Radiation is something many people don’t know much about. Radiation is in fact everywhere, you just can’t see it – well, most of it that is,and the kind you can see, you can’t see all that well. Most people have a general idea of what it might be, and might even know a few examples of how it manifests itself – such as from a microwave or x-ray.
With the recent threat of nuclear disaster here in Japan, sparked from the damage inflicted by the massive earthquake and subsequent tsunami, many people have no doubt been caught not knowing as much as they would have liked about nuclear radiation effects, types of radiation, and what a “safe” radiation dose amounts to.
But before we can understand what makes radiation so potentially harmful, we have to understand what it is and where it comes from. There are two main types of radiation: ionizing radiation and non-ionizing radiation. Ionizing radiation is what we’re looking for here, and it is the kind of radiation that turns atoms into ions, or atoms with uneven amounts of protons and electrons.
“So What Exactly does Ionizing Radiation do?”
Ionizing radiation is simply radiation that has the energy-capacity to ionize atoms, and is often the sole form of radiation implied when speaking of harmful radiation. The ionizing of an atom occurs when ionizing radiation collides with an atom, “knocking out” an electron and causing an uneven amount of electrons and protons. This leaves the atom with a net positive charge – also called a cation.
Conversely, a net negative charge occurs when an atom gains an electron due to a free electron that is energetic enough to literally force its way into an atom, also called an anion. These two processes are the basis of ionization. Alpha particles, beta particles, neutrons, x-rays, gamma rays, and cosmic rays are all examples of ionizing radiation.
Now that we understand the process of ionization and what ionizing radiation is, we can uncover its potential dangers. As simple as this may sound, the adverse physical effects of radiation are caused by the alteration of atoms by this ionization process to the point of manifesting physical symptoms, such as cell death, genetic mutations, cancer, and ultimately, even death.
“Where does this ionizing radiation come from?”
Think of ionizing radiation as invisible particles or waves of energy that are emitted from either radioactive atoms or radiation-producing machines such as nuclear reactors. Radioactive atoms, also called radioisotopes or radionuclides, are atoms with an unstable nucleus and are therefore experiencing radioactive decay at a rate expressed by its half-life.
During radioactive decay, the atom emits ionizing radiation in the form of gamma rays and/or subatomic particles. However, the amount of ionizing radiation emitted from most naturally occurring radioactive decay is within safe limits. Nuclear reactors on the other hand, are responsible for the perpetual emission of large amounts of ionizing radiation via nuclear fission.
Of course, this radiation is contained within the housing structure so long as it does what it’s supposed to. – Which is exactly the point in question! If something happens that is outside the control and foresight of engineers, such as earthquakes, and other natural disasters, and the radiation is somehow allowed to leak out – or God forbid, flood out – there will be a major disaster.
The ionizing radiation in the form of radioisotopes such as iodine-131 and caesium -137 will then be dispersed by the wind which will carry them far and wide. The lifespan of a radioisotope is determined by its half-life, therefore you can say that the amount of damage it can inflict is in large part based on its half-life. Iodine-131 for example, only has a half-life of about 8 days, whereas caesium-137 has a half-life of about 30 years.
But half-life is not the only factor involved in determining potential danger and its extent. The shorter the half-life of an atom, the higher its release of ionizing radiation per unit time. So although the time during which it’s doing damage is shorter, it’s also more concentrated and intense. Another factor is the atomic mass of the radioisotope.
The heavier it is the quicker it will sink to the ground, and the smaller the radius of contamination. Conversely, the “lighter” the radioisotope, the further it will be carried by the wind. Of course, penetration into the ground water and any nearby bodies of water such as rivers can be particularly dangerous due to the range of contamination and potential imbibing of radioactive water.
Absorbed Dose vs. Equivalent Dose (Gray vs. Sievert)
There are two main ways to measure radiation doses depending on the context: The absorbed dose and the equivalent dose. The absorbed dose, measured in the SI unit, gray (Gy), is a measure of the energy deposited in a medium by ionizing radiation per unit mass. It makes no distinction between radiation types, but simply is a measure of the absorbed dose of energy within a given material.
The gray, therefore, is not an accurate representation of the adverse biological effects of radiation as it measures the dose in terms of net energy per unit mass without taking into consideration what is called the relative biological effectiveness (RBE, covered in more detail below). The sievert on the other hand, is a more accurate representation of radiation effects.
The sievert, or equivalent dose, is equal to the absorbed dose, the gray, multiplied by a “weighting factor”, which is dependent on radiation type and its energy range. Alpha particles for example, have a weighting factor of 20, as opposed to photons and electrons, which only have a weighting factor of 1. Thus, an absorbed dose of alpha particles equal to 1 gray will in turn, exact an equivalent dose of 20 sieverts.
Relative Biological Effectiveness (RBE)
The relative biological effectiveness is equal to the weighting factor mentioned above, and essentially defines the effectiveness of ionizing radiation to cause damage to biological tissues. For example, although alpha particles are the most damaging IF they can make it into your body, because of their size, they can’t penetrate very far into things. Even a single sheet of paper will stop an alpha particle!
So although alpha particles have a high relative biological effectiveness, they still have to make it into your system before they can cause the damage they’re capable of inflicting. In contrast, gamma rays and neutrons are capable of deeper penetration and are therefore more dangerous to the human body from the outside.