Radiation And You (GM)

Units and definitions

SI-units Formerly used Conversion
Designation of quantity Name Unit designation Name Unit designation Old to SI
Activity (A) Becquerel (Bq) 1/s1 Curie Ci 1 Ci = 37 GBq
Ionization dose Coulomb (C) C/kg Röntgen R 1 R = 2.58 x 10-4 C/kg
Ionization dose rate Coulomb (C) Ampère (A) C/kg.s or A/kg R/s
Absorbed energy dose Gray (Gy) J/kg (J = Joule) Rad Rad 1 Rad = 0.01 Gy
Equivalent dose (H) H = D X RBE2 Sievert (Sv) J/kg Rem Rem 1 Rem = 0.01 Sv

α, β, γ, n

Everything is radioactive. Everyone in the world is constantly exposed to low levels of ionizing radiation. The bulk of this exposure comes from radon gas, found as a trace element almost everywhere. The other main sources are medical X-rays and cosmic rays.
Fun fact: the higher you are, the more cosmic radiation you will receive, because there is less atmosphere to absorb the incoming rays. This includes travelling in airplanes.

Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionized.

Radiation is the characteristic of certain elements to emit alpha (α), beta (β) or gamma (γ) rays or a combination thereof. Alpha and beta rays consist of electrically charged particles, whereas gamma rays are of an electromagnetic nature.

Alpha (α) particles

Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. They are generally produced in the process of alpha decay, but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α.

Alpha particles are not, in general, dangerous to life unless the source is swallowed or inhaled, in which case they become extremely dangerous. The particles themselves are readily stopped by a few centimeters of air, a piece of paper or even the outermost layer of dead cells of the human skin.

When alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest. This is due to the high relative biological effectiveness of alpha radiation to cause biological damage after alpha-emitting radioisotopes enter living cells. Ingested alpha emitter radioisotopes are an average of about 20 times more dangerous, and in some experiments up to 1000 times more dangerous, than an equivalent activity of beta emitting or gamma emitting radioisotopes.

Beta (β) particles

Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. They are designated by the Greek letter beta, β. There are two forms of beta decay, β− and β+, which respectively give rise to the electron and the positron.

Of the three common types of radiation given off by radioactive materials, alpha, beta and gamma, beta has the medium penetrating power and the medium ionizing power. Although the beta particles given off by different radioactive materials vary in energy, most beta particles can be stopped by a few millimeters of aluminium. Being composed of charged particles, beta radiation is more strongly ionizing than gamma radiation. When passing through matter, a beta particle is decelerated by electromagnetic interactions and may give off x-rays.

In water, beta radiation from many nuclear fission products typically exceeds the speed of light in that material and thus generates blue Cherenkov radiation when it passes through water. The intense beta radiation from the fuel rods of pool-type reactors can thus be visualized through the transparent water that covers and shields the reactor

Beta particles are able to penetrate living matter to a certain extent and can change the structure of struck molecules. In most cases such change can be considered as damage with results possibly as severe as cancer and death. If the struck molecule is DNA it can show a spontaneous mutation.

Gamma (γ) radiation

Gamma rays arise from the disintegration of atomic nuclei within some radioactive substances, also known as isotopes. The energy of gamma-radiation cannot be controlled; it depends upon the nature of the radioactive substance. Nor is it possible to control its intensity, since it is impossible to alter the rate of disintegration of a radioactive substance.

Gamma rays are emitted in an isolated line spectrum, i.e. with one or more discrete energies of different intensities. These energies, together with the wavelength, determines the radiation hardness. Radiation is called hard when its wavelength is small and soft when its wavelength is long. As a rule of thumb: the harder the radiation, the higher the penetrating capabilities of the rays.

All ionizing radiation causes similar damage at a cellular level, but because rays of alpha particles and beta particles are relatively non-penetrating, external exposure to them causes only localized damage, e.g. radiation burns to the skin.

Gamma rays and neutrons are more penetrating, causing diffuse damage throughout the body (e.g. radiation sickness, cell's DNA damage, cell death due to damaged DNA, increasing incidence of cancer) rather than burns. External radiation exposure should also be distinguished from internal exposure, due to ingested or inhaled radioactive substances, which, depending on the substance's chemical nature, can produce both diffuse and localized internal damage.

Neutron (n) radiation

Neutron radiation is a kind of ionizing radiation which consists of free neutrons. These neutrons may be emitted from nuclear fusion or nuclear fission, or from any number of different nuclear reactions such as radioactive decay. Large neutron sources are rare and are usually limited to large-sized devices such as nuclear reactors or particle accelerators.

