What is Radiation?
Everything we interact with in our world is made of matter and energy. Some matter, like mercury, is toxic, while other types, like gold, are totally harmless. The types of matter we can interact with most of the time are atoms, like many are taught about in grade school. Atoms have one or more electrons orbiting them. These electrons carry a negative electrical charge and are responsible for much of our electronic technology, as well as magnets, static “cling” holding your paints against your skin, and lighting. The electrons orbit the central “nucleus” of the atom. This force, which holds electrons in orbit around the nucleus, is called the electrostatic force. This nucleus is composed of one or more protons and one or more neutrons. There are examples of nuclei (plural for nucleus) were there are no neutrons at all, but most of the time there are both. On such example is simple hydrogen, which has only a single proton orbited by a single electron.
The nucleus of an atom contains only protons and neutrons. Protons carry a positive electrical charge and attract the electrons, holding them in orbit. The neutrons effectively carry no electrical charge and are held tightly to the protons by another form of energy, the Strong Nuclear Force. Our world is created from elements, each with different chemical properties. For example, mercury is a heavy metal which is liquid a room temperature. Next to mercury on the periodic table of elements is gold. In fact, gold is almost the same as mercury, the only difference being that mercury has one proton more than gold. This single extra proton significantly changes the nature of gold.
Changes in the nature of atoms have been studied and given different names, each producing different results. If an atom loses some of its electrons, it is said to be “ionized”. The atom remains the same, but may be attracted to an electrical field as a result. If the nucleus of an atom changes, however, the resulting atom is different from the original atom. If you recall, an atom’s nucleus is composed of neutrons and protons. Neutrons add mass (sort of like weight) to the atom, while the protons change the nature of the atom. While a change of protons significantly changes the chemical properties of an atom, a change in the number of neutrons does not. To explain this more easily, recall mercury and gold. The removal of one single proton changes an atom of mercury into an atom of gold. This process is not easy (alchemists have sought a way to make gold for centuries). The strong nuclear force hold the nucleus tightly together.
All atoms of the same element have the same number of protons. Uranium, for example, always has 92 protons. The number of neutrons in an element can be changed, however, without changing into another element. These variations of each element, equal in protons but not equal in neutrons, are called Isotopes. To keep isotopes separated from each other, scientists often add the number of protons and the number of neutrons to the end of their name. For example, most natural uranium contains 92 protons and 146 neutrons, per nucleus. 92 + 146 = 238. Thus, we would call this variation, this isotope, of uranium: Uranium 238. There are other versions as well, such as Uranium 235. Uranium 235 has 92 protons, like any uranium atom, but only 143 neutrons. When an atom loses or gains a proton, it changes to another atom. When an atom gains or loses a neutron, it changes into another isotope of the same atoms. These are fundamental characteristics of our world.
Radioactive Decay and Radiation
The various configurations of protons and neutrons, which make up the nucleus of an atom, cause various effects in the way each atom behaves. Many of these interactions are quite complex, but in general some atoms are stable and some are not. Stable atoms have a nucleus which remains the same, indefinitely. They may gain or lose electrons, but their nucleus does not change. An unstable atom exists where the nucleus does not remain the same over time. Analogy: Think of a waiter at a restaurant. The waiter is the strong nuclear force. A waiter holding a few plates may be stable, but a waiter with plate in the wrong position may start to wobble around, trying not the fall. The plates are like the protons and neutrons. Depending on the configuration of the plates, the waiter might wobble for a short or long time before dropping the plates. Often, the waiter will drop only a few plates and become stable again. The waiter with plates is like a nucleus in this way. An unstable nucleus must find a way to become stable and it often does this my losing a few protons or neutrons, and sometimes by even changing a neutron into a proton!
When an atom changes the state of its nucleus and loses one or more neutrons or protons in order to become more stable, it is said to have “decayed”. Decay results in several forms of radiation, each of these changing the nucleus in some fashion. Sometimes an atom can even “catch” a free roaming neutron and break into two parts, fission, releasing energy:
Primary Decay – Primary decay occurs when an unstable atom discharges particles from its nucleus to become stable. The result is not always stable, unfortunately, and subsequent decays may be required.
