READ: Radioactive Decay

READ: Radioactive Decay

Have you seen the symbol to the left?  Are you familiar with food irradiation? Food irradiation is a sensitive subject for many people. The practice involves exposing the food to ionizing radiation in order to kill harmful bacteria (such as salmonella) that cause sickness. The food is essentially unchanged and does not lose any nutritive value. Parasites and insect pests are easily destroyed by this process, while bacteria take longer to kill. Viruses are not affected by the radiation treatment. But don't worry - the food is not radioactive and you will not glow in the dark if you eat it.   If you would like to learn more about food irradiation, then click on the link in the sidebar and read through slides.  These slides were created by the UW Food Irradiation Education Group.

Unstable nuclei spontaneously emit radiation in the form of particles and energy. This generally changes the number of protons and/or neutrons in the nucleus, resulting in a more stable nucleus. A nuclear reaction is a reaction that affects the nucleus of an atom. One type of a nuclear reaction is radioactive decay, a reaction in which a nucleus spontaneously disintegrates into a slightly lighter nucleus, accompanied by the emission of particles, energy, or both. In this lesson, you are going to learn about three different types of decay that can occur.

Alpha Decay

Alpha (α) decay involves the release of helium nucleus from the nucleus of an isotope. An alpha particle (α) is a helium nucleus with two protons and two neutrons. The symbol for an alpha particle in a nuclear equation is  He.   Release of an alpha particle produces a new atom that has an atomic number two less than the original atom and an atomic mass that is four less. Alpha decay typically occurs for very heavy nuclei in which the nuclei are unstable due to large numbers of protons and neutrons. A typical alpha decay reaction is the conversion of uranium-238 to thorium:

 U  Th +  He

In the equation above, you see a decrease of two in the atomic number (92 to 90, which determines the element uranium to thorium) and a decrease of four in the atomic mass (238 to 234).  Note that in a balanced nuclear equation, the sum of the atomic numbers and the sum of the mass numbers must be equal on both sides of the equation. In these nuclear equation isotopic notation is used, which shows both the atomic number and mass number along with the chemical symbol as seen diagramed below:

Beta Decay

Some nuclei are unstable because their neutron to proton ratio is too high. To decrease that ratio, a neutron in the nucleus is capable of turning into a proton and an electron. Since electrons do not belong in the nucleus of an atom, this electron is immediately ejected at a high speed from the nucleus. A beta particle (β) is a high-speed electron emitted from the nucleus of an atom beta decay.The symbol for a beta particle in an equation is e.   In beta decay, the atomic number increases by one while the atomic mass stays the same since the mass of an electron is incredibly small.   A typical beta decay reaction involves carbon-14, often used in radioactive dating techniques. The nuclear reaction forms nitrogen-14 and an electron as seen in the equation below:

 C N +  e

Gamma Ray Emission

Gamma rays (γ) are very high energy electromagnetic waves emitted from a nucleus. Simply put, gamma radiation is high energy. Gamma decay is the process in which an excited atomic nucleus kicks out a photon of light and releases some of its energy. The makeup of the nucleus doesn't change, it just loses energy. There is no change of atomic number or atomic mass in a simple γ-emission. It can be useful to think of this as energy of motion - think of a shuddering nucleus that only relaxes after emitting some light. Below is a nuclear reaction equation for gamma emission:

 Ba*  Ba + y

Detecting Radiation

You generally can't see, smell, taste, hear, or feel radiation. Fortunately, there are devices such as Geiger counters that can detect radiation. A Geiger counter, like the one pictured below, contains atoms of a gas that is ionized if it encounters radiation. When this happens, the gas atoms change to ions that can carry an electric current. The current causes the Geiger counter to click. The faster the clicks occur, the higher the level of radiation.

Using Radiation

Despite its dangers, radioactivity has several uses. For example, it can be used to determine the ages of ancient rocks and fossils. It can also be used as a source of power to generate electricity. Radioactivity can even be used to diagnose and treat diseases, including cancer. Cancer cells grow rapidly and take up a lot of glucose for energy. Glucose containing radioactive elements can be given to patients. Cancer cells take up more of the glucose than normal cells do and give off radiation resulting in killing the cancer cells. For example, thyroid cancer can be treated with a radioisotope of iodine because under normal conditions the thyroid absorbs iodine. If the radioactive iodine is absorbed to a large enough dose it can kill the cancer cells in the thyroid.

Additionally, a home smoke detector uses a weak radioactive source that is strong enough to ionize air molecules but not strong enough to be harmful. The free electrons and positive ions created will create a closed circuit and allow a current of charges to flow in the smoke detectors. If smoke is present in the air, the ions will have an electrostatic attraction  to the smoke particles and will either stop the current or decrease the current through the smoke detector's circuit. Any change in the current in the smoke detector will trigger the smoke alarm.    

As you saw at the beginning of this lesson, a common practice now in the food industry is to radiate foods that spoil easily with gamma radiation to kill any bacteria that will begin the decomposition and decay process.  This increases the shelf life of the food and prevents it from spoiling too soon before or after you buy it.  Although the food does not become radioactive, this process is somewhat controversial as it has been found to change the molecular bonding of the food.

Radioactive tracers are radioactive molecules with short half-lives that can be detected in different areas after being released by a technician. Radioactive tracers are used industrially to determine if there are any leaks in pipes or to check flow rates and other plumbing problems in piping that may be hidden in walls or under ground. Detecting any rust or other corrosion problems early can avoid catastrophic problems due to pipes breaking or leaking, especially in gas or fuel lines. Chemists can also use tracers to follow a particular chemical reaction pathway or they can be used to help in identifying particular elements in compounds. Biologists and doctors can place radioactive tracers on food particles and follow them throughout the body and detect them later in parts of the body they end up in, or to locate cancer tumors that consume those particular food molecules.

Another use of radiation is called nitrogen activation. To help eliminate the threat of unwanted explosives on airplanes or other places that people gather, this process uses neutron activation that radioactivates the targets the target container or suitcase of interest. Because most explosives contain nitrogen, a gamma ray with specific amount of energy will be emitted only by nitrogen when bombarded with the activated neutrons. This specific energy can be detected and the bag can be searched further for explosives.

Summary and Review

The table below summarizes the main types of nuclear radiation, including charge, mass, symbol, and penetrating power. Penetrating power refers to the relative ability of the radiation to pass through common materials. Radiation with high penetrating power is potentially more dangerous because it can pass through skin and do cellular damage.

Practice filling in the decay equations below by placing the correct isotope in the blank in the equation. Also, identify each decay reaction as either alpha decay or beta decay.

Georgia Virtual, Nuclear Chemistry, CC BY-NC-SA 3.0

After you have completed this part of the lesson, you can check the associated box on the main course page to mark it as complete