Radiobiology

Discussion

radiation

as a general term in physics

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.

• 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 in 1915 and confirmed in 2015.
• Cosmic radiation is a blanket term that includes all forms of radiation from outside the Earth. This includes…
  • cosmic rays: high energy, ionized nuclei and electrons from all sorts of celestial sources
  • solar wind: cosmic rays originating from the Sun
  • cosmic microwave background: the remnant thermal radiation of the Big Bang that permeates the universe
• 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.

• Alpha particles: Helium nuclei emitted from large, unstable nuclei or other nuclear reactions
• Beta particles: Energetic electrons produced when an unstable neutron transforms into a proton
• Gamma rays: The most energetic electromagnetic waves yet discovered
• X-rays: Electromagnetic waves that are not as energetic as gamma rays
• Free neutrons: Neutrons that are not part of a nucleus but have somehow managed to break free
• Fast ions: Atoms missing one or more electron with an energy greater than several thousand electronvolts — cosmic rays and the daughter nuclei of fission reactions, for example

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

• Neutrinos
  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 without any interaction whatsoever.
• Microwaves
  People who use the term "nuke" when heating foods in a microwave oven may sometimes need to be reminded that this is a joke. Yes microwaves are a form of radiation, but then so are infrared and visible light. The same people that say "I'm going to nuke these leftovers" when they reheat food in a microwave would probably 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. In these situations too much energy is being delivered too quickly over too small an area for exposures like these to be considered safe.

ionizing vs. non-ionizing radiation

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 — to remove electrons from on atom and deposit them on another. 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.

Ultraviolet radiation is 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-rays and gamma rays are worse. They are electromagnetic waves that are both ionizing and penetrating.

effects of ionizing radiation

Any radiation that can disrupt the normal chemistry of a cell is dangerous to living things. There are four mechanisms by which nuclear radiation can do this. Listed below in order of generally increasing energy, the are…

1. Ionization: Energetic particles leave a trail of ions in their wake when they pass through living things. Atoms and molecules with unbalanced charge do not behave the same as their neutral parents did.
2. Dissociation: A well placed, sufficiently energetic particle can break the bonds between atoms in a molecule. Two half molecules do not behave the same as one whole one.
3. Displacement: An even more energetic particle can displace an atom from its position in a molecule. A molecule with parts missing or rearranged is obviously going to behave differently than its parent molecule did.
4. 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. Different elements have different chemical properties. That's kind of their definition.

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 a 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.

dose

The amount of energy absorbed (E) from a source of radiation by some material per mass (m) is called the absorbed dose (D).

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 (w_R) that varies according to the type of radiation. The product is called the equivalent dose (H).

H = w_RD

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 (w_T) that varies according to the organ or tissue exposed. The product is called the effective dose (E).

E = w_TH

Radiation exposure can include a mix of radiation types and be spread across multiple organs. When this happens, the effective dose is a sum.

units

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 quantities differ by a factor of 100, with the gray being the larger unit.

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".

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.

Weighting factors

Free neutrons

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.

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.

Cummulative effects

Discuss

Random notes:

• Not quite the same as the absorbed dose is the kerma: kinetic energy released per unit mass, kinetic energy released in material, kinetic energy released in matter".
• The banana equivalent dose (BED) is an informal unit of exposure equivalent to eating one banana.
• v0
  • On page 620 of the CRD Handbook on Rad Measurement and Protection, the concentration of ⁴⁰K in a "Reference Banana" is listed as 3520 picocuries per kilogram of banana. For those of us who are stuck in certain unit ruts, this is equivalent to 3.52E-6 microcuries of ⁴⁰K per gram of banana.
  • An average "Reference" banana weighs (masses) about 150 grams (I think.) So, the ICRP Reference Banana contains about 5.28E-4 microcuries of probably deadly ⁴⁰K.
  • Federal Guidance Report #11 lists the ingestion dose (committed effective dose equivalent) for ⁴⁰K as 5.02E-9 Sv/Bq or (again, for those of us who are "unit-challenged," 1.86E-2 rem per microcurie ingested.)
  • Thus, the CEDE from ingestion of a Reference Banana is 5.28E-4 x 1.86E-2 = 9.82E-6 rem or about 0.01 millirem.
• v1
  • One banana contains approximately 422 mg of potassium
  • The naturally occurring radioactive isotope of potassium, ⁴⁰K, has a natural occurrence of 0.0117%
  • About 0.05 mg is radioactive or 13.3 Bq (about 400 pCi) of radioactivity per banana
  • If a "bunch" = 10 bananas, that's 130 Bq (4000 pCi) of ⁴⁰K per bunch of bananas
  • Your body contains about 4000 Bq of radioactive ⁴⁰K (250,000 decays every minute)
  • 5 oz of uranium ore of 0.1% grade, contains as much radioactivity from uranium as ⁴⁰K in ten bananas (60 to 80 oz)
• v2
  • The average radiologic profile of bananas is 3520 picocuries per kg, or roughly 520 picocuries per 150 g banana. The equivalent dose for 365 bananas (one per day for a year) is 3.6 millirems (36 µSv).
  • Some other foods that have above-average levels are potatoes, kidney beans, Brazil nuts and sunflower seeds. Among the most naturally radioactive food known are Brazil nuts with activity levels that can exceed 12,000 picocuries per kg. But this does not mean you should stop eating these – they are healthy foods that pose no threat to people!!
• The biologically effective radiation dose, BED, refers to the true biological dose that a tissue receives and it depends on the total dose, fraction per dose and specific tissue characteristics
• List of related terms
  • dose equivalent
  • ambient dose equivalent
  • directional dose equivalent
  • personal dose equivalent
  • organ equivalent dose
• The linear no threshold theory (LNT)

Environment

• potassium 40 makes up about 3 millionths of the Earth's crust
• Cosmic Rays
• Food & Water
• Radon

medical

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