Alpha Decay – Two protons and two neutrons are expelled at a very high rate of speed. They move the emitting atom “up” the periodic table of elements, by two positions, towards the lighter elements. Alpha radiation is very deadly inside of your body, but nearly harmless outside. This is because, having two protons, the alpha particle is so very positively charged that natural electromagnetic fields and particles around it slow alpha particles to a halt within a few inches. Even a sheet of paper can block alpha particles.
Beta Decay – The most common trigger for beta decay occurs when a neutron changes into a proton, though other forms of this decay do occur. Since a neutron is more massive than a proton, some energy is left over. This energy turns into a beta particle and quickly fly’s off. Beta’s come in two forms: positive electrical charge, called a positron, and negative electrical charge, called an electron. In the case of negative charged beta decay, called beta minus, the negative beta (a high energy electron) is not the same as the electrons around the atom (low energy electrons). When an atom undergoes beta decay, it moves “up” the periodic table of elements, becoming a higher element, but a lesser nuclide. This change in nuclide position is the result of the loss of a neutron (recall that changes in neutrons change the nuclide, not the element). The change in element is due to the addition of a proton (recall that changes in protons change the element). Both beta’s leave from the atom at a high rate of speed and can move many feet in the air before slowing down. Beta radiation cannot typically penetrate very deeply into the body, but can harm the skin and the eyes, or if ingested.
Secondary Emissions – After a primary decay, an atom is often left in an excited state and must release energy to “relax”. This energy can come from excited electrons or an excited nucleus. While both relaxing processes produce a high energy photon, similar to normal light be orders of magnitude more powerful, their origins are different and their names reflect these differences.
Gamma Rays – A high energy photon emitted by an atom’s nucleus is called a gamma ray. This photon, the same particle which causes light, radio waves, and other forms of electromagnetic radiation, can be dangerous because of how much energy is possesses. A single gamma carries many thousands of times the energy of normal light and can move through normal matter, only rarely hitting anything. Gamma rays require thick lead or equivalent shielding to block them. The emission of a gamma ray does not change the atom into anything else, it merely relaxes the atom. Therefore, gamma rays, though very important to nuclear physics, are not a form of decay. Gamma rays are the result of decay.
X-Rays – An X-ray is a high energy photon, just like a gamma ray, but they are emitted from electrons orbiting the atom. These electrons are not involved in the primary decay, but as a result of primary decay they are left excited and must “calm down” by emitting an x-ray photon. X-rays are often much lower in energy than gamma rays, though the only technical difference between an X-ray and a gamma ray is their origin. X-rays are also the result of decay.
Fissile Emissions – When an atom absorbs a neutron it can become unstable. If an atom becomes extremely unstable, the atom can split into two pieces. Each piece is a new atom. The division of these atoms is not often uniform. Normally, the asymmetric (meaning, not equal) split results in two atoms of differing sizes, as well as a few free neutrons and other radiation. Nuclear reactors and weapons exploit this property of atoms. Isotopes of atoms which have an affinity for capturing neutrons are chosen and great quantities of these atoms are bombarded by neutrons, causing them to become unstable and split. Two great examples of such nuclides are Uranium 235 and Plutonium 239. Each of these two elements readily accepts a low energy neutron, called a “thermal neutron”, and becomes unstable, splitting into two lesser atoms, called fission products, and emitting a few extra neutrons, which can go forth to cause additional fissions, as a chain reaction. The resulting atoms from a fission, the fission products, are sometimes stable and sometimes not. Sometimes, these fission products can be bombarded by neutrons from other nearby fissions and made unstable! As an example, cesium 133 is a stable fission product. A nearby fission could force a neutron into cesium 133 and change its isotope number to cesium 134, an unstable isotope! It is these unstable fission products which decay rapidly, generating heat, and requiring spent fuel rods form nuclear reactors to be cooled for many years.
