The term radiation refers to energy that is emitted from a source. Although the term is normally reserved for wave phenomena (like electromagnetic radiation) it can also be used to describe emitted particles (like alpha and beta radiation).
Radiation is not automatically a bad or dangerous thing. A radiator in a home heats a room by radiating thermal energy in the infrared. A light bulb emits visible electromagnetic radiation more commonly known as light. These things are certainly not dangerous.
Unfortunately, the term radiation has now become hopelessly muddled with nuclear processes and is automatically associated with danger. Before we go any further, we should discriminate between the many different things called radiation.
- Electromagnetic radiation is energy transferred from one place to another by means of electromagnetic waves. This encompasses a wide variety of waves or rays with different names. In order from lowest to highest frequency they are: power waves, radio waves, microwaves, infrared radiation, light (sometimes called visible light), ultraviolet radiation (sometimes called ultraviolet light), x rays, and gamma rays. This sequence is known as the electromagnetic spectrum.
- Thermal radiation is the subset of the electromagnetic spectrum emitted by an object due to its temperature. If an object is hotter than its surroundings it will experience a net loss of internal energy. Thermal radiation is one of the processes by which this can happen. (The others are conduction and convection.) This transfer of internal energy is called heat. Heat can be given off as electromagnetic radiation with any wavelength. The hotter the object, the shorter the peak wavelength. At temperatures that seem "hot" to us, the peak wavelength of the emitted electromagnetic radiation will be in the infrared to visible portions of the spectrum. Because of this, infrared radiation is often mistakenly called "heat waves". Use of this term should be avoided, however.
- Gravitational radiation is energy transferred from one place to another by means of a gravitational wave. The existence of gravity waves is predicted by Einstein's Theory of General Relativity. Their existence has not yet been confirmed.
- Cosmic radiation (or cosmic rays) consists of high energy, ionized nuclei and electrons whose origin is outside of the earth. Sources include the sun, our galaxy, and other galaxies.
- Nuclear radiation are the energetic particles and energetic electromagnetic waves released during the radioactive decay of unstable isotopes.
This last form of radiation is what most people are referring to when they use the word "radiation". It is generally considered dangerous and, as we shall see, it largely deserves this reputation. Nuclear radiation comes in several forms. These include…
- alpha particles, helium nuclei emitted from large parent nuclei;
- beta particles, energetic electrons emitted from neutrons;
- gamma rays, the most energetic electromagnetic waves yet discovered;
- x‑rays, electromagnetic waves that are not as energetic as gamma rays;
- neutrons, free neutrons that are not part of a nucleus; and
- ions, this includes cosmic rays and the daughter nuclei of fission reactions.
Some items should not be added to this list of dangerous forms of radiation. Two examples are provided below.
These weakly interacting, nearly massless particles may have their origin in nuclear reactions, but they certainly pose no threat to humans or other living things on earth. Trillions or more pass though us every second with absolutely no effect.
People who use the term "nuke" when heating foods in a microwave oven need to be told that they sound like complete idiots when they do this. Yes, microwaves are a form of radiation, but then so are infrared and light. The same people that say "I'm going to nuke these leftovers" when they reheat food in a microwave would never say "I'm going to nuke my clothes" when they turn on a dryer or "I'm going to nuke my eyeballs" when they turn on a light switch. Not all forms of radiation are nuclear in origin.
Any form of electromagnetic radiation is dangerous at large intensities. One shouldn't operate a broken microwave oven, or place one's hand in boiling water, or stare at the sun. Too much energy is being delivered too quickly over too small an area for these exposures to be safe. But microwaves, infrared, and visible light are not in the same league as ultraviolet, x rays, and gamma rays. The former are "normally harmless", while the latter are "generally dangerous". The difference lies in the ability of the more dangerous forms of radiation to ionize atoms (remove electrons) or to dissociate molecules (break them in two). The division between ionizing and non-ionizing electromagnetic radiation lies in the visible portion of the spectrum. Roughly speaking, ionizing radiation has a frequency higher than visible light and non-ionizing radiation has a frequency lower than visible light.
Visible light is also unique in that photons around this portion of the electromagnetic spectrum possess just enough energy to excite the outermost electrons of an atom, but not enough to strip them off completely. This has special consequences for life on earth. Plants have evolved special pigments to absorb radiant energy in the visible spectrum and convert it into chemical energy through the excitation of an electron. This process is known as photosynthesis and the most famous of these pigments are chlorophyll a and chlorophyll b.
|← infrared and below ←||visible light||→ ultraviolet and above →|
|non-ionizing radiation||excited electrons||ionizing radiation|
starting to loose it …
Ultraviolet radiation is not very penetrating, however. This makes it dangerous to our skin and corneas (UV is a major cause of skin cancer and cataracts), but its effects on the deep interior of our bodies is hard to demonstrate. UV is something to be wary of, but x and gamma rays are worse.
