Radiation Playhouse! Pre-Lab: Articles and Simulations and Videos, Oh My! A Bit of History Henri Becquerel spent the first half of his life studying phosphorescence, the phenomenon by which a material absorbs and then (after a delay) re-emits light of a certain wavelength. The delayed emission occurs because the energy levels involved are associated with a forbidden quantum mechanical transitions (this is how objects glow-in-the-dark). When Willhelm Rӧntgen discovered X-rays in 1895, Becquerel thought they might somehow be related to his beloved phosphorescence. Becquerel’s experiments the next year centered on exposing uranium salts on photographic plates to sunlight. This caused the salts to emit X-rays and leave an image on the photographic plate - or so Becquerel thought. One morning he had his experiment all set up and ready to go when the weather refused to cooperate, and instead decided to rain all day (weathermen in 19th century Paris were about as accurate as meteorologists today). Grumpy that his experiment was ruined for the day, Becquerel put the uranium salts in a drawer. When he came back the next morning, he found that an image had still been created on the photographic plate, despite the distinct lack of sunlight in the drawer (Figure 1). Becquerel had discovered natural radioactivity. His discovery is a perfect example that great scientific achievements don’t always come about from meticulously designed experiments intended to answer burning scientific questions; they can also come about purely by a serendipitous accident (similarly, the discoveries of Teflon, penicillin, and vulcanized rubber were all happy accidents). The common denominator is that a scientist took the time to figure out what was going on and why it was happening. As Louis Pasteur (who himself got lucky on several important occasions) declared, “In the field of observation, chance favors only the prepared mind.” Becquerel’s prepared mind was awarded the Nobel Prize in Physics in 1903 for the discovery of natural radioactivity. Figure 1: Image of Becquerel’s photographic plate, showing the effects of exposure to radioactive uranium salts. Becquerel wrapped the photographic plate in several sheets of thick black paper to prevent it from reacting with light in the room. He then placed a copper Maltese cross (distinctly visible in the lower black blob) on top of the paper and covered the whole thing in uranium salts. The radiation exposed (i.e., darkens) the photographic plate wherever it was spread, except where the metal cross shielded the plate. The light area around the two big black blobs wasn’t covered in uranium salts, and therefore wasn’t exposed to radiation. Fast forward 110 years, and radioactive substances are everywhere. Despite what the news may have you believe, these substances rarely pose a danger and are actually advantageous in many cases. We naturally encounter radioactive elements in the food we eat, the buildings in which we attend class, and the air around us. Radionuclides illuminate watches, alert us to the presence of smoke, allow us to date objects older than the earth, help us to diagnose all manner of illnesses, and power unmanned satellites to the farthest reaches of the Solar System. Radioactivity will not, however, turn you into a superhero or a supervillain (which may or may not be a positive, depending on your affinity for spandex). For additional information on radioactivity, see Appendix A, as well as Moore Q12 – Q15 and Young & Freedman Chapter 43. The Story The energy released by the decay of a radioactive substance can power a circuit, just like a battery does. These devices, called radioisotope thermoelectric generators (RTGs), have been used since the 1960s to power spacecraft because they provide a lot of energy for fuel while adding relatively little mass to the spacecraft. Midway through your first week as a summer intern at NASA, your mentor informs you that NASA is moving forward with a project to create robots that can hop around the surface of Mars, like a wacky hybrid of Tigger and Wall-E. This would allow NASA to more efficiently explore the rocky terrain of Mars [Note: this is an actual idea floating around at NASA and the European Space Agency – the robot, that is, not the Disney cross-over event. If you’d like to learn more, go to www.space.com/9547- radioactivity-power-hopping-robots-mars.html]. These robots would require a sustained power source (you can’t very well plug them into a charging station on Mars) that is also light (jumping upwards of 1 km at a time only gets harder the more mass you have on board). RTGs are very likely the best way to do this. Unfortunately, a previous intern (who wasn’t as