Radioactivity and Nuclear Reactions Chapter 24 Radioactivity and Nuclear Reactions

24 - 1 Radioactivity Henri Bequerel accidentally discovered radioactivity when he left some uranium salt in a desk drawer with a photographic plate. The image left on the film he hypothesized was from the uranium giving off some “invisible energy” that exposed the film. Two years after Bequerel's discovery Marie Curie and her husband Pierre discovered the elements polonium and radium which are even more radioactive than uranium.

24 - 1 Radioactivity Remember that all elements are made of atoms and atoms are made of protons, neutrons and electrons. Protons and neutrons are held in the nucleus by what scientists call the strong force. This force is strong enough to prevent the protons from pushing each other out of the nucleus but the force only acts across very small distances. Therefore elements with high atomic numbers are held together less securely. Elements with atomic numbers of 84 or higher have nuclei that allow particles or energy to escape. This process is called radioactive decay.

24 - 1 Radioactivity Elements with atomic numbers of 84 or higher are unstable and therefore undergo radioactive decay. Radioactivity is the term used to describe the emission of high energy radiation or particles from the nucleus of radioactive atoms. Elements with atomic numbers 93-109 were created in labs. These synthetic elements are very unstable. Isotopes of some elements are radioactive as well. Recall an isotope of an element differs in the number neutrons in the nucleus. The nucleus of an isotope with a certain atomic number and mass is called a nuclide. (Carbon has 16 isotopes, Carbon14 or C14 is a nuclide)

24 – 2 Nuclear Decay How can you tell one isotope from another of the same element? Because they will have a different amount of neutrons. Remember the atomic number is the number of protons and the mass number is the number of protons plus neutrons.

24-2 Nuclear Decay There are three types of nuclear radiation: -alpha, beta and gamma radiation Radiation in the form of alpha particles is given off when a nucleus releases two protons and two neutrons. Symbolized “α” alpha particle radiation is the largest and slowest form of radiation. It is the least penetrative. These particles can be stopped by a sheet of paper. (used in some smoke detectors) Beta radiation occurs when a neutron decays into a proton by emitting an electron at high speed. Beta particles symbolized “β” are much faster and more penetrating than alpha but can be stopped by a sheet of aluminum foil.

24 - 2 Nuclear Decay An atom that loses an alpha or a beta particle undergoes transmutation. Transmutation is the process of changing one element to another through nuclear decay. The most penetrating and potentially dangerous form of radiation is not made of particles at all. Gamma rays are electromagnetic waves with very high frequency and energy. They have no mass, no charge and they travel at the speed of light. They are usually released along with alpha or beta particles. Gamma rays can be stopped by lead and concrete.

24 - 2 Nuclear Decay All radioactive isotopes or radioisotopes decay or “give off” radiation until they become stable nuclides. The time it takes for some to decay is mere seconds for others it may take millions of years. A measure of the time required by the nuclides of an isotope to decay is called half-life. The half-life of an isotope is the amount of time it takes for half of the nuclides in a sample to decay. Half-lives vary widely.

24 - 2 Nuclear Decay Our bodies contain a constant amount of radioactive carbon or carbon-14. This isotope is in some of the carbon dioxide molecules plants take in. As a result, you have C- 14 inside of you from the plants and animals you eat. C-14 emits a beta particle and decays into nitrogen. All living things contain a somewhat constant amount of C-14 because it is replaced in living organisms but when you die the C-14 decays without replacement. C-14 has a half-life of 5,730 years. By measuring the % of C-14 remaining in a fossil or skeleton, scientists can determine the approximate age of the material. This process is called C-14 dating.

24 – 3 Detecting Radioactivity One method of detecting radiation is called a cloud chamber. A cloud chamber can be used to detect charged nuclear particles as they leave cloud tracks. Beta particles leave long, thin trails and alpha particles leave shorter and thicker trails. Another method is a bubble chamber. A bubble chamber holds a superheated liquid, which doesn’t boil because of increased pressure. When a moving particle leaves ions behind, the liquid boils along the trail. The pat shows up as tracks of bubbles. A simple method of counting radioactivity is to use a Geiger counter which is a device that produces an electric current when radiation is present.

24 - 4 Nuclear Reactions Scientists were experimenting with larger atoms by shooting neutrons at their nucleus. What happened was the unstable nucleus split into two nuclei of nearly equal mass. The process of splitting a nucleus into two nuclei with smaller masses is called nuclear fission. When a nucleus is split, neutrons break free of the nucleus and bombard other nuclei which creates a chain reaction unless another material is present to absorb the extra neutrons. A chain reaction is an ongoing series of fission reactions. This can cause billions of reactions to occur in each second, resulting in the release of tremendous amounts of energy.

24 – 4 Nuclear Reactions Splitting one uranium-235 nucleus produces several million times more energy than exploding one molecule of dynamite. Nuclear fusion releases even more energy than nuclear fission. Nuclear fusion is the combining of two nuclei with low masses to form one nucleus of larger mass. This is difficult to do and requires extremely high temperatures (like in the sun and stars.) When the sun first formed, most of its nuclei were hydrogen nuclei. As thermonuclear fusion occurs, four hydrogen nuclei fused to become one helium nucleus.

24 - 5 Nuclear Medicine Radioactive isotopes can help locate and even kill cancerous tumors in the body. This field of medicine is called nuclear medicine and involves using radioactive isotopes in the diagnosis and treatment of many medical problems. Radioisotopes that are put in the body to monitor a bodily process are called tracers. Ex: diagnosing thyroid problems using a radioactive isotope of iodine in order to help locate cancerous cells. Cancerous cells reproduce very rapidly so more of the radioactive isotope gathers in these cells. The exact location of the tumor can be determined by using tracers, thus eliminating the need for risky exploratory surgery.

24 - 5 Nuclear Medicine Tracers can help diagnose cancer but the problem of treating the cancer remains. Radiation can be used to treat cancer not just to diagnose. A beam of radiation specific to the type of cancer is aimed at the cancerous spot to destroy the cancerous cells. This treatment can shrink or even eliminate some cancerous tumors. Even though using radiation in medicine has some risks, the risks are often smaller than the possible complications of surgery and are often more effective in preventing the disease from reoccurring.

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