As a nuclide decays it changes from one element to another and one isotope to another. This second nuclide is known as the “daughter” nuclide, or the progeny. It is sometimes said to be, “progenic”, meaning that it comes from something else. A child is progenic of its parents, and we often ask if a chicken is progenic of an egg, or if an egg is progenic of a chicken. When isotope A decays into isotope B, the resulting isotope B may additionally decay into further generations of subsequent isotopes, C, D, … , X, Y, Z, etc. Each one of these isotopes may decay at a differing rate. The change in the quantity of an isotope as time changes is called the rate of decay. Each isotope has a different rate of decay. Some rates of decay require seconds while other may require eons. When many groups of isotopes decay, one after the other, in a series, this is called a decay chain or decay series. The decay chain is often labeled by its first member, thus all elements which decay from isotope A, such as B and C, would be referred to as members of the A decay series.
Scientists often concern themselves with knowing the amount of time it takes for half of a given quantity of an isotope to decay into another isotope. This is because decay, being somewhat random, is determined statistically rather than directly. If you know the average rate at which isotope A changes to isotope B, you can determine how long it will take to change half of A to B, which is called the half-life of A. A half-life is inversely related to the radioactivity of a sample. The shorter the half-life of an isotope, the faster it must be decaying. Since each decay results in the release of energy, as radiation, the shorter the half-life, the more radiation released. Example: Uranium 238 has a half-life of over four billion years! This is why the uranium 238, found naturally in most soil, is not considered especially dangerous in small quantities. Conversely, polonium 210 has a half-life of 138 days and is considered very dangerous. One gram of polonium decays about 166 trillion times per second, while one gram of the long-lived uranium 238 decays at a much slower rate of around 12 thousand decays per second.
Naturally Occurring Radioactive Material
Many Natural Occurring Radioactive Materials, often referred to as NORM, exist in our environment. These naturally radioactive materials are either primordial (they have existed since before the Earth) or are created from natural sources. The two major natural sources for the creation of natural radioactive material are as a result of decay from primordial elements, or as a result of high energy cosmic rays, from space, interacting with our atmosphere. Uranium 238 and Thorium 232 are primordial elements, meaning that they were created before the Earth. Each of these elements creates a long decay series with many members. Much of the Earth is covered with trace amounts of these long-lived isotopes and their progeny. Radon, a nasty gas which may exist in basements and low lying ground, actually comes from the decay of uranium 238 and is responsible for the radioactivity detected in freshly fallen rain, though this radioactivity quickly becomes undetectable. Below is the uranium 238 decay series. Some decays can actually have more than one possible resulting isotope, but only the most common have been listed below.
The Uranium 238 Decay Series
Some rare possible decays omitted and some simplification provided for ease of understanding and brevity.
* Gamma energies provided for only the most abundant gamma energies. Many more may exist, but are not included for brevity. Energies are provided in thousands of electron volts (keV) and are accompanied by their percentage of emission. E.g, 100 keV (0.80) means that a 100 thousand electron volt gamma is emitted for approximately 80% of the decays of this isotope.
Some unstable isotopes are actually created new from cosmic rays slamming into our planet’s atmosphere. When a powerful high-energy particle from space impacts our atmosphere, the particle may deflect off of otherwise stable atoms with so much energy that it causes pieces of those atoms to literally be torn off. Tritium, the third isotope of hydrogen, is created in this way. Tritium is radioactive and short lived, but quite useful for self-lighting devices and nuclear fusion (combining atoms to cause energy, the opposite of fission). The primary isotopes associated with NORM are Uranium 238 and progeny, Uranium 235 and progeny, Thorium 232 and progeny, and potassium 40 (a radioactive isotope of normal potassium which occurring with all stable potassium, in trace amounts).
