MARIST P 19 - TEACHER NOTES & GUIDELINES

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I. TEACHER NOTES & GUIDELINES(takes about ninety minutes)About the SN-7995 IsogeneratorDISCUSSION OF PRECONCEPTIONSPROBLEMV. EVALUATIONSOTM LAB: P19I. TEACHER NOTES & GUIDELINESTITLE OF LAB Half-life of an Isotope (Half-life of a Radioactive Substance(Nuclear sensor))DEVELOPERS OF LABTed Brown, JD806Kirk Reinhardt, JD738OVERVIEW OF LABDESCRIPTIONIn this lab students will learn how to determine the half-life of Barium 137m by measuring its rate of decay with a G-M Probe interfaced to a PASCO 500 or 750 interface.(Ref: Sws Library Experiment #GS17)CURRICULUM CONSIDERATIONSStudents should have been introduced to the basic structure of the atom and the various forms of natural radioactivity. SAFETY CONSIDERATIONS 1. Follow all instructions for using the equipment in this activity.2. Wear an apron, gloves, and safety goggles when handling radioactive substances. 3. Follow all instructions about handling and using radioactive substances.CAUTION: Do not let radioactive substances get on your skin or into your eyes or mouth. Inform your instructor of any spills or if you come in contact with the radioactive substance. 4. The solution containing the radioactive isotope can be disposed of by flushing down the drain since its half-life is short (2.6 min).BACKGROUND INFORMATIONA. SCIENTIFIC VIEWPOINT1. Radioactive decay is a random process. It is not possible to predict when a particular nucleus will decay, but we can make fairly accurate predictions regarding how many nuclei in a large number will decay in a given period of time.2. 2. Half-life is the length of time it takes for half the nuclei in a sample to undergo radioactive decay.3. 3. The number of nuclei decaying in a given length of time is proportional to the number of nuclei present at the beginning.B. COMMON MISCONCEPTIONS1. All the nuclei will decay in two half-lives.2. The half-life concept can be applied to any number (even a small one) of nuclei.OBJECTIVES1.To introduce students to the nature of a radioactive decay curve.2.To measure the half-life of Barium 137.EQUIPMENT/MATERIALSPROVIDED BY SOTMLaptop and PASCO 500 or 750 InterfaceG-M tube and clampIsogenerator Kit PROVIDED LOCALLYRing stand Small beakerBuret ClampRight Angle ClampDe-ionized or distilled water ADVANCE PREPARATION(takes about ninety minutes)About the SN-7995 IsogeneratorThe isogenerator (short for “isotope generator”) is a device that has a radioactive substance cemented into a porous plug of epoxy. The radioactive substance itself cannot come out of the plug of epoxy. However, the radioactive substance (Cesium-137) produces a radioactive by-product (Barium-137) that has a short half-life. The Barium-137 can be “flushed” out of the plug of epoxyusing a special liquid. You can measure the half-life of the Barium-137 by measuring the counts of radioactivity coming from the liquid. NOTE: Because the special liquid drains through the isogenerator and picks up the radioactive Barium-137 along the way, the isogenerator is sometimes referred to as a “nuclear cow”.The isogenerator kit includes three parts:- ion exchange column- stopcock- 0.1 Molar EDTA eluant (special liquid) with pH = 9The ion exchange column is filled with de-ionized water when it is shipped from the factory.Preparing the IsogeneratorREMINDER: Be sure to wear protective clothing (gloves, apron, goggles) when handling or using the isogenerator or the special liquid after it drains through the isogenerator. Follow all instructionsfor handling and using the isogenerator.You will need the following items:- isogenerator kit- beaker, 50 ml- base and support rod- buret clamp1. Remove the large cap at the top of the reservoir on the ion exchange column. Hold the column upright securely in one hand. Use your thumb to push the large cap upward. Do this slowly so that none of the liquid inside spills out. - Keep the large cap so you can cover the ion exchange column again when you are done with it.2. Put a buret clamp on a base and support rod. Put the ion exchange column into the buret clamp. Turn the ion exchange column so you can see the small marks that are on the back of the reservoir.3. Place the beaker under the small cap at the bottom of the ion exchange column.4. Carefully remove the small cap from the bottom of the ion exchange column. Let the de-ionized water drain out of the ion exchange column into the beaker. 5. Put the stopcock onto the end of the ion exchange column. Turn the valve on the stopcock so the valve is horizontal and the stopcock is closed. 6. Remove the small cap from the top of the squeeze bottle that contains the EDTA eluant. Fill the ion exchange column with the EDTA eluant to the 5 ml mark on the reservoir of the ion exchange column.7. Allow the ion exchange column to sit for one hour with the EDTA eluant in it. (Thisallows more of the Barium-137 to move down the column.)ION EXCHANGE COLUMNBEAKERBURET CLAMPSTOPCOCKII. PRE-LABPRE-LAB EXERCISE TO ELICIT STUDENTS’ PRIOR KNOWLEDGE ANDMISCONCEPTIONS1. Hand out 16 M&M’s to each pair of students and ask them to thrown them onto thetable and count the number of M&M’s that have the M showing.2. After tabulating the results, ask students why not everyone got 8 M’s. Discuss, andpoint out that, although the probability of an M showing is one-half, not all groups got8 M’s out of 16 because, for small numbers, there can be quite a lot of variability inthe results.3. Next, find the total number of M’s for all the student groups. This number should beclose to one-half the total.4. Have the student groups try a few more trials to show that the variability is less forgreater numbers of trials.5. Ask students to write a few sentences to summarize what they have learned from thedemonstration. Have some of the students read their summaries to the class anddiscuss.6. Now have the students throw out the M&M’s as before and remove from the group allthose that show an M. Tabulate the results. Next, have the students throw theremaining M&M’s on the table and count the number of M’s. Tabulate the results.7. Ask how many times they would have to throw the M&M’s after removing the one’swith the M’s showing to not have any left. Discuss.DISCUSSION OF PRECONCEPTIONSThe teacher should try to address the misconceptions listed above in the course of the discussion of the results of the M&M demonstration.Students should be instructed to carefully dispose of the


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MARIST P 19 - TEACHER NOTES & GUIDELINES

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