Tuesday, April 19, 2011

Radiation: levels, exposure, dosage

The continuous coverage of the ongoing nuclear catastrophe centered in the Fukushima prefecture of the Japanese island of Honshu often includes a lot of information related to radiation levels and their effects on human health. The purpose of this entry is to cover the basic physics of the radiation's affect on living tissues. We won't have the occasion here to cover in any detail how exposure to radiation is related to increased incidence of cancer. Hopefully, we'll get to that soon, in a later entry.

Let's begin first with what radiation is, physically speaking. Radiation is a collection (perhaps a collection of just one) of subatomic particles. 'Subatomic' means things smaller than the atom. Recall that (a cartoon of) the atom looks like a solar system, with the nucleus at the center, like the sun, and electrons going around the center, like the planets. Also recall that while the structure is similar to the solar system, the analogy ends there. The forces and speeds acting in the atom are much larger than those in the solar system.

Now, when we're discussing radiation, we're often talking about the things that make up the atom -- electrons and nuclei -- so they have to be smaller than the atom itself. This is the origin of the term, "subatomic," which means "below the atomic scale." And the atomic scale is about 100 million times smaller than the diameter of a an American penny, which is much too small to see with visible light from, say, a flashlight or even in full sunlight. Radiation can not be seen by human eyes because the particles are so small. Visible light is of too large a wavelength to resolve these subatomic particles. Incidentally, the overwhelmingly vast majority of radiation doesn't glow either. You can't see it, taste it, smell it, hear it, or feel it -- unless you suffer a huge dose, that is. But we'll return to this.

Now that we understand the length scale -- subatomic -- that we're discussing, we forget about the atom as a whole and we just consider its parts. Because it is these parts of the atom, the electrons and the nucleus (made of protons and neutrons) and some other types of subatomic particles not found in the atom, that can be flying through space at tremendous speeds, which damage the cells of living (and dead, incidentally) tissue. This damage is the exactly the damage that radiation causes. And this radiation damage can lead to severe sickness and death and to higher incidence of cancer and other diseases.

Let's talk in a little more detail about the microscopic physical process that corresponds to radiation damage. Suppose that we could follow a single neutron flying through space. Typical speeds are tens-of-thousands to millions of miles-per-hour. And suppose that we can see this neutron hitting a collection of tissue, like muscle tissue in a person's arm or bone marrow. Then what we would most often "see" (we're imagining all of this because you can't see things this small with visible light) as the neutron impinged upon the surface of the tissue, which is made of closely-packed cells touching each other, is that it would pass undisturbed through the cell membrane, the outermost portion of the cell, through the inner parts of the cell -- the cytoplasm, organelles, and other cellular features, like the nucleus -- and right out the other side. Since we're looking at a single neutron this is a common scenario. In this scenario, there would be no damage to the cell.

But suppose conversely that the alignment of the flight path of the neutron were just right and it strikes one of the nuclei of one of the atoms in one of the many molecules making up some part of the cell. In that case, the neutron would essentially blast apart the nucleus in the atom of the nucleus in the cell that it hit. This is (neutron) radiation damage. And this is what can be so devastating to living tissue at the cellular level when it happens frequently.

Taken one at a time like we're considering here, the neutron hitting the matter that makes up the cell tissue isn't going to do any significant damage (statistically speaking, at least) to that tissue. But suppose instead of a single neutron flying into the cell and blasting its tissue apart at the subatomic level we have a hundred neutrons in a dense packet that flies into the cell. Then we've just raised the odds (or probability) that some damage can be done. Damaging levels of neutron radiation correspond to several hundreds of neutrons per second falling on an area about the size of an American penny. With this high number of neutrons bombarding the matter that makes up the cellular tissues, the damage can be catastrophic for the cell and it may stop functioning. Or it may, at lower levels of prolonged radiation, suffer a transformation to a cancerous cell.

The above understanding of the microscopic characterization of the physical process of radiation damage allows us to understand the issues of radiation levels, exposure, and dosage.

The radiation level is how many subatomic particles are flying around in some region of space. In order to specify the radiation level we have to say how many particles there are in some given amount of time and how densely they present themselves in space.

The next two terms can be a little confusing because they're often conflated in the media -- radiation exposure and dosage.

The radiation exposure is how much radiation at a given level was applied externally to a given sample of tissue. The radiation dosage is how much radiation is absorbed the exposed tissue. It's the dosage that is a measure of the potential damage that high radiation levels can 'achieve' because this means that the neutron (or other subatomic particle) hit something in a cell. But recall from our discussion of what happens when a single neutron approaches a cell. It might just pass right through. So this particular "exposure" to the neutron radiation didn't result in a "dose" of neutron radiation.

Calculating dosage is a pretty tricky affair. That's because we first need to know the radiation level and the exposure in order to determine the dosage. And the dosage also depends on the type of radiation (neutron or alpha-, beta-, or gamma-radiation, for example) and the type of tissue that received the dose.

These topics will be the subject of subsequent entries.

Thanks for reading. --Mark