One of the most severe hazards of neutron radiation is neutron activation: the ability of neutron radiation to induce radioactivity in most substances it encounters, including body tissues.
This process accounts for much of the radioactive material released by the detonation of a nuclear weapon. It is also a problem in nuclear fission and nuclear fusion installations, as it gradually renders the equipment radioactive; eventually the hardware must be replaced and disposed of as low-level radioactive waste.

Neutron radiation protection relies on radiation shielding. Due to the high kinetic energy of neutrons, this radiation is considered to be the most severe and dangerous radiation to the whole body when exposed to external radiation sources.

In comparison to conventional ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and slowed (absorbed) by light nuclei, so hydrogen-rich material is more effective than iron nuclei. The light atoms serve to slow down the neutrons by elastic scattering, so they can then be absorbed by nuclear reactions.
However, gamma radiation is often produced in such reactions, so additional shielding has to be provided to absorb it. Care must be taken to avoid using nuclei which undergo fission or neutron capture that results in radioactive decay of nuclei that produce gamma rays.

Neutrons also degrade materials; bombardment of materials with neutrons creates collision cascades that can produce point defects and dislocations in the materials. At high neutron fluxes this can lead to embrittlement of metals and other materials, and to swelling of some of them. This poses a problem for nuclear reactor vessels, and significantly limits their lifetime.

X-rays vs. γ -rays

X-rays and γ-rays have the following properties in common:

• Invisibility: they cannot be perceived by the senses.
• They travel in straight lines at the speed of light.
• They cannot be deflected by means of a lens or prism, although their path can be bent (diffracted) by a crystalline grid.
• They can pass through matter and are partly absorbed in transmission.
• They are ionizing, that is, they liberate electrons in matter.
• They can impair or destroy living cells.

The key difference is the source: x-rays are emitted by the electrons outside the nucleus, and gamma rays are emitted by the excited nucleus itself.

Additionally, they differ in frequencies and wavelengths: X-rays have frequencies ranging from 30 PHz to 30 EHz and wavelengths from 10 nm to 10 pm. γ-rays have frequencies higher than 30 EHz and wavelengths smaller than 10 pm.

Irradiation vs. contamination

Outside the nuclear and scientific community, there is a general misunderstanding that everything exposed to ionizing radiation has been contaminated and is dangerous forever. This is not so. Many of the things you use or eat every day have been irradiated to, for example, sterilize them.

When something has been irradiated, by x-rays, gamma rays or electron beams for example, the irradiation stops as soon as the source of ionizing radiation has been removed or terminated. Think of it this way: when you turn a light on, the room is filled with electromagnetic radiation in the form of visible light. The instant you turn the light off, the electromagnetic radiation is gone. The same can be said about the energy of ionizing radiation.
(Note: However, even though the irradiation stops, the biological effects may still occur if unrepaired cell damage has been inflicted).

Contamination is much different. When contamination has occurred, the source of the ionizing radiation itself is transferred, such as when radioactive isotopes in solid, liquid or gaseous forms are introduced into the environment.
When something has been contaminated with radioactive isotopes, it will remain radioactive until the radioactive isotope has decayed to a safe level.
If the contamination occurs in a controlled environment such as a building or over a fairly small area, the radioactive isotopes can be cleaned using specialized techniques, equipment and procedures. The contaminated materials must then be properly stored until the isotope involved has decayed into a stable state.

A tragic case from Brasil illuminates the difference between irradiation and contamination: the Goiânia accident

Most of the problems with disasters like Chernobyl, Fukushima and various nuclear testing sites stem from the problems of contamination. Usually, the background radiation in these areas is lower than the internationally agreed safety levels.
The main radiation risk in these areas is the food and water: you’d take in radioactive particles each with each mouthful.

At least, more than usual. Enjoy your potassium-40, people.

Risk of contamination is also the reason why you seen people milling about in hazmat suits near sites of nuclear accidents. These suits do very little to stop all but alpha radiation but they DO stop you inhaling or ingesting radioactive gas, dust and other particles.

Lethality and ALARA

First off, there is one very important point to remember: there is NO safe level of radiation exposure below which one will not experience any ill effects. This doesn’t mean one should have an unreasoning fear of radiation, but one should always try and minimize exposure.
This is the basis of the ALARA-principle: As Low As Reasonably Achievable.

The understanding of the effect exposure to radiation has on human health has grown over the past 50 years and has led to a substantial reduction of the maximum permissible dose. These values have been established by the ICRP (International Commission on Radiation Protection). The values given below apply to external radiation of the whole body.

  • Radiation workers, category A: 20 mSv/year
  • Radiation workers, category B: 5 mSv/year
  • general public: 1 mSv/year

These levels are acceptable but it is not to be automatically assumed that people working with radiation actually should receive these doses.

