Radiation Basics

What is radiation?

Generally speaking there are many types of radiation such as radio waves, UV from the sun, heat from a fire, and what some people call “nuclear radiation” but is more precisely known as “ionizing radiation.” It’s called ionizing radiation because it has enough energy to break molecules apart into ions. Too much of that in human tissue can result in things like an increased risk of cancer or, in extreme cases, immediate health consequences. For simplicity this “ionizing radiation” is referred to just as “radiation” in some contexts, including this web site. There are many exotic types of radiation, but for most people the only ones that matter are alpha, beta, X-ray, and gamma.

Where does radiation come from?

Radiation is everywhere and ordinary amounts of it are nothing to fear. Some natural sources include cosmic radiation originating from the sun, terrestrial radiation from ordinary materials in the earth, and radon gas in the air we breathe. Some people are also exposed to radiation due to a need for X-ray imaging or nuclear medicine treatments. Other normally minor sources of radiation exposure include the food we eat, small quantities of leftover fallout in the atmosphere from past nuclear weapons testing, and some consumer products like smoke detectors. Depending on factors such as where you live, the materials in your home, and your medical needs – this baseline level of radiation exposure can vary significantly from one person to another.

The previous paragraph describes where radiation typically “comes from” in our daily lives, but to be a bit more precise radiation usually comes from specific “isotopes.” Every material is fundamentally composed of one or several different elements. For example, water contains the elements hydrogen and oxygen. The smallest piece of an element is an atom – for example, one balloon filled with the element helium contains many, many individual helium atoms (a number almost too large to comprehend). Each atom can have different variations called isotopes, and each of those isotopes is defined by a number (that number is equal to the total number of protons and neutrons in that isotope).

Most materials we encounter are composed of stable isotopes and do not emit radiation. A few materials, however, contain radioactive isotopes - meaning the isotope is unstable and in a constant state of gradually transforming into a different material. During that transformation, radiation is emitted (such as an alpha particle, a beta particle, or a gamma ray) each time one atom changes. For example, we can consider potassium. Potassium ordinarily consists of three isotopes: potassium-39, potassium-40, and potassium-41. One of these, potassium-40 (also noted as K-40 since K is the symbol of potassium), represents 0.012% of all potassium and is an unstable, radioactive isotope. Bananas contain potassium, and therefore all bananas are emitting radiation – although an extremely tiny amount. How much and what type of radiation given off from an object depends on the type(s) and quantity of radioactive isotopes it contains.

What are the types of radiation?

Aside from exotic applications, there are only a few types of radiation most people need to be familiar with: alpha, beta, X-ray, and gamma.

Alpha particles are positively charged and, in radiation terms, they are very heavy (even if one particle is still unimaginably small). Because they are heavy and charged, they tend to travel more slowly than other radiation types and are stopped in a very short distance by any ordinary material. Alpha particles would not typically pass through a piece of paper or even the very thin outer layer of dead skin we all have. Even just in air alpha particles gradually lose energy and are stopped after traveling only a few inches or centimeters. Alpha particles are, therefore, not much of a health danger to humans unless a material which emits alpha particles is inhaled or otherwise ingested. If the alpha particles are emitted inside someone’s lungs, for example, they can directly reach the tissue of the lung and potentially pose a health risk.

Beta particles are basically electrons, negatively charged, travelling with a lot of energy. They weigh much less than alpha particles they tend to travel much faster and further. Still, because they are charged they are stopped fairly easily by any material. Any solid material needs only a thin layer to stop most beta particles, such as a piece of aluminum foil or an ordinary glass window. The range of beta particles in air is typically several feet or a couple meters. Beta particles can pass through your skin to reach sensitive tissue in your body, but only if you are close enough to the source without any material in between.

The last two types of radiation are X-rays and gamma rays (or “gammas” for short). These are both energetic photons, like the visible light photons coming from your desk lamp but with higher energy and much smaller quantities. Any X-ray or gamma photon has a specific energy. There is literally zero physical difference between an X-ray photon and a gamma photon with the same energy. One is called an X-ray and another a gamma ray only according to what physical process created it. X-ray production is associated with the electron shell of an atom and gamma production is associated with the nucleus of an atom. Despite popular belief, a photon does not fall into one category or the other according to its energy - both exist over a wide range of energies. Most of the X-rays we encounter in our daily lives come from devices specifically designed to produce them – such as the X-ray generator in a hospital or the X-ray luggage scanner at an airport. Away from those special devices, most of the energetic photons we encounter in our daily lives are gamma photons.

