4 α or 4 2 He. Radioactivity. Exercise 9 Page 1. Illinois Central College CHEMISTRY 132 Laboratory Section:

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1 Exercise 9 Page 1 Illinois Central College CHEMISTRY 132 Laboratory Section: Radioactivity Name: Equipment Geiger Counter Alpha, Beta, and Gamma source Objectives The objectives of this experiment are to develop an understanding of nuclear equations and to illustrate the first-order decay of radioactive isotopes. Background In the formation of molecules or ionic compounds, only the distribution of the electrons is affected. The nuclei of the atoms involved remain unchanged. However, in nuclear reactions, the composition of the nucleus will change, most often changing the identity of the element itself. The driving force for nuclear reactions is naturally to form a more stable nucleus. This is accomplished by "ejecting" particles from the nucleus to bring about a more stable arrangement of its nucleons. Typical particles ejected by unstable nuclei include: Alpha Particles: Alpha particles consist of two protons and two neutrons (essentially the same as a helium atom that has been stripped of its electrons). The nuclear symbol for the alpha particle includes its atomic number (the number of (+) charges it carries) and its mass number (the sum of its protons and neutrons). The mass number The atomic number 2 4 α or 4 2 He

2 Exercise 9 Page 2 Beta Particles: These particles resemble, at least in mass and charge, a very high speed electron that appears to be ejected by the nucleus of some unstable atoms. Since electrons do not reside in the nucleus, there are several theories as to how an "electron-like" particle could originate there. One plausible theory is that a neutron is actually a proton and electron "stuck" together. If the neutron were to eject its "electron" half, it would be converted into a proton. Since elements that undergo beta decay do increase in atomic number as they radiate, this is consistent with the theory. Since beta particles are negatively charged, they are assigned an atomic number of -1 and a mass number of 0 (considering their mass to be amu, their mass is negligible compared to protons and neutrons). So, its symbol is 0 β or e Positrons: A positron is similar to a beta particle, having the same mass but carrying a positive charge β or 0 +1 e Gamma Radiation: Gamma rays have been demonstrated to be electromagnetic waves similar to x-rays, but of much shorter wavelength (and consequently, higher energy). Since gamma rays have no charge and, as light, have no rest mass, the symbol used for gamma rays is: 0 0 γ

3 Exercise 9 Page 3 Nuclear Equations Equations for nuclear reactions are similar to regular chemical reactions in that they are subject to a mass balance. In nuclear equations we use nuclide symbols to represent given isotopes of elements. The nuclide symbol includes the atomic number and the mass number of that particular isotope. In balancing a nuclear equation, one must simply ensure that the sum of atomic numbers and mass numbers on both sides of the arrow are equal. For example, the alpha decay of Uranium-238 would be represented as U Th + 4 2He In this example, Uranium is the "parent" nuclide, while thorium-234 is considered the "daughter" nuclide. Reactant and product nuclei are always represented using nuclide symbols. Other particles that are involved in many nuclear equations are given the following symbols in which the subscript equals the charge and the superscript equals the mass number. proton neutron electron positron gamma photon 1 1 p 0 1 n 1 0 e +1 0 e or or or 0 0 γ 1 1 H 1 0 β +1 0 β Beta emission of a nucleus involves the ejection of a beta particle from the nucleus. If you recall, this produces a daughter element with an atomic number one greater than the parent nuclide. For example, the equation for the beta decay of Technicium-99 produces Ruthenium-99 as its daughter Tc Ru e Note that in all nuclear equations, the total charge is conserved. This means that the sum of the subscripts on both sides of the arrow must tally. Likewise, the total mass is conserved requiring that the sum of the superscripts on both sides of the arrow must tally.

