radiation. Charged particles such as ¥, ¥+
, and ¥- particles are highly interacting with electrons, and as
such will travel only a short distance in a material before their energy is deposited. Photons such as x-rays, UV-rays,
and ¥ radiation are weakly interacting with matter, and as such will penetrate deeply into a material, distributing their energy over a comparatively long distance. Uncharged particles like neutrons also weakly interact with matter and can travel a long d
istance before an interaction occurs. For the purposes of describing the effects of ionising radiation we will follow
the notation of the National Academy of Sciences BEIR VII model and categorize the radiation as either high or low linear energy transfer (LET)
ionising radiation. Low-LET radiation is defined as radiation that deposits less energy along its path of travel, and is considered less destructive then
high-LET radiation, which deposits all of its energy over a relatively short path. Gamma and X-rays are considered examples of low-LET radiation,
while alpha and beta radiation are considered to be high-LET. As such, we consider alpha and beta radiation to be much more damaging to the human body then Gamma or X-rays. Sources of Ionising Radiation There are two main categories of ionising radiation sources: natural background radiation, and man-made radiation. Natural Background Radiation
As we go about our daily lives we are constantly bombarded with radiation, the majority of which is natural background
radiation. In fact, in the United States background radiation constitutes 82% of the total radiation exposure for the
average citizen. The average annual background exposure in the US and Canada is approximately 3.0 m
Sv, with the worldwide average at about 2.4 mSv and levels varying from 2 to 4 mSv in Canada. This increased level in Canada
and the US is due to higher then average Radon levels in the soil. The worldwide dose varies from 1 to 10 mSv, with some strange anomalies occurring. For example, in Ramsar, Iran, the annual dose of background radiation is up to 260
mSv. Despite having lived for generations in an area with background radiation more then one hundred times higher
then the global average, there are no appreciable cytogenetic difference between the people and populations of areas
with normal levels of background radiation. This suggests that the body is much better able to survive increases in background radiation levels then abrupt exposures to radiation.
Of the worldwide background radiation level, the single greatest contribution comes from Radon. Radon levels vary
greatly throughout the world and even between different parts of the country. Radon is only one of many sources of
ionising radiation occurring in the Earth. Since formation the planet has contained a stock of radioactive elements
including potassium, thorium and uranium. These stocks are continually diminishing as these elements naturally decay.
The products of these decays are other potentially radioactive isotopes as well as alpha, beta and gamma radiation.
These naturally occurring sources, along with Radon, make up 72% of the natural background radiation dose.
Cosmic radiation is the next largest contributor to the background radiation dose. Included in this category is radiation
originating from both inside and outside the solar system. Incoming radiation ranges from iron nuclei to simple protons,
some of which interacts with elements in the atmosphere to produce secondary radiation that sometimes reaches the
surface of the Earth. The dosage from this component varies depending on altitude and the geomagnetic field (which
will repel charged ions depending on its relative strength). This component is much larger at high altitudes, so much so
that according to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the highest dose of ionising radiation for any profession is that of airline workers.
The smallest major component of the background radiation dose comes from ingested sources. These sources are
mostly thorium and uranium radioisotopes found in food and drinking water as well as elements already within the
human body. For example, Carbon 14 is a naturally occurring radioactive isotope created in the atmosphere when
nitrogen atoms react with incoming cosmic radiation. This isotope is found within all living things.
Shown below is a breakdown of all these naturally occurring sources of background radiation. Man-Made Radiation In spite of popular public sentiment, man-made radiation is a comparatively tiny portion of a person's total ionising
radiation exposure, with a total contribution of only 18%. Not surprisingly, the single greatest source of man-made
radiation exposure is from medical procedure such as diagnostic x-rays, and radiation therapy. These are sources are explored will be explored in detail elsewhere
. The next largest contribution comes from consumer products. Airport x-rays, smoke detectors, televisions, fossil fuels, tobacco, luminous dials and watches, and electron tubes are just a few
of the many consumer products that expose the user to some form of ionising radiation. The nuclear fuel cycle (which
includes everything from the mining of uranium to the disposal of its waste products) is responsible for about 1% of a
person's man-made radiation dose. Nuclear fallout (the result of the use of nuclear weapons) contributes to the total dose, as does occupational exposure. All of these contributions are summarized in the figure below. |
