Radioactive Decay

1. Radioactive Decay

2. Generally, there are four main concepts that students struggle with when thinking about radioactive decay: 1. the spontaneity (or randomness) of radioactive decay, 2. the reason isotopes are important, 3. the concept of half-life, and 4. knowing which system is appropriate Radioactivity: A steady but unpredictable (spontaneous) process Radioactivity and radioactive decay are spontaneous processes. Students often struggle with this concept; therefore, it should be stressed that it is impossible to know exactly when each of the radioactive elements in a rock will decay. Statistical probability is the only thing we can know exactly. Often students get bogged down in the fact that they don't "understand" how and why radioactive elements decay and miss the whole point of this exercise. If they can begin to comprehend that it is random and spontaneous, they end up feeling less nervous about the whole thing.

3. Radioactive decay involves the spontaneous transformation of one element into another. The only way that this can happen is by changing the number of protons in the nucleus (an element is defined by its number of protons). There are a number of ways that this can happen and when it does, the atom is forever changed. There is no going back -- the process is irreversible. This is very much like popping popcorn. When we pour our popcorn kernels into a popcorn popper, the is no way to know which will pop first. And once that first kernel pops, it will never be a kernel again...it is forever changed! (And coincidentally, much yummier!)

4. Isotopes: same element, different atomic mass Often students struggle with the concept of isotopes. The Oxford English Dictionary defines an isotope as: "A variety of a chemical element (strictly, of one particular element) which is distinguished from the other varieties of the element by a different mass number but shares the same atomic number and chemical properties (and so occupies the same position in the periodic table)." That definition may not mean anything to them. They may ask, "What's the difference between an isotope and an atom?" Another way of explaining it is that when geologists talk about isotopes, they are talking about one element of differing masses. Isotopes of an element are atoms that all have the same atomic number (or number of protons in the nucleus) but have different atomic masses (hence different numbers of neutrons in the nucleus). For example, all atoms of oxygen have 8 protons in the nucleus and hence have an atomic number of 8. However, oxygen atoms can have between 8 and 10 neutrons in the nucleus and therefore the isotopes of oxygen have atomic masses of 16, 17, and 18 a.m.u.(and none are radioactive!). Samarium (Sm) has 7 naturally occurring isotopes (3 are radioactive). Remind them that geologists only use certain radioactive isotopes to date rocks.

5. The atoms that are involved in radioactive decay are called isotopes. In reality, every atom is an isotope of one element or another. However, we generally refer to isotopes of a particular element (e.g., Rubidium-87 (87Rb) or Lead-206 (206Pb)). The number associated with an isotope is its atomic mass (i.e., protons plus neutrons). The element itself is defined by the atomic number (i.e., the number of protons). Only certain isotopes are radioactive and not all radioactive isotopes are appropriate for geological applications -- we have to choose wisely. Those that decay are called radioactive (or parent) isotopes; those that are generated by decay are called radiogenic (or daughter) isotopes. The unit that we use to measure time is called half-life and it has to do with the time it takes for half of the radioactive isotopes to decay (see below). Half-life: a useful way of telling geologic time Half-life is a very important and relatively difficult concept for students. Mathematically, the half-life can be represented by an exponential function, a concept with which entry-level students may not have much experience and therefore may have little intuition about it. I find that entry-level students in my courses get stuck on the term "half-life". Even if they have been given the definition, they interpret the term to mean one-half the life of the system. Instead, it is really the lifetime of half of the isotopes present in the system at any given time. Marie and Pierre Curie.

6. Problem solving in the geosciences was forever changed with the discovery of radioactivity. Radioactive elements can be used to understand numerical age of geological materials on time scales as long as (and even longer than) the age of the Earth. In order to determine the age of a geologic material, we must understand the concept of half-life. Half-life is a term that describes time. The definition is: The time required for one-half of the radioactive (parent) isotopes in a sample to decay to radiogenic (daughter) isotopes. --modified from Webster's Third International Dictionary, Unabridged In otherwords, it is the lifetime of half the radioactive isotopes in a system. The units of half-life are always time (seconds, minutes, years, etc.). If we know the half-life of an isotope (and we can measure it with special equipment), we can use the number of radiogenic isotopes that have been generated in a rock since its formation to determine the age of formation. Radiometric dating is the method of obtaining a rock's age by measuring the relative abundance of radioactive and radiogenic isotopes.

