CHEMISTRY 170 Radioisotopes Positron Emission Tomography or PET scans use the radioisotope 18 F to create an image of the brain. DEPARTMENT OF CHEMISTRY UNIVERSITY OF KANSAS
Radioisotopes Introduction Radioisotopes are isotopes that are unstable and can emit nuclear radiation, large amounts of energy released from an atomic nucleus, compared to the energy in chemical reactions. The three most common types of nuclear radiation are assigned the first the letters of the Greek alphabet: alpha (α), beta (β), and gamma (γ). In this lab, you will look at three different radioisotopes, each emitting a different type of radiation. One radioisotope ( 210 Po) emits alpha radiation, one ( 90 Sr) emits beta radiation, and one ( 60 Co) emits gamma radiation. Alpha, beta, and gamma radiation differ in several important ways. For example, one is electromagnetic and two are charged particles. (See the pre-laboratory exercise.) Another way these types of radiation differ is in their ability to travel through a material, such as air, concrete, or a person. Because different types of nuclear radiation generally carry the same amount of energy (a few million electron volts, or MeV), the differences in how far each type of radiation travels through matter must come from differences among the three types of radiation. Electric charge and mass are two very important physical properties that lead to different penetration into or through a material. You will be asked to summarize these differences for your short report after completion of this lab. Why learn about nuclear radiation? The large amount of energy released by radioisotopes can be used a number of ways. For example, nuclear power plants, like the Wolf Creek Generating Station (Fig. 1) near New Strawn, KS, convert the energy from radioisotopes to electrical energy for everyday use. Additionally, radioisotopes play major roles in medicine, including cancer treatment and medical imaging. In each case, the radiation must be able to travel the right distance (for example, to get to a cancer cell in the body), but not so far that it becomes generally harmful. This means that containing radiation is an important consideration when using radiation. Figure 1: Wolf Creek Generating Station In today s lab, you ll measure and determine how well different types of radiation move through different materials. The radioisotope samples in the lab are specially manufactured to make them safe to handle, as the samples release only very low levels of radiation. From your data, you may be able to infer what types of materials make for good storage of radioisotopes. 2
Pre-lab Safety: Goggles must be worn at all times. Gloves are highly recommended. When handling the radioisotope samples, hold them as far from the body as you reasonably can. Your TA will distribute the radioisotope samples during lab and will collect them from you at the end of lab. ALL materials MUST be returned by the end of lab. Pre-lab Assignment: Please write out the following in your lab notebook. This assignment must be completed before the beginning of lab. You will not be allowed to start the experiment until this assignment has been completed and accepted by your TA. 1. Make a table in your notebook. The first column should list the three different types radiation (alpha, beta, gamma). Then include columns for the example radioisotopes used in this lab, example radioisotopes not in this lab, the electric charge for that type of radiation, and the mass for that type of radiation. Complete your table. 2. Summarize the experimental procedure in your lab notebook according to the Guidelines for Keeping a Laboratory Notebook handout. 3. A particular radioisotope emits 21,763 counts of radiation over 120 seconds with no barrier present. When a sheet of gelatin is placed between the source and the detector, 8,598 counts are recorded over the same amount of time. What is the percent change in the signal? In addition to these pre-lab requirements, a short quiz may be given at the beginning of lab based on the material in this lab write-up. Experimental Procedure to Vernier radiation probe Part 1 Differences between alpha, beta, and gamma radiation sources In this part of the experiment, you will collect data using different radioisotopes that emit different types of radiation: alpha, beta, or gamma. If you have questions, be sure to ask your TA. 1. If not already connected, connect the devices: a) Log into the computer using one group member s KU e-mail username and password. b) Connect the Vernier LabPro device to the computer using the USB cable. (See Figure 2.) c) Connect the Vernier LabPro device to the power supply. to power supply to computer USB Figure 2. Connecting the Vernier LabPro device 3
d) Connect the Vernier LabPro device to the Vernier radiation probe by plugging the probe cord into the DIG / SONIC 1 port on the LabPro device. 2. Ready the software. a) Open the Logger Pro 3.8.7 software toward the left on the Desktop. b) In the LoggerPro software, go to Experiment > Set Up Sensors > Show All Interfaces. A diagram of the LabPro device will appear in a new window. Click the icon for DIG/SONIC1. The go to Choose Sensor > Radiation. (This is the second option from the bottom.) c) When Radiation Counts appears on the main window toward the bottom left corner of the screen, you can close the Interface window. d) (OPTIONAL) Go to Experiment > Extend Collection. Each time you do this, you should see the maximum value on the X axis of the graph on the screen increase. This will give you a bit of extra time to collect your data, if you feel you need it. 3. Collect your data. a) Hit the Green arrow button labeled Collect toward the top and middle of the main window. It may take several seconds before anything happens. You should see that values begin to appear in the column toward the left of the screen. Allow the probe to sit exposed to the room and collect baseline data for approximately 120 seconds. Have one group member add all the counts during this 120 s interval. Record this number in your data table. b) Place the 210 Po sample (α emitter) on the table so that the flat side is down. Then place the radiation probe directly on top of the sample. Then wait a few seconds. You should see the red line increase in value into the hundreds of counts. (This will shrink the previous parts of the graph, and you ll see that the Y axis now covers a new range of values.) Collect data for another 120 seconds. Then remove the probe from the sample and make sure that the red data line on the screen drops. c) Repeat step b) for the 90 Sr sample (β emitter). d) Repeat step b) for the 60 Co sample (γ emitter). NOTE: the default time limit for data collection is a total of 600 seconds, so don t delay too long between different samples! At the end of Part 1 of the experiment, you should have a graph that looks similar to (but not exactly the same as) Figure 3. 6000 5000 90 Sr sample 4000 3000 2000 No sample 210 Po sample 60 Co sample 1000 0 Figure 3: Example data for Part 1 of the experiment. 4
