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The Bringing It Together section explains how radiation affects all life, including people, and how it is measured. If you have not read the The Science section, it's recommended that you do. There will be many illustrations and references to examples used on that page in this section.
Our example in the The Science section was regarding sunburn’s effect on human tissue and how radiation is similar. Consider the fact that in most cases, the sun and minor sunburns are fairly harmless. The sunburn will go from red to brown, sometimes in a day, and no permanent damage has been incurred. In some cases, though, long exposure to the sun can burn your arm and cause permanent damage.
Radiation reacts in much the same way. The environment and the beings that exist within it can process and handle the existence of radiation without issue. In some cases, though, people can be harmed and physical ailments can occur. This happens if people are exposed to too much radiation in a short time. If there are short times in intense sun exposure, there is less of a chance of burning than the inverse.
So then, how does one know just how much radiation is too much before they get burned and have scars from it? Before we can answer this, we need a way to measure the various aspects of radiation. We can't simply set a limit without any way to have people relate to what we're saying, otherwise all we'd be saying is, "My sunburn is redder than yours, " and we'd have no way to tell how much damage there is in any one case.
Radiation is measured by two important figures. The first denotes how hot the sun is. Remember that we can't control or measure the temperature of our skin due to exposure from the sun – it’s almost impossible to tell how much of the heat is from our bodies and how much is from the sun. Both of those factors on their own tell us nothing since both of them together are what gives us a burn. However, we can measure the amount of radiation emitted from the sun in a given amount of time. These two measurements together, in reality, will tell us not only how dangerous the current state of the sun is, but also what precautions we must follow in order to protect ourselves from getting harmed by it.
These same two types of measurements are used in reference to radiation. The measurement of "how hot our skin is" is referred to as activity, and a measurement of "how radioactive is the sun" is referred to as dose rate. These terms are very common in the radiological industry and often define how dangerous a given radioactive material is.
As with distance or any other quantifiable measurement, there are multiple units that are used based on the preferences of a particular region on the Earth. The SI, or global standard, units that identify the activity level and dose are the Bequerel (Bq) and the Gray (Gy), respectively. Notice that I did not refer to the dose as the "dose rate." More on this late. The Gray does not indicate the effects that that steam has on our arm or hand, however. It is a simply an indication of how much heat is transferred unto the objects the steam particles collide with. As such, it is defined to be what's called the absorbed dose, since the heat transfer over a period of time is absorbed by whatever material is around. What does this have to do with us as humans? Well, think about when you burn yourself. In some cases, there must be much more heat transfer to burn some parts of your body than others. For instance, a warm piece of metal will hurt much less if put on the back of your hand than the front, just as radiation will affect your skin before it affects your bone. There must be a way to determine the human equivalent - the effects that the heat particles have on different parts of our body. In fact, there is such a method. The SI unit for what's called equivalent dose is the Sievert (Sv). The sievert incorporates a "quality factor" into the amount of heat transfer to get a relative concept of how that heat transfer will affect the human body. Essentially, it's like saying "this 80 degree object is perceived to be 120 degrees on this part of my body, and only about 70 on another." It is the perception of radiation to the human body, and directly affects how elements of the human body react to the absorbed dose present.
Of course the United States has developed their own standards pertaining to radiation measurement. The United States definitions for activity, equivalent dose, and absorbed dose are the Curie (Ci), the Rem, and the Rad respectively. These definitions measure slightly different concepts, but can easily be concerted between each other. That is, the Becquerel can be converted to the Curie, the Rem to the Sievert, and the Gray to the Rad.
Technically speaking, the Becquerel measures the amount of radioactive material that will have one transformation per second. On the other hand, the Curie measure the amount of radioactive material that has 37,000,000,000 transformations per second. Quite the difference! As such, the Becquerel is often referred to with the prefix Mega- meaning on million (1,000,000) or much less commonly Tera- meaning one trillion (1,000,000,000,000). A terabecquerel is easily a lethal amount of radiation if one is exposed over a short time. Curies are often referred to in the form of microcuries, or more often millicuries. A microcurie is a very small amount of radiation, whereas a millicurie is fairly standard.
The technical definitions or the Gray and the Sievert are fairly straighforward. A Gray is equal to one joule of energy deposited on one kg of material. Remember that a joule is simply a form of energy measurement and does not necessarily imply harmfulness or not. Just as water can be hot, but depending on where it touches the human body the heat will be felt more or less. As such, the Sievert is simply the Gray dose times a "quality factor," as mentioned earlier. There are 100 Rads in one Gray, and 100 Rem in one Sievert. A value of 20mSv per year is considered safe for professional radiation employees, above 100mSv is considered a deadly amount.
Most often, you will hear these radiological terms used as complex units. That is, Bq/cm^2 or mRem/hr. mRem/hr is probably the most common measurement in the radiological field. It is use to measure dose rate, or the amount of relative radiation entering the human body over a measured period of time. Other measurements, like Bq/cm^2 simply measures the amount of radioactivity present in one square centimeter of material, or the number of disintegrations per second (as activity measures disintegrations per second to begin with) of one square meter of material. This is often used to estimate the amount of activity present on a surface that is too large to measure. One would take the measurement of the Bq per square meter and then multiply the activity in that square centimeter by the entire surface area of the object, obtaining a value that would be a rough estimate of the surface area.
Two other terms you may hear are terms not yet addressed but easily conceptualized. They are counts per minute, or cpm, and disintegrations per minute, or dpm. Counts per minute is a measurement that counts the number of atoms that have decayed, or emitted any form of radiation, in one minute, just as becquerels measures the counts per second. There are 60 becquerels in one cpm. However, since atoms are round and radiation can fly off in any direction, there will always be some missed counts, since the instrument can only obtain a measurement from on point in space - a fraction of the possible locations where radioactivity could go. As a result, there must be a different, more efficient way to measure the disintegrations. Disintegrations per minute is the solution. DPM attempts to estimate the total number of disintegrations of a given amount of radioactive material in one minute. It does so by using an efficiency factor and retrieving the counts per minute as a normal detector would. The efficiency factor is usually a percentage under 50 and is derived through very particular means, but takes into account all possible direections that each piece of radiation could go.
Hopefully this answered many of the questions and curiousities surrounding the topic of radiation. Please feel free to visit the contact page and forward any questions or comments to the e-mail addresses located there.
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Last updated: Thursday, March 5, 2009 9:19 AM