Detecting radiation and Identifying Nuclides
Various detectors exist for the detection and identification of radioactive isotopes and their resulting radiation. The ability to detect and even identify some of these isotopes has existed for over 100 years. In days of old, scientists would burn the samples and observe the visible colors of light, their visible spectrum, they produced. After a time, the ability of nuclear radiation to ionize gases and complete an electrical circuit was discovered allowing for the creation of Geiger counters and ionization detectors.
Geiger Mueller Detectors
A Geiger counter is a tool which allows a person to detect the presence of nuclear radiation. In the Geiger counter, a low pressure gas tube contains a central wire with a powerful current applied to it. The inner walls of the tube, not connected to the wire, have an equally powerful opposite electrical current. If the wire could touch the wall of the tube, the circuit would complete and a count would be recorded (you might hear a single tick sound). When radiation enters the tube and impacts one of the atoms of gas in the tube, electrons are liberated from their orbits around the gas atom. The liberated electrons quickly start moving towards the casing of the tube, building energy and velocity as they go. These electrons are being accelerated by the electrical field produced between the central wire and the casing. They will knock free other electrons on their way, building a cloud of electrons, which all impact the wall of the Geiger counter as one and produce a short pulse of current. This pulse of current is recorded as a count. This is how a Geiger counter works. Geiger counters can detect the presence of nuclear radiation, but they cannot determine how much energy a single radioactive particle has. A very low energy particle and a very high energy particle will both produce a single pulse of current. Geiger counters are especially good at detecting beta radiation, due to the negative or positive charge of the beta (electron or positron).
When an atom decays it is often left in an excited state which requires the emission photons as x-rays and gamma rays to relax. These photons are emitted at very specific energies for each type of isotope. The ability to determine the energy of a single particle of radiation allows scientists to determine the origin of the particle, like a finger print. To do this, an energy spectrometer is used. Spectrometers produce a spectrum of energy identifying how much energy was detected from a certain event. Alpha, beta, gamma, and x-ray spectrometers exist to perform these functions. The most common method of isotope detection relies on a gamma spectrometer. Gamma spectrometers detect energies from gamma rays and record them, grouped by energy. A gamma spectrometrist can interpret these energies to determine the isotopes which emitted them. In the above uranium 238 decay series, the associated energies are provided for common gamma rays emitted by or as a result of each member of the uranium decay series. When testing a sample of soil, for example, energies from two or three of the members of the uranium 238 decay series provide an excellent finger print for the presence of the entire series. Other common isotopes are easily detected, such as cesium 137 and iodine 131.
A Practical Example of What Was Learned Above
Cesium 137 is an unstable isotope of cesium which is commonly found in the area of nuclear disasters such as Fukushima or Chernobyl. Cesium 137 has a half-life of 30.1 years, which means that a sample which decays at a rate of 100 decays per second will only decay by 50 decays per second in 30.1 years’ time, slowly decreasing as time moves on. In another 30.1 years (60.2 years since we first obtained our sample) only 25 decays per second will occur. This continues to halve every 30.1 years until it is gone. Cesium 137 decays by beta minus decay. One of the neutrons of the cesium 137’s nucleus changes into a proton, moving the atom down the periodic table of elements to barium 137, and emitting an electron-like beta particle at high energy. A Geiger counter may detect this beta, and any resulting gammas or x-rays. This will tell the Geiger counter operator that radiation exists in their immediate vicinity.
Sometimes the beta carries away most of the energy from the decay, but sometimes it does not. About 85% of the time the newly formed barium 137 atom will be “excited” and in a meta stable state (meta stable means that the atom is “stable” but still full of energy which needs to be released as a photon). The newly formed barium 137, if left excited, will relax by emitting a single gamma ray from its nucleus with a specific energy of 661.66 keV (thousand electron volts), which might be detected by a gamma spectrometer and used to identify that specific isotope. The electrons around the atom are also quite excited from the decay and on occasion some of the electrons will change their orbits to lower orbits and emit x-rays, often at 32 keV (thousand electron volts). A gamma or x-ray spectrometer may detect these photons and identify that specific isotope.