e) Save your data so you have access to it. You will not be able to return to lab to get your data from the local computers. To save your data, go to File > Export As > CSV. Name your file so you know what it is ( Chem130_Lab2_Part1 or something similar would probably serve you well) and save it to the Desktop. You can sign in to your e- mail account through mail.ku.edu. E-mail the files to yourself and your labmates. Delete the files from the local desktop. Part 2 Barriers to radiation In this part of the experiment, you will determine how well each type of radiation travels through paper, aluminum foil, and gelatin sheets by measuring changes in the amount of radiation that hits the detector. If you have questions, be sure to ask your TA. 1. Collect data that will help you determine how well the radiation from 210 Po moves through different materials. a. Select File > New to start a new file. b. Collect your data for the 210 Po sample: i. With the Vernier Radiation Probe away from any samples, hit the green Collect arrow on the screen. Place the 210 Po sample (alpha emitter) on the table so that the flat side is down. Then place the piece of paper on top of the sample. Then place the radiation probe on top of the paper, so that it covers the same region as the 210 Po sample below. Collect data for about 120 seconds, noting the time you start and stop collecting data. Then remove the probe from the sample. Add the counts during this time and record this number in your data table. ii. Repeat i. using aluminum foil. iii. Repeat i. using the gelatin sheet. c. Determine the total number of counts during the 120 s interval when the probe was over the paper, aluminum, and gelatin barriers. Record your data in the table in your notebook. d. Similarly to 1e), go to File > Export As > CSV. Save and distribute your data to all members of your group. Do not leave files on the local machine. 2. Collect data that will help you determine how well the radiation from 90 Sr moves through different materials. Do this by repeating steps 1a-1d above for the 90 Sr sample. 3. Collect data that will help you determine how well the radiation from 60 Co moves through different materials. Dot his by repeating steps 1a-1d above from the 60 Co sample. 5
Lab clean-up: 1. Confirm that you have access to your data files. (Check your e-mail, your flash drive, or wherever you saved your data.) 2. Close the software. It may ask you if you want to save your changes to something.cmbl. Click No. 3. Delete your files from the local Desktop. 4. Replace all your radioisotope samples in their plastic storage container. Return the set to your TA. None of your group members will be allowed to leave until all of your radioisotope samples have been returned. 5. Please arrange devices and cords neatly on the lab bench for the next group. You do not need to disconnect them. Data and Results Provided below is an example data table that you should copy into your notebook. Complete the table as you collect your data. Counts in 120 s interval no shielding shielding source (air) paper aluminum foil gelatin sheet none alpha ( 210 Po) beta ( 90 Sr) gamma ( 60 Co) Table 1: Data for radioisotope experiment. For your data analysis, you will use your raw data above to calculate the percent change for the different materials. Mathematically, we can define the percent change as the change in the measurement (the difference) all divided by the measurement under the original condition and multiplied by 100%. Here s an example for the percent change in 210 Po signal when with and without paper present: % change = N paper N air N air 100% = 112 counts 6705 counts 6705 counts 100% = 6593 counts 6705 counts 100% = 98.31% This means that the counts decreased by 98.31% by placing the paper over the sample. This example is a fairly large change. Your numbers, based on your data, will be a bit different for 210Po, but probably still large. Your numbers for 90 Sr and 60 Co will probably be smaller than the 98% decrease calculated here. Use your raw data and perform this type of calculation to complete the following table. 6
Percent change shielding material source paper aluminum foil gelatin sheet alpha beta gamma ( 210 Po) ( 90 Sr) ( 60 Co) Reference(s) 1. Gastineau, J. Nuclear Radiation with Computers and Calculators, 3 rd ed.; Vernier Software & Technology: Beaverton, OR, 2003; sections 1, 6. 2. https://en.wikipedia.org/wiki/positron_emission_tomography, accessed Jan. 18, 2016. 3. http://www.kansastravel.org/wolfcreek.htm, accessed Jan. 17, 2016. 7
Glossary Radioisotope an isotope that can release high-energy radiation or particles during a spontaneous change in the nucleus. CPM counts per minute or number of decays or particles actually measured by a detector. Radiation energy emitted as particles or waves. 4 Alpha radiation, alpha particles; α, 2 α a particle consisting of 2 protons and 2 neutrons (helium nucleus) that can escape with high energy (MeV) from a larger nucleus ALARA The University of Kansas is committed to keeping doses As Low As Reasonably Achievable (ALARA). This goal serves as the overall controlling aim of radiation safety, and commits laboratories to steps that reduce exposure. Beta radiation, beta particles; β, β for β, this is an electron that can escape with high energy (MeV) from a nucleus when a neutron transforms into a proton, also releasing an antineutrino; β + is another type of beta radiation that releases a positron, rather than an electron. Radiation Safety Officer appointed by the Provost and approved by the State of Kansas and responsible for radiation protection at the University. Gamma radiation, gamma ray; γ a high-energy (MeV) unit of light (photon) that escapes from a nucleus when the nucleus changes from a higher-energy state to a lower-energy state. KDHE Radiation Control the regulatory agency in Kansas that oversees the use of radiation in the State. detector a device or instrument designed to identify or record presence of a stimulus, such as a particular object or substance; this presence is usually converted to an electrical signal that is sent to and interpreted by a computer. Authorized User laboratory workers or classroom participants who are certified by Radiation Safety to work with radiochemicals. 8