What is Radiation?
Published on Oct 10, 2009 at 10:15 pm.
3 Comments.
Filed under physics, space radiation.
As the second installment in my space radiation series, I thought that I’d take a moment to ask (and answer!) the question, “What is radiation?” After all, at heart, I am a physics and astronomy professor, so it makes little sense to me to talk about something without first introducing the topic defining the terms.
At first thought, it would seem that everyone should have at least some idea what radiation is. After all, radiation has been something that is listed in science curricula for most of the Twentieth Century. It is a topic that is in everyday life. People eat irradiate food. They go to the dentist and the X-ray machine has a radiation warning trefoil on it. If you open a smoke detector to change the batteries, you’d often see a small metal container with a radiation trefoil warning sticker. Most people cook food using microwave radiation. And, of course there are occasional news reports that involve nuclear radiation topics. For that matter, since the 1940s, radiation has been in the news. So, with all of this, people should know what radiation is, right? I’ve been a college professor for far too long to believe that! I know that a great many of my students (in fact most of my non-science majors) don’t know what radiation is, or they have serious misconceptions about radiation. The demographics of my regular readers are such that they probably have some knowledge about radiation. However, a lot of people are finding their way to Astroprof’s Page using search engines, and they may be here simply because they typed in the question “What is radiation?” just to find the answer to the question. So, it is useful to do my professorial thing and teach about radiation.
If you try to read about radiation on the internet, you find a bewildering assortment of things. One of the first difficulties is that the term appears to be used in many different ways to refer to different physical phenomena. Secondly, and perhaps even more difficult for the layman trying to understand radiation is that there is a bewildering assortment of ways of measuring and reporting radiation. You wind up with units such as Grays, Sieverts, Rads, Roentgens, REMs, Curies, Becquerels, Kermas, Counts, Watts, Electron Volts, and a host of others. I’ve got some friends who are pilots and flight attendants. They are often exposed to quite a bit of radiation on the job, but the few who have tried to figure out how much exposure they get quickly become confused trying to sort through all of the literature. The problem is that there are many different ways of measuring radiation, and each of the different ways of measurement measures a different thing. There are often ways to go from one measure to the other, but sometimes it is not easy to do that. So, it is no wonder that there is so much confusion about radiation.
First of all, we need to cover just what we mean by the term radiation. Whenever radiation is used in the media, it normally comes with a negative connotation. But, in physics the term is completely neutral. The term radiation ultimately comes from the term ray. Remember, physics is a very mathematical science. 
In mathematics, a ray can be thought of as half of a line. While a line extents infinitely far in both directions, a ray starts at one point and goes off in an infinite direction in the other direction. Light appears to move in rays. Light travels away from a light emitting body in straight lines. (Note: technically, this is not true. The path of light can be altered by passing from one medium to another via a process known as refraction, and general relativity shows that the path of light can be bent by gravitational effects. However, for most purposes, light appears to travel in straight lines.) When something moves away from a source in rays, it is said to be radiating. The following image (courtesy Grant Goodge and Wikimedia Commons) shows crepuscular rays radiating from the Sun.
To a physicist, this process of radiating is known as radiation. Unfortunately, not only is the act of radiation called radiation, but so is the thing being radiation. So, light, in this instance, is radiation. In fact, visible light is only one instance of radiation. Ultraviolet light, infrared light, and even radio waves are all different forms of electromagnetic radiation — waves of electric and magnetic fields. Microwaves are also examples of electromagnetic radiation.
In the Nineteenth Century, another form of radiation was discovered. Certain vacuum tubes were found to be emitting radiation of some sort from the cathode, or negative electrode, of the tube. Eventually, it was shown that these cathode rays were negatively charged particles streaming away from the cathode in straight lines. In essence, they were radiating from the cathode, and thus were a form of radiation. Physicist began to think of radiation as both being from electromagnetic waves and from particles streaming away from a source. In 1895, Wilhelm Roentgen was experimenting with Crooke’s tubes, the devices in which cathode rays had been discovered, and found that some mysterious radiation was being emitted by the anode (the positive electrode) of the tube. This mysterious radiation was designated X-radiation (or X-rays, for short). This X-radiation was far more penetrating than the other forms of radiation known. But, it was absorbed by some materials. So, it would pass through flesh easier than through bone. This led to the possibility of using X-rays as non-invasive diagnostic techniques for medicine and industry.
Soon, it was found that certain elements tended to emit forms of radiation. This radiation comes from the nuclei of the atoms, so it is called nuclear radiation. The nuclear radiation was found to be either positive, negative, or neutral in electrical charge. The positive charged radiation was called alpha radiation. The negative charged radiation was labeled beta radiation. And, the neutral was called gamma radiation. Alpha, beta, and gamma are the first three letters in the Greek alphabet. Later studies showed that alpha radiation was the result of the emission of what we now regard to be essentially helium nuclei moving at very high speeds. Beta radiation is the emission of very high energy electrons. As it turns out, cathode rays are also high energy electrons. The biggest difference between beta rays and cathode rays is simply the source. The neutral gamma rays turn out to be more electromagnetic radiation. X-rays, too, turn out to be electromagnetic radiation. But, so is visual light. So, what’s the difference? The only difference between the different forms of electromagnetic radiation turns out to be the frequency at which they oscillate (and hence the energy that they carry). All forms of electromagnetic radiation are oscillating electric and magnetic fields, and all travel at the speed of light.