meticulous as you are) labeled the radioactive isotopes that NASA is considering but forgot to record his labeling scheme. You have three radioactive sources labeled A, B, and C. Further, you know that one source is Polonium-210, one source is Strontium-90, and one source is Barium-133. But which is which is which? They also lost some crucial data on an additional source NASA is considering. Before the project can move forward, the scientists at NASA need you to figure it out, and give them some preliminary information on the feasibility of each one as a power source. An Article: Getting to Know the Types of Radiation All parts of the electromagnetic spectrum as well as many other types of energetic particles can be referred to as radiation. An important distinction to make is whether the radiation is ionizing or non- ionizing. Ionizing radiation has sufficient energy to remove an electron from an atom or molecule. A Geiger counter works by detecting the current that’s created when radiation passes through and ionizes an otherwise insulating gas. If ionizing radiation passes through your body, it can create dangerous free- radicals, which can damage DNA and lead to cancer. Ionizing radiation includes the high-frequency part of the electromagnetic spectrum (i.e., gamma, X-ray, and some UV radiation), along with alpha and beta particles and cosmic rays. The relatively low-frequency end of the spectrum (i.e., microwave, infrared, visible, and some UV radiation) doesn’t have enough energy to ionize particles and isn’t known to damage DNA or cause cancer. All the radiation we’ll be dealing with in this lab is ionizing radiation. While this may seem a bit frightening, don’t forget that you encounter naturally occurring ionizing radiation in your everyday life and probably don’t think twice about it. Ionizing radiation can even be beneficial – the same processes that can harm healthy cells can be targeted at tumors to disrupt their growth. And although you may not realize it, ionizing radiation benevolently watches over you every day and may even save your life. Do This: Read about smoke detectors at home.howstuffworks.com/home- improvement/household-safety/fire/smoke.htm. You may skip page 2 of the article if you want. PL1. What type of ionizing radiation is used in smoke detectors and what is the typical activity (in whatever units of activity you prefer)? PL2. Briefly describe how an ionization chamber works, either in words and/or with a picture. PL3. What happens when smoke is present? In other words, why does the alarm sound? Do This: Let’s delve into the types of radiation in a bit more depth by watching this video: www.youtube.com/watch?v=27qSAqafQ6o. PL4. Now complete the following chart in your lab notebook. You should use the information you’ve learned thus far in class and in this lab. Be sure to look at the notes for each column before responding: Type of Actual Relative Distance ChargeB Massc Reasoningf Radiation Particlea Speed d Travelede Alpha Beta Gamma a. For historical reasons, we refer to each type of radiation by a Greek letter (when scientists first discovered radioactivity, they didn’t know what the radiation really was, so they started sequentially naming them after Greek letters). You, however, know what these particles really are – list them here (e.g., photons, neutrons, neutrinos, etc). b. What is the charge of each type of radiation (sign and total charge, in whatever units you deem convenient)? c. What is the relative mass of each radiation type? You don’t need to give an exact value (although you can if you’d like). It’s sufficient to say that one radiation type is much, much lighter than the others – or something similarly qualitative. d. How fast, in general, will each type of particle be moving? Once again, you can give qualitative comparisons. e. Based on what you’ve summarized about each particle, how far is the particle likely to travel before it interacts with and loses energy to matter? Qualitative comparisons are fine. f. In a word or two, explain how you decided on your response in the previous column. The Statistical Nature of Radioactivity Do This: To get a feel for what the half-life of a radioisotope really represents, go to the Pre-Lab links on the course website to visit http://phet.colorado.edu/en/simulation/beta-decay and click the “Run Now!” icon (if you’re having trouble getting it to run, you may need to first select “Download”). Do This: Choose the “Single Atom” tab and select your favorite radioactive atom from the list on the right side. This program will allow you to observe the simulated decay of an atom. each time you want to see how long it takes an atom to decay, click the “Reset Nucleus” button.