4 Exercise 9 Page 4 Positron emission involves the ejection of a positron from an unstable nucleus. A positron is similar to a beta particle, having the same mass but carrying a positive charge. For example, the positron decay of Bismuth Bi Pb +1 0 β Gamma emission is simply the emission of a gamma photon, which carries no mass and no charge consequently has no effect on the parent nuclide's atomic number or mass number. Gamma emission generally accompanies the emission of other radioactive particles, such as alpha or beta. In general, the radioactive decay of a nuclide results in an "excited" product nuclei. This excited "daughter" nuclide will return immediately to its ground state by emitting electromagnetic radiation (generally in the gamma region of the spectrum). Although this emission usually happens immediately, some nuclides exist for a short time (10-9 sec) in a metastablestate (denoted by the "m" in the equation below) before they emit a gamma photon. A metastable nucleus is one in an exited state with a lifetime of at least 1 nanosecond. The following is an example of how this decay is depicted in equation form. Technecium-99, used in medical diagnosis, is an example of an exited nucleus with a significant lifetime. 99m 43 Tc 99 43Tc γ

5 Exercise 9 Page 5 Procedure Measuring "Activity" 1. Your Instructor will demonstrate the Geiger Counter by taking a 60 second "background" reading with nothing in the probe. This background count is primarily due to the decay of naturally occuring radioactive isotopes in the Earth's crust and consequently in the materials from which the college itself is built. Note: any subsequent readings of radioactive samples must be "corrected" for this background count. 2. Your Instructor will now take "unshielded" 60 second readings of three radioactive isotopes. (one alpha, one beta, and one gamma emitter). Record these values on your Report Sheet. 3. With a heavy bond paper shield placed between the radioactive sources and the Geiger probe, the readings will be repeated. Record these values on your Report Sheet. 4. A third set of readings will now be taken using a lead shilding. Record these values on your Report Sheet. 5. The alpha source used is polonium-210. Write the equation for its decay on your Report Sheet. 6. The beta source is thallium-204. Write the equation for its decay on your Report Sheet. 7. The gamma source is cobalt-60m. Write the equation for its gamma radiation on your Report Sheet. First-Order Kinetics and Half-lives All radioactive decay reactions follow first-order kinetics. Consequently, they obey the first-order time-concentration equation. log [A] t [A] 0 = kt This equation rearranges to give a linear relationship between log[a] t and time. log[a] t = ( k ) t + log[a] 0 y = m x + b (General equation for a straight line)

6 Exercise 9 Page 6 So a plot of the log(activity) of a sample versus time should yield a straight line with a slope equal to (-k/2.303) and an intercept of log(initial Activity). A close examination of this plot reveals an easy way to determine the half-life of the isotope measured. 1. In the LabWorks Program window click Analyze and select Create a New File. Manually input the data in Table 1. Time (minutes) Column A Counts per minute (corrected) Column B Have the spreadsheet calculate the log[a] t in Column C. 3. Select Graph Setup and create a plot of log[a] t vs time. 4. Select Curve Fit from the menu bar and perform a first-order linear regression on the plot. Attach a printout of this graph to your Report Sheet. 5. From this plot, determine the half-life of the unknown isotope. Record this value on your Report Sheet.

7 Exercise 9 Page 7 Illinois Central College CHEMISTRY 132 Laboratory Section: REPORT SHEET Radioactivity Name: Background reading (counts per minute) Radiation Activity in Counts Per Minute unshielded paper shield lead shield alpha source beta source gamma source 1. The alpha source used is polonium-210. Write the equation for its decay. 2. The beta source is thallium-204. Write an equation for its decay. 3. The gamma source is cobalt-60m. Write an equation for its gamma radiation. 4. From your plot of log(activity) versus time, what is the half-life of the unknown isotope. t 1/2 =

8 Exercise 9 Page 8 5. If you had a radioactive sample of table salt, how might you determine very quickly whether it was the sodium or the chlorine that was unstable? 6. The following data might have been taken from a sample of highly radioactive thorium-226. Using this data, calculate the half-life of thorium-226. time Counts per minute 9:10 am :45 am 270 t 1/2 = 7. Fill in the missing pieces in these nuclear equations. a) Np Pu + b) U Th +

Chemistry 132 NT. Nuclear Chemistry. Not everything that can be counted counts, and not everything that counts can be counted.

Chemistry 132 NT. Nuclear Chemistry. Not everything that can be counted counts, and not everything that counts can be counted. Chemistry 132 NT Not everything that can be counted counts, and not everything that counts can be counted. Albert Einstein 1 Chem 132 NT Nuclear Chemistry Module 1 Radioactivity and Nuclear Bombardment