7. So many systems, how do we choose? Most students don't really know how isotopes are used to determine age. In particular, they have a hard time understanding that different systems are appropriate for different types of radiometric dating and why. There are several important points that can be emphasized to help avoid confusion when talking about the various systems: Geologists have a plethora of choices for calculating the age of a rock using big and complicated systems. Check out this table of isotope systems and half-lives (Excel 18kB Jun24 04); all of these are used to date rocks or sediment! With all these systems, how do we choose? Geologists use a number of criteria to decide which of the systems to use: showShow caption hideHide Zircon, which is a useful mineral for U-Pb dating. Details 1. Is the half-life of the system appropriate for the rock that you are trying to date? Most geologists have an idea of the age of a rock (if age is less than 6 half-lives, it'll work). 2. Does the rock have minerals that can be used for the isotope system you want to use? You need to have minerals in your rock that contain the element(s) you want to use. 3. What event do you want to date? Some systems are very good for dating igneous events, others are very good for dating metamorphic events. (Remember, it is impossible to date sedimentary minerals because they eroded from some igneous or metamorphic rock.) Together, the answers to those questions help geologists decide which system they should use.

8. What about Carbon-14? Most students have heard of Carbon-14; yet, it doesn't appear in the table of isotopes used to date rocks and minerals. Why not? Carbon-14 is not appropriate for rocks because it must involve organic carbon. Rocks are made of minerals that are by definition inorganic. The discussion of 14C below is a great way to illustrate important points of how to choose a system. Carbon-14 is special for two reasons. 1. With 14C, we can only calculate the age of something that was once living (contains organic carbon). Since (most) rocks were never alive, we can't use this to date a rock. 2. The half life of 14C is geologically short -- 5730 years -- and is therefore not useful for materials older than about 35,000 years. That's well over 4 billion years of geologic history that we can't touch. So, what geologists and archaeologists date when they use 14C is the death of an organic lifeform. Most geologists want to know the age of crystallization or metamorphism of rocks that are millions or billions of years old -- 14C won't work for that.

9. The exact nature of the radiation emitted from the disintegrating elements remained a mystery until a series of papers by Ernest Rutherford in 1899 and Paul Villard in 1900. After determining that the radiation emitted from uranium was composed of two different components, Rutherford unsuccessfully attempted to separate them using prisms of glass, aluminum and paraffin wax. Eventually, using two oppositely charged plates, he identified the components as positive particles (alpha particles) and lighter mass negative particles (beta particles). Villard identified a third primary type of radioactivity, gamma rays, from a radium sample. Gamma rays have no mass and possess no charge. The behavior of the three types of particles as they pass through the electric field between two charged plates is shown below.

10. The are at least two important points to notice: 1. The positive particles are bent toward the negative plate, the negative particles are bent toward the positive plate and the neutral particles are not bent in either direction. 2. The extent to which the path of a particle is bent as it passes through an electric field depends on its mass and its charge * the larger the charge on the particle, the further it is bent. * the larger the mass of the particle, the less it is bent.

11. Three Types of Radioactive Decay There are three main types of radiation: * Alpha radiation * Beta radiation * Gamma radiation

12. Alpha Decay The reason alpha decay occurs is because the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a Helium nucleus is emitted. The way it works is that the Helium nuclei are in constant collision with the walls of the nucleus and because of its energy and mass, there exists a nonzero probability of transmission. That is, an alpha particle (Helium nucleus) will tunnel out of the nucleus. Here is an example of alpha emission with americium-241:

13. Beta Decay Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted. Here's a diagram of beta decay with hydrogen-3:

14. There is also positron emission when the neutron to proton ratio is too small. A proton turns into a neutron and a positron and the postiron is emitted. A positron is basically a positively charged electron. Here's a diagram of positron emission with carbon-11:

15. The final type of beta decay is known as electron capture and also occurs when the neutron to proton ratio in the nucleus is too small. The nucleus captures an electron which basically turns a proton into a neutron. Here's a diagram of electron capture with beryllium-7:

16. Gamma Decay Gamma decay occurs because the nucleus is at too high an energy. The nucleus falls down to a lower energy state and, in the process, emits a high energy photon known as a gamma particle. Here's a diagram of gamma decay with helium-3:

17. What is meant by the "decay" of a radionuclide? Remember that a radionuclide represents an element with a particular combination of protons and neutrons (nucleons) in the nucleus of the atom. A radionuclide has an unstable combination of nucleons and emits radiation in the process of regaining stability. Reaching stability involves the process of radioactive decay. A decay, also known as a disintegration of a radioactive nuclide, entails a change from an unstable combination of neutrons and protons in the nucleus to a stable (or more stable) combination. The type of decay determines whether the ratio of neutrons to protons will increase or decrease to reach a more stable configuration. It also determines the type of radiation emitted.

