Lesson 28: Radioactive Decay Radioactive decay is a process in which an unstable nucleus undergoes change by emitting radiation. Radiation is a general term for anything that can be radiated out of the nucleus. Radiation can come in the form of particle ( ) or energy ( ) emission. Radioactivity was discovered by accident by Henri Becquerel in 896. Becquerel placed a rock containing uranium on a piece of photographic paper inside a dark drawer. He found that the paper developed an image as if it has been exposed to a strong light source. Common types of radiation Antimatter: Beta negative particle Beta positive particle alpha particle neutrino antineutrino gamma photon
Nuclear Equations Nuclear equations are written like chemical equation, with the original material on one side of the equation and the resulting material on the other. The two sides are separated by an arrow. Conservation laws and nuclear equations: Conservation of nucleons Conservation of charge Conservation of antiness Example: Common types of radioactive decay Alpha Decay: Example:
Beta Negative Decay: Beta Positive Decay: Gamma Radiation: Example: Example: Example:
The energy released in radioactive decay If you find the total mass of the parent isotopes for a nuclear decay and compare it to the total mass of the daughter isotopes, there will often be some missing mass. This mass has been converted into energy, which is released in the kinetic energy of the alpha and beta particles as well as gamma radiation. Example: Calculate the energy released during the alpha decay of - Mass of thorium-230 is 230.03334 u - Mass of radium-226 is 226.02540 u - Mass of alpha particle is 4.002603 u Radiation Hazards Ionizing radiation consists of particles that have sufficient energy to knock an electron out of an atom or molecule, thus forming an ion. A single alpha or beta particle has enough energy to ionize thousands of molecules. Nuclear radiation is potentially harmful to humans because the ionization it produces can significantly alter the structure of molecules with in a living cell. Risks of radiation exposure: - - - - - 230 Th 90 Type of Radiation Mass Speed Penetrating Power Ionization Alpha Beta Gamma
Radioactive Decay rates A stable nucleus is one that is much less likely to undergo radioactive decay. In order for a nucleus to be stable, there must be a balance between the number of protons and the number of neutrons in the nucleus. Different isotopes decay at different rates. Half life: Example: Carbon-4 Uranium-238 Uranium-235
Decay Curve: Formula: Example: Polonium-20 has a half life of 38 days. After 3.8 years, how much of a 765 g sample of polonium-20 is left? Example: The half-life of a radioactive isotope is 6.8 years. If the activity of the original sample of this isotope was 4.9 x 0 5 Bq, what would its activity be after 00 years? Example: You have 75 g of lead at the start of an experiment. How many half lives have passed when 9.3 g remains?
Practice Problems: Remember u =.660539 x 0-27 kg. Write the alpha-decay process for. 2. Which type of beta decay transmutes carbon-4 into nitrogen? Write the process for this decay. 3. Which type of beta decay transmutes the sodium isotope into? Write the process for this decay. 4. Explain whether the atomic number can increase during nuclear decay. Support your answer with an example. 5. What isotope will β + decay of thallium-202 produce? Write the process for this decay. 6. Following are some equations of artificial transmutations produced by particle bombardment. Using a periodic table, if necessary, determine the other product in each case. a. 27 4 3 Al + 2He b. 2 2 6 C + c. 9 2 4 Be + d. 4 4 7 N + 0n 6C e. + 0n? f. 23 2 Na + g. 4 4 7 N + 2He 7. In each of the following equations, determine the emitted particle(s) in each case. a. 222 28 4 4 86 Rn 84Po e. 57 La 58Ce b. 238 234 38 38 92 U 90Th f. 60Nd 59Pr c. 34 35 22 22 7Cl 6S g. 82 Pb 83Bi d. 226 222 25 2 88 Ra 86Rn Po Pb h. 84 82 8. Radium-226 decays to polonium-24 as follows: (a) (b) (c) (d) (e) 226 222 28 24 24 24 Ra Rn Po Pb Bi Po 88 86 84 82 83 84 What kind of particle is emitted in each of the transmutations labeled (a) to (e). 9. Find the energy in MeV released when alpha decay converts thorium-228 (228.02875 u) into radium-224 (224.02086 u). The mass of an alpha particle is 4.002603 u. 234 Pa 9 22 Na 22 Ne 0
0. Find the quantity of nuclear energy liberated in each of the following nuclear reactions: + > + + 2 (235.043925 u) (.00867 u) (39.926 u) (93.95367 u) (.00867 u) 3 3 4 2He + 2He > He + 2 2 (3.0603 u) (3.0603 u) (4.00260 u) (.00728 u) 235 U 40 94 92 0n 54 Xe 38Sr 0n (Answers: 2.96 x 0 - J, 2.2267 x 0-2 J). The half-life of a radioactive isotope is 2.5 years. If the activity of the original sample of this isotope was 3.2 x 0 3 Bq, what would be its activity after 5.0 years? [8.0 x 0 2 Bq] 2. What percentage of a polonium-20 sample will remain after 72 days if it has a half-life of 38 days? [42.2%] 3. A 2.0 g sample of a radioactive isotope undergoes radioactive decay. If the half-life of this isotope is 45 mins, how much of this isotope remains after 5.0 hours. [0.020 g] 4. In 9.0 days the number of radioactive nuclei decreases to one-eighth the number initially present. What is the half-life (in days) of the material? [3 days] 5. Strontium-82 has a half-life of 25.0 d. If you begin with a sample having a mass of 40 g, in how many days will you have only 7.5 g of strontium-82 left? [75 days] 6. The half-life of a radioactive isotope is 2.5 years. If the activity of the original sample of this isotope was 3.2 x 0 3 Bq, what would its activity be after 5.0 years? [8.0 x 0 2 Bq]