So, as you can see, there is more than one type of radiation. Over time, even more forms of radiation have been found. Some of that radiation is in the form of oscillating fields (electromagnetic radiation, gravitational radiation, etc), and some of the radiation is in the form of highly energetic particles (alpha radiation, beta radiation, neutron radiation, proton radiation, etc). There are quite different physical properties involved in the generation of all of the different forms of radiation, so why do we use the term radiation to describe all of it? Well, in physics, we use the term radiation to describe any process in which energy radiates away from a source in rays. For electromagnetic radiation, the waves travel in straight lines. For extremely energetic particles, the particles travel in straight lines. Naturally, the paths of the particles can be altered by fields (electric or magnetic fields for particles with charge, gravitational fields for particles with mass, etc), but they move in straight lines away from their source. Their deflection occurs later. The particles are moving, so they have kinetic energy. The electromagnetic fields can affect charged bodies so they, too, carry energy. Conservation of energy, a basic premise in physics, tells us that if energy is being carried by the radiation, then it must have come from the source, which thus must lose energy in this process. Electromagnetic radiation can be absorbed when it runs into something. The particles can collide with something and slow down. Energy is thus transferred. In a macroscopic view, all of this looks superficially similar, so it is useful to call it all the same thing: radiation. It doesn’t matter exactly what the source of the radiation is, nor the physical process in which the energy is transferred (particles moving carrying kinetic energy or electromagnetic waves carrying energy). What matters is that energy is transferred in rays, straight lines (excluding external influences), as in this diagram.
When the rays of the radiation stop, that is when the energy carried by the radiation is absorbed by something, that energy can change the object being absorbed. The degree of change depends upon how much energy is absorbed. Visual light only carries enough energy to adjust energy levels of electrons in atoms. That can trigger chemical reactions in some cases. Of course, that is ultimately how you see! Radio waves can cause electrons to run up and down electrical conductors, and if you have the appropriate electronics connected, you can detect them. X-rays, gamma rays, and the high energy particles can carry plenty of energy that can knock electrons out of atoms. This leaves the atom ionized. The ionized atoms then behave differently than they did before being ionized. Thus, ionization can cause damage to complex molecules (such as DNA) or to physical structures (such as crystal structures). Certain gemstones can be colored by exposure to radiation of sufficient energy, and some integrated circuit chips can be damaged. But, from a human point of view, the damage to molecules such as DNA is important. When DNA is damaged, then cellular damage can occur. Such damage can cause either cell death or mutation. That can lead to radiation sickness or death. Mutations can also lead to cancer at a later time. I’ll talk more about the radiobiological issues later in this series on space radiation.
Radiation can be in the form of electromagnetic waves, particles, or any other means that carries energy away from a source in an essentially straight line. It can be subatomic particles such as electrons, protons, or neutrons. Radiation can also be in the form of fast moving atomic nuclei, such as alpha particles. Many of the forms of cosmic rays are high energy atomic nuclei. Many of these nuclei are quite heavy nucleons, such as iron nuclei. Because these nuclei are so heavy, they carry much more energy than lighter particles moving the same speed. However, cosmic rays move very fast indeed, many of them moving at very close to the speed of light. This is much faster than many more familiar forms of radiation, such as alpha or beta radiation. The energy is very sensitive to the speed of the particles. The kinetic energy goes as the square of the speed. So, going twice as fast has four times the energy. Going 10 times faster gives 100 times the energy. And, going 1000 times faster gives 1,000,000 times the energy. So, you can see how the cosmic rays, moving near the speed of light, can be very energetic indeed!
Now, I want to finish up this particular posting on the measure of radiation. How would you go about measuring radiation? Since radiation carries energy, one unit of measure would be the amount of energy carried. The SI unit for energy is the Joule. Often the rate of energy transmittal is more important than just the amount of energy. In that case, the appropriate unit is the Watt. One watt equals one joule per second. Typically, the energy is distributed over an area, so a better measure might be the intensity, which is the energy per second per area. The units of intensity would be watts per square meter. These units are fine, and they are typically quite sufficient for most forms of radiation, such as visible light or microwaves. However, they fail to convey the damage potential of ionizing radiation, so there are different units typically used to measure ionizing radiation.
But, what counts as ionizing radiation? Clearly, from what I’ve said earlier, alpha rays, beta rays, gamma rays, and X-rays can be considered ionizing radiation. But, ultraviolet light can also ionize some atoms. Since UV is not as strongly ionizing as X-rays, UV is not typically considered ionizing radiation by most people (though there is little difference between high energy UV and low energy X-rays). But, you may wonder if intensity makes any difference. After all, if low energy UV does not ionize well, could you just crank up the intensity to ionize something? The short answer is “No!” As it turns out, light also behaves like it travels as particles, called photons. The shorter the wavelength of light, the higher energy each light photon has. So, gamma rays have inherently more energy than X-rays, and X-rays are inherently more energetic than ultraviolet light. In fact, some gamma rays are so energetic that they can cause electrons to be ejected from atoms with enough energy to ionize other atoms. The energy of individual photons can be measured in terms of a tiny bit of energy called an electron volt (eV). An electron volt is 1.602×10-19 Joules. X-rays are often in the range of thousands of electron volts (keV) and gamma rays are often in the range of millions of electron volts (MeV). This tells how ionizing the radiation is, but not how much ionization the radiation produces.
