What is Radiation?
Radiation refers simply to energy moving through space or through matter. There are many different kinds of radiation: radio waves, heat, visible light, ultraviolet rays, to name just a few. However, usually when people refer to ‘radiation’ they are referring specifically to “ionizing radiation”. This type of radiation has the ability to kick an electron out of a neutral atom or molecule. Now charged instead of neutral, the atom or molecule may behave differently in its local environment. In addition, the freed electron, which has been given some energy by the incoming radiation, can ionize atoms and molecules as it passes through the material. While radio waves, heat, visible light, ultraviolet rays, and microwaves are examples of non-ionizing forms of radiation, x-rays and gamma-rays have the ability to ionize. Several types of particles emitted by radioactive atoms including alphas, betas and sometimes neutrons also ionize the material through which they travel.
Where does Ionizing Radiation come from?
1. Natural Radiation:
The earth contains a large number of different radioactive atoms. Many of these atoms were created in supernova explosions billions of years ago and have been in existence since long before the earth condensed from a gaseous cloud into a solid mass. These atoms are unstable (radioactive) due to the particular forces at play inside the atomic nucleus1. To get to a lower energy configuration the nucleus ‘transmutes’ – it changes into a different nucleus by emitting a small particle (an alpha or beta particle, often accompanied by a gamma ray)2. For instance, Uranium is an extremely common element found in rocks and soil all over the world. All uranium is radioactive; the most common type of Uranium, 238U, has a half-life of 4.5 billion years, about the estimated age of the earth. [So about half the 238U that was around at the beginning of the earth is still here today.] When 238U decays (transmutes) it ejects an alpha particle and becomes an atom of 234Thorium. This thorium is also unstable (radioactive) and so will also decay. In fact, there are a number of decays as first U, then Th, etc. decay until finally we get to lead (206Pb), which is stable. Our natural radiation background comes primarily from these decay chains found in the environment, each starting with a heavy, primordial, radioactive element and decaying through several steps (emitting several alphas, betas and gamma rays) until the daughter has reached a stable configuration. The various elements in these chains are found naturally in the constituents of our planet. They are part of our soil and our air and become part of our building materials so we are surrounded by radiation sources all the time. These elements are also found in our water and in our foods and thus become part of every life form on earth, including humans. It would be impossible to eliminate our exposure to these natural radiations.
Radiation also comes from outside our planet. Cosmic rays come from the sun and from unknown sources beyond our galaxy. At sea level the atmosphere above us provides a great deal of protection from cosmic rays, but as we move to higher altitudes (eg. airline travel) we remove some of this natural shielding and thereby increase our exposure to cosmic rays.
2. Manmade Radiation:
Although radiation has been around a lot longer than people (and indeed longer than any life form on earth) we only found out about natural radiation once we were able to make radiation ourselves. Firing a beam of electrons into a solid target generates a very penetrating kind of energy. Discovering this by accident in 1895, Wilhelm Roentgen called this unusual and mysterious phenomenon “x-rays”. Other investigators, trying to understand the nature of these mysterious x-rays, soon learned that many natural substances also emit energetic rays, some very penetrating (gamma-rays) and some not so penetrating (alpha and beta particles). Trying to understand the natural radioactivity of the earth soon led to the discovery of cosmic radiation3.
More sophisticated versions of Roentgen’s device are used today for many different industrial and medical applications [irradiation of food to prolong shelf-life and kill bacteria, sterilization of medicines and bandages, dental x-rays, CT scans, whole-body scanners at the airport, etc.] We have also learned to manipulate the high energy levels present in the atomic nucleus. Nuclear power reactors are based on using neutrons to induce a heavy nucleus (eg. U or Pu) to split in two, releasing a very large amount of energy per unit mass, much larger than we get from chemical reactions. The two “fission products” are radioactive and begin a chain of radioactive decays in their quest to become stable. Usually the fission process is tightly controlled and we make sure the fission products stay contained within the fuel assembly. It is the inadvertent release of these products into the atmosphere that results in environmental contamination following an accident at a nuclear power plant.
What does Ionizing Radiation do to people?
