Nuclear Radiation

Nuclear Radiation Nucleus has protons (+ charge) and neutrons (no charge). Electrons are not in the nucleus and do not influence nuclear processes. When discussing nuclear radiation, we are only concerned with the very small nucleus that consists of positive protons and neutral neutrons. There are no electrons in the nucleus and electrons do not make an atom radioactive. http://commons.wikimedia.org/wiki/File:Atom-struc.svg

Isotopes Isotopes: Atoms of the same element (same number of protons) but with different nuclei due to different numbers of neutrons. n n + + n + 1H 2H 3H These are all isotopes of hydrogen. Hydrogen-1 has one proton and no neutrons. H-2 still has one proton, but also has one neutron. It is heavier (more massive) than H-1. H-3 has one proton and two neutrons and has 3 times the mass of H-1. H-1 is very common, H-2 is uncommon but not unusual, H-3 is quite rare. A common student misconception is that H-1 is not considered an isotope. They think only H-2 and H-3 are isotopes of hydrogen and that H-1 is the “standard”. In fact, all three are isotopes of each other. Chemical tests can’t differentiate between isotopes, but mass spectrometers are sensitive instruments that can separate isotopes by mass. Why important? For example, nuclear reactors need Uranium-235 but the most common form of Uranium is U-238. A mass spectrometer could be used to separate these isotopes and produce enriched Uranium. More massive!

Naming Isotopes – 14N Isotopes are named by their element name and their mass number. 14N has 7 protons (because it is nitrogen) and 7 neutrons (7 p + 7 n = 14) How many p and n in 14C? + n Carbon has six protons so C-14 has (14 – 6 = 8) eight neutrons.

Why are isotopes important? Radioactivity depends on neutron/proton ratio. Big atoms: can be stable if n > p. Lead-206 is stable. It has 82 p and 124 n. Uranium 238 is unstable. It has 92 p and 146 n. Small atoms: stable when p  n. Oxygen-16 is stable. It has 8 p and 8 n. The ratio of neutrons to protons determines if an atom is radioactive. Larger atoms are stable when they have more neutrons than protons because the strong and weak nuclear forces (the attractive forces that act between all nucleons) overcomes the electrostatic repulsion of the positive protons. Too many neutrons, however, and the nucleus will decay. Bigger atoms usually decay by getting rid of 2 protons and 2 neutrons (a Helium nucleus). Smaller atoms (having a smaller volume nucleus) are stable when the number of protons is about the same as the number of neutrons. Too many neutrons and a smaller atom decays by changing a neutron into a proton and emitting an electron from the nucleus. http://commons.wikimedia.org/wiki/File:Radiation_warning_symbol.svg

Summarizing Questions Part 1 Does the number of neutrons have any apparent effect on the name or behavior of an atom? If your uncle asked you what an isotope is, what would you tell him? Have students share answers to the Summarizing Questions part of Radioactivity -1. These are not in their guided notes because they are on the simulation handout. Answers 1: The number of neutrons determines the mass number, which is part of the name of an isotope. The number of neutrons, compared to the number of protons, determines whether a nucleus is stable or radioactive. Answer 2: An isotope is an atom of an element that has the same number of protons as all other atoms of that element, but has different numbers of neutrons as some atoms of that element. Answer 3: No Does an atom’s positive or negative charge affect its radioactivity?

What is radiation? Radiation: High energy particles that come out of the nucleus of some atoms. A Geiger counter detects these Each is a quantum (plural quanta) A quantum is a bundle (often a bundle of energy). When something is emitted as quanta, that means it comes out as a chunk. Electrons, protons, and neutrons are also quanta. All students need to know is that each time a particle comes out of the nucleus of the atom it makes a single click on the Geiger counter. The terms quantum and quanta are here only as an introduction to the terms. http://commons.wikimedia.org/wiki/File:Geiger_counter_2.jpg

Alpha Particles 2 protons + 2 neutrons. High energy! Remaining nucleus has less mass. Alpha One of the high energy particles that come out of the nucleus is the alpha particle. It is the most massive of the ejected particles and is actually a Helium nucleus (2 protons and 2 neutrons). Because it is so massive, it can actually be stopped by a piece of paper! Alpha radiation is most dangerous when a radioactive material (like Radon gas) is breathed into the lungs. There the alpha particles that are emitted as Radon turns into Polonium can harm the tissue in the lungs. When an alpha particle is ejected from the nucleus, the mass number of the original nucleus decreases by 4. The number of protons and the number of neutrons each goes down by 2. A new element is formed. Nuclear reactions release HUGE amounts of energy (think atomic bombs and fusion in the sun) because the nuclear forces (strong and weak) are very strong. Energy is released when these bonds are broken. The energy comes out as kinetic energy of the alpha particle. The amount of chemical energy typically released (or converted) in a chemical explosion is: 5 kJ for each gram of TNT The amount of nuclear energy typically released by an atomic bomb is: 100,000,000 kJ for each gram of uranium or plutonium http://commons.wikimedia.org/wiki/File:Disintegration_(PSF).png Radon-222 p: 86 n: 136 Polonium-218 p: 84 n: 134