18. As mentioned previously, radioactive decay is the disintegration of an unstable atom with an accompanying emission of radiation. As a radioisotope atom decays to a more stable atom, it emits radiation only once. To change from an unstable atom to a completely stable atom may require several disintegration steps and radiation will be given off at each step. However, once the atom reaches a stable configuration, no more radiation is given off. For this reason, radioactive sources become weaker with time. As more and more unstable atoms become stable atoms, less radiation is produced and eventually the material will become non-radioactive. The decay of radioactive elements occurs at a fixed rate. The half-life of a radioisotope is the time required for one half of the amount of unstable material to degrade into a more stable material. For example, a source will have an intensity of 100% when new. At one half-life, its intensity will be cut to 50% of the original intensity. At two half-lives, it will have an intensity of 25% of a new source. After ten half-lives, less than one-thousandth of the original activity will remain. Although the half-life pattern is the same for every radioisotope, the length of a half-life is different. For example, Co-60 has a half-life of about 5 years while Ir-192 has a half-life of about 74 days.

19. Nuclear Fission An atom's nucleus can be split apart. When this is done, a tremendous amount of energy is released. The energy is both heat and light energy. Einstein said that a very small amount of matter contains a very LARGE amount of energy. [Drawing of fuel rod assembly] This energy, when let out slowly, can be harnessed to generate electricity. When it is let out all at once, it can make a tremendous explosion in an atomic bomb. A nuclear power plant (like Diablo Canyon Nuclear Plant shown on the right) uses uranium as a "fuel." Uranium is an element that is dug out of the ground many places around the world. It is processed into tiny pellets that are loaded into very long rods that are put into the power plant's reactor. The word fission means to split apart. Inside the reactor of an atomic power plant, uranium atoms are split apart in a controlled chain reaction. In a chain reaction, particles released by the splitting of the atom go off and strike other uranium atoms splitting those. Those particles given off split still other atoms in a chain reaction. In nuclear power plants, control rods are used to keep the splitting regulated so it doesn't go too fast. If the reaction is not controlled, you could have an atomic bomb. But in atomic bombs, almost pure pieces of the element Uranium-235 or Plutonium, of a precise mass and shape, must be brought together and held together, with great force. These conditions are not present in a nuclear reactor. The reaction also creates radioactive material. This material could hurt people if released, so it is kept in a solid form. The very strong concrete dome in the picture is designed to keep this material inside if an accident happens. The absorption of a neutron by 238U induces oscillations in the nucleus that deform it until it splits into fragments the way a drop of liquid might break into smaller droplets

20. This chain reaction gives off heat energy. This heat energy is used to boil water in the core of the reactor. So, instead of burning a fuel, nuclear power plants use the chain reaction of atoms splitting to change the energy of atoms into heat energy. This water from around the nuclear core is sent to another section of the power plant. Here, in the heat exchanger, it heats another set of pipes filled with water to make steam. The steam in this second set of pipes turns a turbine to generate electricity. Below is a cross section of the inside of a typical nuclear power plant.

21. The first artificial nuclear reactor was built by Enrico Fermi and co-workers beneath the University of Chicago's football stadium and brought on line on December 2, 1942. This reactor, which produced several kilowatts of power, consisted of a pile of graphite blocks weighing 385 tons stacked in layers around a cubical array of 40 tons of uranium metal and uranium oxide. Spontaneous fission of 238U or 235U in this reactor produced a very small number of neutrons. But enough uranium was present so that one of these neutrons induced the fission of a 235U nucleus, thereby releasing an average of 2.5 neutrons, which catalyzed the fission of additional 235U nuclei in a chain reaction, as shown in the figure below. The amount of fissionable material necessary for the chain reaction to sustain itself is called the critical mass.

22. Nuclear Fusion [ Nuclear fusion drawing ] Another form of nuclear energy is called fusion. Fusion means joining smaller nuclei (the plural of nucleus) to make a larger nucleus. The sun uses nuclear fusion of hydrogen atoms into helium atoms. This gives off heat and light and other radiation. In the picture to the right, two types of hydrogen atoms, deuterium and tritium, combine to make a helium atom and an extra particle called a neutron. Also given off in this fusion reaction is energy! Thanks to the University of California, Berkeley for the picture. Scientists have been working on controlling nuclear fusion for a long time, trying to make a fusion reactor to produce electricity. But they have been having trouble learning how to control the reaction in a contained space. What's better about nuclear fusion is that it creates less radioactive material than fission, and its supply of fuel can last longer than the sun.