Since they are the same thing, any physical property of gammas is equally true for X-rays, so for the rest of this section both will be referred to simply as gammas. Gammas do not have a positive or negative charge, which allows them to travel much longer distances than alpha or beta particles. Alpha and beta particles gradually lose energy as they pass through any material until they are absorbed. Each gamma photon, on the other hand, travels for a while, then interacts. One of those interactions might be absorption, or it might bounce off in a different direction. It is not a gradual process. Each gamma ray follows a unique and somewhat random path, although the probabilities are defined by physics principles, based on things like the energy of the gamma ray and the material it is travelling through. If many gammas enter a brick wall, some might be absorbed, some might bounce off of the wall towards a new direction, and some might pass straight through without interacting. As the wall gets thicker, fewer and fewer will pass through, but even with a very thick wall a small number will pass through - even if this is a tiny amount. This can be viewed as a problem in the sense that  sources of gamma radiation are difficult to shield, but it also means that gamma sources are easier to locate because they can be “seen” or measured from a greater distance away.

What kind of radiation is important to measure?

In the previous section, four types of radiation were described – alpha, beta, X-ray, and gamma. It was mentioned that gammas and X-rays are physically identical phenomena (with semantic differences in their origin) so in this section only gammas will be discussed - though all statements about gammas equally apply to X-rays (including how they are measured and their effect on human health).

As previously described, measuring alpha particles is a challenge because they don’t travel far. By far the most important and well-known alpha emitting isotope is radon because it is common around us, takes the form of a gas, and mixes into the air we breathe. Radon can in particular collect in confined places with stagnant air, like some basements, and pose a health hazard if the concentration is too high. Measuring radon concentration in air is a difficult technical challenge. In the same way that a person would usually have to breathe in radon for it to be harmful, a measuring device generally needs to somehow “breathe in” air to measure how much radon it contains. As air passes through such a device, it measures the radon inside. This type of specialized apparatus is typically capable of measuring radon concentration in air and nothing else.

The reason a normal radon detector can do only that one task is that technology suited for that task is, by its nature, not suitable for other radiation detection tasks. It will not be sensitive to beta or gamma radiation. A radon measuring device is not even suitable for identifying most other alpha emitters – for example a surface contaminated with an alpha-emitting isotope. That is because the contaminant on a surface is not in the air and therefore will not be “seen” by the device, because it is looking at the air which flows through it.

Devices do exist which can identify such a surface contamination of an alpha-emitting isotope - they are radiation detectors with one “window” of an extremely thin material which allows alpha particles to pass through the thin window into a sensitive region of the device. For a device like that to sense something, it must be very close to the source of the alpha particles because – as previously mentioned – a few centimeters or a few inches of air is enough to stop the alpha particles entirely, beyond which point they cannot be measured. This kind of detector is generally very expensive and is intended for specialized professional uses, such as in a nuclear facility where radioactive materials are being handled and surfaces need to be checked for contamination. Furthermore, a device like that will not be well-suited to measuring radon concentration in air because the thin window used to allow alpha particles to pass through it does not allow air to flow inside.

To make a long story short: if you want to measure radon, purchase a dedicated radon sensor. Do not expect that device to be suitable for any other purpose, and do not expect any general purpose radiation detector to be suitable for measuring radon in air.

Beta particles travel further so they are easier to detect than alpha particles. Most radiation detectors are sensitive to beta particles. Like alphas, though, if the distance between the source of the radiation and the detector is too large, or if there is any material between them, the measurement is likely to have zero sensitivity to the source.

The previous paragraphs paint a concerning picture – alpha and beta particles are hard to measure at a distance - even if gamma particles are easier to sense. For a person who wants to be able to search for and locate sources of radiation, however, there is positive news. Beta and/or alpha radiation is almost always accompanied by gamma radiation. If a radioactive isotope gives off beta or alpha radiation, it almost always gives of gamma radiation at the same time. Pure beta emitters or pure alpha emitters are extremely rare and almost never encountered in ordinary circumstances. This means that if a person is searching for unknown radiation emitters, measuring gamma rays alone is generally enough.