Beta Particles An electron leaves the nucleus …but there are no electrons in the nucleus! A neutron breaks into a proton + an electron …the proton stays behind. Proton # increases, neutron # decreases. In the diagram, the electron is the beta particle. It does not come out of the atomic orbitals, but comes out of the nucleus. Beta particles can go through paper, but can be stopped with Aluminum foil or a few cm of flesh. They have so much kinetic energy that they will burn the skin or tissues that they strike. Carbon-14 decays by emitting a beta. One neutron becomes a proton with a resulting atom of nitrogen-14 with 7 p and 7 n. The electron is emitted from the nucleus with a very high kinetic energy. http://commons.wikimedia.org/wiki/File:Beta-minus_Decay.svg Carbon-14 p: 6 n: 8 Nitrogen-14 p: 7 n: 7 Electron +

Gamma Particles Gamma particles are pure energy - electromagnetic radiation (like x-rays) No change in the number of p & n. Most alpha and beta decay also has gamma. Gamma rays, gamma particles, gamma decay – all the same thing. Gamma rays are the highest energy wave on the electromagnetic spectrum. They are thought of as particles (wave/particle duality of quantum physics) because they can be counted as they are emitted from the nucleus. When a gamma ray is emitted, there is a decrease in nuclear vibration (the nucleus “settles down”, but the number of protons and neutrons in the nucleus do not change. With an energy higher than x-rays, gamma rays are very dangerous to cells in the human body, and are usually emitted along with alpha and beta particles. It takes a block of lead several feet thick to stop a gamma ray.

Check your understanding Are alpha, beta, or gamma particles radioactive? Is radioactivity like dirt? When radiation gets on you do you become radioactive? Do you agree with Rob or Maria? Rob: “An atom is not radioactive - it only emits radiation for a short time.” Maria: “But radiation comes from the atoms so they must be radioactive.” The questions are animated – press enter to show the next question. These questions are not on the guided notes but are on the handout for Simulation 2. Answer 1: No, they are the radiation that comes off a radioactive material, but are not radioactive themselves. If they land on you, you do not become radioactive, however, they can do damage to your cells because they hit you with so much kinetic energy. Answer 2: Need to be careful here with answers – if the alpha, beta, gamma particles do not make things radioactive, but if you rubbed radioactive Uranium on you, some of the U might get on your skin and it will be radioactive. If you drink radioactive Barium, you become radioactive because the Barium is in you. Answer 3: Maria is correct. Atoms are radioactive. They only decay once, then become a different element (which might also decay), but they are radioactive before they decay. Radioactive materials keep emitting particles because there are so many atoms in the materials (might be a good time to mention half-life).

Practice 1 (use periodic table) 1. Uranium-235 has an atomic number of 92 and a mass number of 235. a. How many protons b. How many neutrons c. It emits an alpha particle (p:2, n:2). What atom is left? How many p & n? Use as many of these slides as needed for practice. 1a. U-235 has 92 protons 1b. U-235 has 143 neutrons (235 – 92 = 143) 1c. Thorium-231 is left. (p: 92 – 2 = 90 so Thorium, n: 143 – 2 = 141 OR 231 – 90 = 141)

Practice 2 (use periodic table) 2. Thorium-231 (90 protons and 141 neutrons) next emits a gamma particle. A gamma is pure energy. What atom remains? Give its name and the number of p and n. Use as many of these slides as needed for practice. 2. Thorium-231 is left! Gamma radiation does not change the number of protons or neutrons in the nucleus.

Practice 3 (use periodic table) 3. Thorium-231 also emits an alpha particle. How many protons and neutrons does the daughter have and what is its name? Use as many of these slides as needed for practice. 3. Radium-227 (227Ra) (p: 90 – 2 = 88 so Radium, n: 141 – 2 = 139 OR 227 – 88 = 139) Radium decays by alpha to Radon-223. The Radon in our basements comes from Uranium in the ground.

Practice 4 (use periodic table) 4. Nitrogen-16 is a radioactive element. a. How many protons? b. How many neutrons? c. 16N decays by beta decay, a neutron turning into a proton and emitting an electron. What is the new atom and how many p and n? Use as many of these slides as needed for practice. 4a. Nitrogen has 7 protons 4b. N-16 has 9 neutrons (16 – 7 = 9) 4c. The new atom is Oxygen-16 (p: 7 + 1 = 8 so Oxygen, n: 9 – 1 = 8). O-8 is stable. Notice that mass number did not change.