There are two categories of biological effects that an overdose of radiation can cause: somatic and genetic effects. Somatic effects are the physical effects. A reduction in the number of white blood cells is an example of a somatic effect. Much more is known about the somatic than about the genetic effects of radiation.
Blood cells are very sensitive and the first signs of radiation are found in the blood, which is why people working with radiation are subjected to periodic blood tests.

The most serious effects of radiation occur when a large dose is received in a short period of time. The table underneath shows doses received over 24 hours and their effect:

Exposure (Sievert) Exposure (Rem) Health Effect Time to Onset (without treatment)
0,05 – 0,1 5-10 Changes in blood chemistry
0,5 50 Nausea Hours
0,55 55 Fatigue
0,7 70 Vomiting
0,75 75 Hair loss 2 – 3 weeks
0,9 90 Diarrhea
1 100 Hemorrhage
2 200 First lethal cases
4 400 Possible death 50% death within 2 months
10 1000 Destruction of intestinal lining, internal bleeding and death Death within 1 – 2 weeks
20 2000 Damage to central nervous system, loss of consciousness and death Loss of consciousness within minutes. 100% death within hours to days.

The consequences of excess radiation are not necessarily noticeable immediately after the irradiation. Frequently, they only show up after some time. The time lapse between irradiation and the moment the effect become apparent is called the ‘latent period’.

Genetic effects can only be assessed over generations.

The protection from unwanted external irradiation is based on three principles:

  • Speed: by working fast, the exposure duration is reduced.
  • Distance: the greater the distance, the lower the rate of exposure (inverse square law)
  • Shielding and collimating: materials with high radiation absorbing properties (lead, concrete, steel, water, boron carbide (neutron radiation), …) reduce the exposure rate to a level that can be calculated in advance.

The protection from unwanted internal irradiation (contamination) is a HAZMAT suit, preferably with SCBA-equipment.

Radiation measurement

From what has been said before, it follows that establishing the presence of ionizing radiation and measuring its levels is of paramount importance. Since ionizing radiation cannot be detected by the senses, detectors and measuring equipment are used. These are various instruments with which one can measure or register radiation.

Dose rate meter:
A portable Geiger-Müller counter is the most commonly used instrument for measuring dose rate, but the more accurate and more expensive ionization charmer is used as radiation monitor as well. Both instruments measure the electric current that is produced by ionization.
The radiation level can be read instantly off a micro-ampere meter with a µSv/h or mSv/h calibrated scale. Some radiation monitors give an audible signal when a pre-set dose is exceeded.
These instruments are used by personnel working with radioactive material or X-ray equipment, to determine safe distance and dose rate.
A GM-counter has a measuring range from 1 µSv/h to 2 mSv/h.

Scintillation counter:
This is an accurate and multifunctional instrument to measure and analyze radiation. The incidence of ionizing radiation on a sodium-iodine crystal is converted into weak light flashes, which are amplified into electric pulses by an integrated photo-multiplier.
By measuring amplitude and number of these electric pulses, energy and intensity (dose rate) of the radiation can be determined.
These instruments are predominantly used for scientific purposes.

Pendosismeters (PDM):
The PDM consists of a quartz fiber electrometer and a simple optic lens system housed in a fountain pen type holder. A small charging unit is used to electrically charge the fiber, which can be viewed through the lens.
The fiber is set on the zero mark of the calibrated scale as initial setting for the work period.
Any radiation will cause the charge to leak away through its ionizing effect and the fiber will move across the scale. The amount of radiation received can be read off the calibrated scale.
This type of instrument is excellent for personal protection as it is small, inexpensive and reasonably robust. It can be easily read and records the total amount of radiation received for the work period with an accuracy of ± 10%.
However, the lack of audible output poses a risk when working on areas with high radiation levels as there is no warning when reaching the maximum periodic dose.

Thermoluminescent dose meter (TLD badge):
The TLD meter consists of an aluminium plate with circular apertures. Two of these contain luminescent crystals. When the meter is read only one crystal is used to determine the monthly dose. The other one is spare and, if necessary, can be read to determine the cumulative dose.
The TLD is sensitive to x- and γ-radiation of 30 keV and higher. The dose measuring rand is large and runs from 0.04 mSv to 100 mSv with an accuracy of ± 5%.
These badges are small, convenient to wear and easy to lose.

Film dose meter (film badge):
The film badge consists of two pieces of X-ray film contained, with filters, in a special holder. At the end of a specified period, the films are developed and the density measured. The radiation dose received by the wearer can then be determined by consulting the density/exposure curves, and the type of radiation received can be established by checking the densities behind the filters. Film dose meters are very cheap and a reasonably accurate method of monitoring personnel in selected areas.
They are small, robust, convenient to wear and easy to lose.

BONUS HORROR: 1958 Tybee Island mid-air collision

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License