The Physics
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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 when used as intended.

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.

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…

Some items should not be added to this list of dangerous forms of radiation. Two examples are provided below.

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.

Visible light divides the electromagnetic spectrum into general regions.
← infrared and below visible light ultraviolet and above →
non-ionizing radiation excited electrons ionizing radiation

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 electronvolts. Of course, particles with energies greater than this are even more dangerous, but their existence on Earth is 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.

  1. Ionization. Energetic particles leave a trail of ions in their wake.
  2. 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.
  3. 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 (E) from a source of radiation by some material per mass (m) is called the absorbed dose (D).

D = E

It is a quantity that applies to any source of ionizing radiation acting on any type of material, be it living or non-living. It only provides a first approximation to the biological damage of radiation on a person.

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 (D) is multiplied by a radiation weighting factor (wR) that varies according to the type of radiation. The product is called the equivalent dose (H).

H = wRD

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 (H) is multiplied by a tissue weighting factor (wT) that varies according to the organ or tissue exposed. The product is called the effective dose (E).

E = wTH


The SI unit of all three types of dose is equal to a joule per kilogram in SI base 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 gray was named in honor of the English physicist Louis Gray (1905–1965) who developed the concept of relative biological effectiveness, which was later quantified as the weighting factor. The gray replaces an earlier unit called the rad, an abbreviation for "radiation absorbed dose". The two quantites differ by a factor of 100, with the gray being the larger unit.

1 Gy =  100 rad
0.01 Gy =  1 rad

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 sievert was named in honor of the Swedish physicist Rolf Sievert (1898–1966) who developed the basic techniques for measuring absorbed dose. The sievert replaces an earlier unit called the rem, an abbreviation for "roentgen equivalent man".

1 Sv =  100 rem
0.01 Sv =  1 rem

The gray and the sievert are rather large for most everyday uses so prefixes like milli (m = 10−3), micro (μ = 10−3), and nano (n = 10−3) are commonly tacked on to these units.

Radiation weighting factors (wR) for equivalent dose (H)
radiation weighting factor
alpha & heavy ions 20
beta & muons 1
gamma & x-rays 1
protons & charged pions 2
neutrons, thermal 2.5
neutrons, fast 2.5–20.7
Tissue weighting factors (wT) for effective dose (E)
tissue/organ weighting factor
bladder 0.04
bone surface 0.01
bone marrow 0.12
brain 0.01
breast 0.12
colon 0.12
esophagus 0.04
gonads 0.08
liver 0.04
lung 0.12
salivary glands 0.01
skin 0.01
stomach 0.12
thyroid 0.04
everything else 0.12
whole body 1.00

The harmful effects of neutrons on the human body depends strongly on their kinetic energy. The International Commission on Radiological Protection (ICRP) recommends the following empirically derived, continuous function for determining neutron radiation weighting factors.

wR =  

2.5 + 18.2e−⅙[ln(E0)]2 E0 < 1 MeV
5.0 + 17.0e−⅙[ln(2E0)]2 1 MeV ≤ E0 ≤ 50 MeV
2.5 + 3.25e−⅙[ln(0.04E0)]2 E0 > 50 MeV

Neutrons with kinetic energy less than 0.001 MeV (1 keV) and greater than 10,000 MeV (10 GeV) have a weighting factor of 2.5, making them about as dangerous as protons. Neutrons are most destructive to human tissues near 1eV where the weighting factor peaks at 20.7, making them about as dangerous as alpha particles.

Line graph

Short term effects of sudden radiation absorption Source: New Scientist and Hiroshima Peace Memorial Museum
dose (Gy)
<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 from accumulated radiation exposure Source: Todd Postma, University of California, Berkeley
mortality one week (Gy) one month (Gy) four months (Gy)
0% 1.5 2.0 3.0
5% 2.5 3.5 5.0
50% 4.5 6.0
Lethal dose for various organisms (LD-50 = 50% mortality)
organism LD-50 (Gy)
dogs, pigs 3
goats 3.5
humans 4
mice, monkeys 4.5
sheep 5.4
fish, shellfish 5.5–1000
cattle, rats, horses 6.3
rabbits 8
chickens 10
insects >50
turtles 150
bacteria, viruses 1000
organism LD-90 (Gy)
cabbage, spinach 140
organism LD-100 (GY)
onions 20
oats 33
barley, rye, wheat, corn 43
fruits, grasses >50
potatoes 120
tomatoes 150
Source: Todd Postma, University of California, Berkeley

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