For visual light, the power of light can be determined by multiplying the intensity by the area. However, ionizing radiation tends to be somewhat penetrating (particularly X-rays, gamma rays, and several other forms). Alpha radiation is one exception. It has very little penetrating capability (which makes it easy to shield against). Low energy beta radiation is not very penetrating, but high energy beta radiation is penetrating. Sometimes penetrating radiation goes completely through a target without interacting with it. Other forms of radiation of similar power and energy may be less penetrating, and thus completely absorbed. Likewise, some radiation may cause more ionization than the same intensity of other radiation. So, the best measure may be some other measure than intensity.
One unit of measure used is Radiation Exposure. The basic unit of radiation exposure is the Roentgen. One Roentgen (R) is the amount of radiation that produces 0.000258 Coulombs of ionization in one kilogram of dry air. Another, smaller exposure measure is the Air Kerma, which is equal to 0.01 R. But, you can sometimes get the same ionization in air from different energies of radiation absorbed. So, it is often better to measure the amount of energy absorbed. That is called the Absorbed Dose. The unit of absorbed dose that I learned and used years ago when I did work in the area was the Rad (standing for radiation absorbed dose). However, the rad is now a deprecated unit. The preferred SI unit for absorbed dose is the Gray (Gy). One Gray is one Joule of energy absorbed per kilogram of material. A Gray is 100 rad.
There are many forms of ionizing radiation. In fact, radiation is one area of physics where the terminology seems a bit sloppy to me. We use the same term for electromagnetic radiation such as gamma rays and for particulate radiation like beta radiation. The thing is that some radiation can do more damage than an equivalent amount of radiation of some other form. For example, alpha particles (the particles of alpha radiation) are very heavy compared with things like electrons. As alpha particles pass through a material, they knock electrons off of atoms ionizing them. But, the alpha particles are so heavy that they only somewhat slow down and they keep moving, ionizing other atoms. X-rays, however, behave differently. A single X-ray or gamma ray photon is absorbed to ionize an atom. It is completely absorbed, so it can’t keep going to ionize other atoms. Thus, a certain amount of energy absorbed of alpha radiation might do more damage than the same amount of energy of X-rays. So, an another measure of radiation is the Effective Dose or sometimes called the Dose Equivalent. To find the effective dose, you multiply the absorbed dose by a term that is related to how many atoms a single particle of radiation can affect. This is sometimes called the quality factor of the radiation. The quality factor is also sometimes known as the Relative Biological Effect (RBE). Numerically, the RBE is the ratio of the dose of 200 keV X-rays needed to produce a certain biological effect to the absorbed dose of some other radiation to produce the same effect. So, alpha radiation has a far bigger RBE than beta radiation. The effect of radiation on biological systems also depends upon the particular body part exposed. Some parts of the body are more susceptible than others to radiation damage. A weighting factor taking into account the body part irradiated also needs to be considered when determining the effect of radiation. By multiplying the RBE and the weighting factor, the actual effect of the radiation can be determined. Then, multiplying the absorbed dose by the RBE yields the effective dose. The preferred unit for effective dose is the Sievert. One Sievert is one Gray times the RBE for the radiation. An older (now deprecated) unit is the REM (standing for Roentgen Equivalent Man). There are 100 rem in one Sievert. Most permitted radiation dose limits are given in Sieverts. The confusion, though, is that the Sievert is a measure of effective radiation absorbed dose, not actual radiation absorbed dose, which is what most detectors measure. And, the radiation that you absorb is not equal to the radiation to which you are exposed. So, it is no wonder that many people in occupations where they are exposed to radiation really don’t even understand the units or how much radiation they receive!
This has been a bit more technical post than I sometimes make. However, I feel that it is relevant to the discussion of space radiation to actually define the terms of radiation. And, after all, I am a college professor, and many of my regular readers expect a certain level at Astroprof’s Page. That is part of what makes this site a bit different from many others on the internet.
-Astroprof
Image Credit: Wikimedia Commons
lfmorgan on October 11, 2009 at 10:12 am: 1
very much appreciate your plain language approach to explaining radiation—-have been at it myself on the internet since 1992. Just want to say that the planck realtion says it all for me if you let n always equal to 1/h as anthropic measure definition. It means we can crudely predict using problematics but in doing so we clind ourselves to frequency pulse level mechanical detail (local dterminism as Einstein inisted).
lfmorgan on October 11, 2009 at 10:15 am: 2
clind should have beeb “blind” & inisted should have been insisted —MORGAN AGAIN!
Sameer Kadam on July 22, 2010 at 11:38 am: 3
excellent post !