Therefore there are two factors that separate the relatively dangerous forms of radiation from the relatively harmless forms: penetration and ionization. This includes alpha, beta, and gamma radiation, x rays, free neutrons, and fast ions — basically any particle with an energy on the order of several thousand to several million electron volts. Of course, particles with energies greater than this are even more dangerous, but their existence on earth is extremely rare under normal circumstances.
Any radiation that can disrupt the normal chemistry of a cell is dangerous to living things. There are three mechanisms by which nuclear radiation can do this.
- Ionization. Energetic particles leave a trail of ions in their wake.
- Neutron Absorption. The absorption of a neutron into the nucleus of an atom may result in the formation of an unstable isotope and its transmutation into a different element.
- Displacement. If the energy of an incident particle is sufficiently high it can displace an atom from its position in a molecule.
When the damage caused by nuclear radiation takes place in one of a cell's DNA molecules it's as if the cell has lost its control center. In extreme cases a cell could be killed outright, but there are more subtle forms of damage. Altering a DNA sequence that controls growth may cause the cell to grow without limit. When such a cell divides it passes this faulty sequence on to its progeny. A group of such damaged cells that divides without regard to the health and well-being of the body as a whole is called a tumor and an animal afflicted with such a condition is said to have cancer. Nuclear radiation is said to be mutagenic in that it can damage or mutate the genetic code of cells. Since these mutations may also lead to cancer in animals, it is also said to be carcinogenic or cancer-causing.
The amount of energy absorbed from a source of radiation by some material per mass is called the absorbed dose (D). It is a quantity that applies to any source of radiation acting on any type of material, be it living or non-living. It only provides a first approximation to the biological damage of the radiation in a human.
Different forms of radiation with identical absorbed doses may differ in their effect on living things. Neutrons and alpha particles are harder on human tissues than are beta particles or gamma rays. To account for this variation, the absorbed dose is multiplied by a radiation weighting factor (Q) that varies according to the type of radiation. The product is called the equivalent dose (H).
Different tissues or organs receiving identical equivalent doses may differ in their response to radiation damage. Skin is durable stuff that is designed to take abuse and be discarded as it wears out. Bone marrow is much less durable and its loss affects the body as a whole since red blood cells are produced within it. To account for this variation, the equivalent dose is multiplied by a tissue weighting factor (Q) that varies according to the organ or tissue exposed. The product is called the effective dose (H).
In symbolic form, the relation between D the absorbed dose, H the equivalent or effective dose, and Q the weighting factor is…
H = QD
The SI unit of all three types of dose is equal to a joule per kilogram in fundamental units. Different names are thus used to distinguish the absolute value of the absorbed dose from the relative values of the equivalent and effective doses. (The weighting factors are unitless.)
- The SI unit of absorbed dose is the gray, which is equal to a joule per kilogram [Gy = J/kg].
- The SI unit of equivalent dose and effective dose is the sievert, which is also equal to a joule per kilogram [Sv = J/kg].
The gray replaces an earlier unit called the rad (an abbreviation for "radiation absorbed dose") which is equal to 0.01 Gy. The sievert replaces an earlier unit called the rem (and abbreviation for "roentgen equivalent, man") which is equal to 0.01 Sv.
The gray was named in honor of the English physicist Louis Gray (1905–1965).
The sievert was named in honor of the Swedish physicist Rolf Sievert (1898–1966) who developed the basic techniques for measuring absorbed dose.
kerma: kinetic energy released per unit mass, kinetic energy released in material, kinetic energy released in matter".
In addition, it is not necessary to correct for radiation type or tissue type. The absorbed dose is sufficient. (??)
- dose equivalent
- ambient dose equivalent
- directional dose equivalent
- personal dose equivalent
- organ equivalent dose
linear no threshold theory (LNT)
|<0.25||short term effects unlikely|
|0.25–1||nausea, temporary sterility|
|1–3||vomiting, diarrhea, rapid weight loss, temporary reduction in white blood cells|
|3–6||damage to bone marrow and digestive tract, sterility, cataracts, 50% mortality|
|10||severe radiation sickness, death within 30 days|
|100||unconsciousness or coma, death within several hours|
|mortality||one week (Gy)||one month (Gy)||four months (Gy)|
|organism||LD-50 (Gy)||organism||LD-90 (Gy)|
|dogs, pigs||3||cabbage, spinach||140|
|sheep||5.4||barley, rye, wheat, corn||43|
|fish, shellfish||5.5–1000||fruits, grasses||> 50|
|cattle, rats, horses||6.3||potatoes||120|
- potassium 40 makes up about 3 millionths of the earth's crust
- Cosmic Rays
- Food & Water