By kicking an electron out of an atom or molecule, a neutral entity now has a positive charge and it may behave differently than it does in its neutral state. Since most of the molecules in the human body are water, water is the molecule most frequently ionized when people are irradiated. Ionized water leads to the creation of ‘reactive oxygen species’. None of these reactive species is new or unusual in the cell, but if the radiation dose is large their number and their spatial concentration within the cell may result in changes to important biological molecules such as DNA. DNA is important since, unlike all other molecules in the cell, there is only a single copy. It also contains the instructions for cellular functions. Damaged DNA could result in one of 4 outcomes. (i) The damage could be noted by the cell and completely repaired by our cadre of DNA repair systems. We know this takes place if the rate of radiation delivery is low allowing time for repair between radiation hits. (ii) On the other hand, if the cell receives a very large radiation dose and there is extensive DNA damage, the cell may be unable to divide. If a great many cells in a given tissue are affected in this way we may see tissue failure and even death from the Acute Radiation Syndrome. This only happens with very large doses of radiation (at least 4 Sv, which is ~1700 years worth of natural background received all at once).
In between these extremes we see DNA damage that is not properly repaired but not extensive enough to end the reproductive life of the cell. (iii) Often the damaged DNA seems to have no impact on the cell or organism (ie that region of the DNA doesn’t seem to code for any important or even noticeable function). (iv) But sometimes the damaged region of DNA leads to a detectable mutation, one that is passed on to daughter cells as the damaged cell divides and proliferates. If this damaged cell is a germ cell (a sperm or egg) we see a radiation-induced genetic change in the offspring (a hereditary effect). If it is any other cell in the body, it could lead to a neoplastic transformation, that is, a cancer. Because radiation-induced cancer has a very long latent period (20-30 years), this cancer won’t be diagnosed until many years after the initial exposure to radiation.
What is Dose? What are rem and Sieverts?
Ionizing radiation only causes harm if it deposits energy in material (via ionizations and excitations). Dose is the amount of energy absorbed by a given mass of material. Dose is measured in units of Gray [1 Gy = 1 Joule of energy absorbed per kilogram of material]. The older unit, still used in the United States, is the rad. [1 Gy = 100 rad].
But just knowing the energy absorbed is not enough to predict the biological damage caused by radiation. We need to know something about how densely ionizing the radiation is – how frequently in a given distance do ionizations occur? Radiations that deposit a lot of energy in just a small distance are much more hazardous than radiations that deposit their energy sparsely in material, even if the total energy dumped into the material is exactly the same. Alpha particles are not very penetrating but are very densely ionizing. The alpha emitted from 238U as it decays to thorium only travels a fraction of a millimeter in tissue but it dumps all of its energy within that distance. The damage caused by these closely spaced ionizations is difficult for our cells to correct. An x-ray, on the other hand, travels large distances, dumping little bits of energy perhaps several centimeters apart. Any damage created is more easily corrected by the cell. The way we deal with these differences in the ionization density is to multiply the energy absorbed (dose) by a number (a Quality Factor, Q) that takes into account how much more dangerous densely ionizing radiations are. We then have a new quantity which we call ‘dose equivalence’. Dose equivalence is given a different unit, the Sievert (Sv). [The old unit for dose equivalence, still used in the United States, is the rem.] Alpha particles have a Q of 20; x-rays have a Q of 1.
Dose Equivalence = Dose x Quality Factor
Sv = Gy x Q 1 mSv = 1/1000th of a Sv 1 microSv (µSv) = 1 millionth of a Sv
rem = rad x Q 1 mrem = 1/1000th of a rem 1 microrem (µrem) = 1 millionth of a rem
1 Radioactivity, a term coined by Marie Cure, is measured in number of decays per second. 1 decay/second= 1 Becquerel (Bq)
2 An alpha particle (α) is a helium nucleus: 2 neutrons and 2 protons. Although it is ejected from the nucleus with considerable energy as part of the decay process, it dumps all of this energy in solid material within a fraction of a millimeter. Most alpha particles can be stopped by a sheet of paper or by the dead outer layer of your skin.
A beta particle (β) is an electron, created in the nucleus as a neutron is converted to a proton as part of the transmutation process. More penetrating than alpha particles, beta particles can still only travel a few mm in tissue or other solid materials.
A gamma ray (γ) has the same properties as an x-ray. Both can be very penetrating. The 364 keV gamma ray from 131I travels an average of 9 cm in tissue or water before its first ionization. The 364 keV gamma ray from 137Cs travels 12 cm, on average, before its first interaction in tissue.
3 Balloon experiments to measure the reduction in terrestrial radiation rate with increasing altitude led to the surprising finding that while radiation rate decreased at first, it soon began to increase as the balloons carrying detectors made it to higher and higher altitudes.
Jacquelyn Yanch has worked with ionizing radiation for 30 years. She was Professor of Nuclear Science and Engineering at the Massachusetts Institute of Technology from 1989-2010 where she performed research and taught courses in the production, detection, uses, and health impacts of ionizing radiation. She can be reached for comments and questions at firstname.lastname@example.org.