Whether natural or unnatural, a gamma signature is nearly always present. The exceptions are typically specialized laboratory environments. This is even more true in the case of a nuclear accident or attack. If a nuclear reactor core melts down and radioactive material is released into the environment – a large variety alpha, beta, and gamma emitters will be all mixed together. Measuring gamma rays in that scenario therefore is a very clear indication of overall radiation levels in a given situation. In the case of a dirty bomb or nuclear weapon incident, the exact same is true.

What do the numbers on a radiation detector mean?

As describe in other sections, there are several types of radiation, and those types can also have different energies. What is important in terms of human health is what type of radiation interacts with your body, where it interacts, and how much energy it deposits. Organs vary in how sensitive to radiation they are, and some radiation types are more or less damaging than others. The overall story can get very complicated but most people ultimately boil a given radiation field measurement down to a single value: effective dose rate or just “dose rate” for short. This is a number which says how fast you are receiving radiation dose if your body is in that location, assuming your entire body is exposed uniformly to that field, wherever it is being measured.

Dose rate alone does not tell the entire story of risk. How long a person is exposed to a given dose rate is extremely important. The combination of dose rate and time exposed is what counts. Depending on the intensity of a radiation field, even if it is above normal a person can safely be exposed to it for a short period of time. The stronger the radiation field, the shorter a person can be safely exposed to it. At slightly elevated radiation levels, if a person is exposed long enough then long-term cancer risks can increase. In the rare case of extremely high dose rates, a person can receive life-threatening amounts of radiation in a short time. It can be thought of a bit like a car driving towards a cliff. If you drive fast, you can’t safely be in the car long. If you drive slowly, you have some time before you need to worry. This is not a perfect analogy, but this basic principle is extremely important. This can be considered, for example, in the common occurrence of an X-ray image at a doctor’s office. The X-ray beam is very powerful, but a person is exposed a very short amount of time – so the health concern is generally negligible.

Two basic units are commonly used to measure dose, the Seivert (Sv) and the rem. The rem is often the preferred unit in the United States, whereas the Sievert is the SI (International System of Units) standard unit and is typically preferred elsewhere. Conversion between the two is simple, 100 rem is equal to 1 Sv. To have a dose rate there must be a time component, usually “dose per hour” – like Sv/hr or rem/hr. If you were exposed to 0.5 Sv/hr for 3 hours, multiplying the two together means your total dose would be 1.5 Sv. This would be an extremely large dose. An ordinary background dose rate might be closer to 0.0000001 Sv/hr. This is not very convenient, so often a “micro” is added to make it one millionth of a Sv, meaning the previous number would become 0.1 μSv/hr. Similarly, instead of rem people often use one thousandth of a rem – one millirem or mrem - as the unit. The previous number of 0.1 μSv/hr would be 0.01 mrem/hr. To relate the two units it’s helpful to remember 10 μSv = 1 mrem.

Having established what a does and dose rate are, and what unit is used to describe it, the next logical question would be what is a “normal” dose rate and what is a “dangerous” dose rate. As previously mentioned, how long a person is exposed is very important, so defining something as “dangerous” does not depend on the dose rate alone. The annual exposure of a typical person can very significantly but a typical number would be roughly 3,000 μSv (=300 mRem). A person who is exposed to radiation as part of their job in the United States is restricted to 50,000 μSv (=5,000 mRem) per year total effective dose equivalent. However, the “ALARA” (as low as reasonably achievable) principle is always in effect, meaning people should minimize their exposure as much as they reasonable can. The vast majority of occupationally-exposed workers get far less dose per year than that. As a frame of reference, though, this can give a first impression of risk. To reach that total dose over the course of a year with continuous 24/7 exposure would require a dose rate of about 6 μSv/hr (far above ordinary background). To reach that level in about one month would suggest a dose rate of about 70 μSv/hr. To reach it in one day would suggest roughly 2,000 μSv/hr or 2 mSv/hr. Danger is hard to quantify, but this should give an impression of the ranges involved in terms of the normal units used.