Name Date Factors that affect detection of radiation: Shielding In this investigation, you will be working with the digital radiation monitor you have used in previous experiments. You be investigating the impact of the shielding materials between the source of ionizing radiation and the monitor. Equipment: Radiation monitor OR disc source wooden source holder meter stick Safety reminder: The thin mica window and wire grid is on the front of the monitor Vernier Digital Radiation Monitor. DO NOT TOUCH OR DAMAGE THE MICA WINDOW Image Source: Digital Radiation Monitor extended manual. AND GRID IN ANY WAY. DO NOT LET ANY SUBSTANCES OR OBJECTS YOU ARE MEASURING TOUCH THE MICA WINDOW AND GRID Remember from your safety discussions that the sources provided by your teacher are safe, but you should still follow ALARA rules to minimize your exposure to even these low levels of radiation. Make certain you return the source to your instructor at the end of the activity. Procedure: 1. Obtain a monitor. Use your monitor to determine the background radiation count rate (in Counts per Minute, CPM). Remember, if you have any sources of radiation nearby, place the source face down on the work surface far from the monitor when doing this. trial CPM Minimum CPM Maximum CPM Average CPM 2. Record what source(s) you have been given (radioactive isotope and type of radiation being produced), and the shielding material you have selected. 3. Place the source in the wooden holder. (You may need to use a bit of transparent tape across the plastic edge of the disk to keep it from slipping out.) Then place the monitor next to the meter stick and the source in front of the monitor and next to the meter stick (see Figure). How should you orient the source relative to the Experimental Setup. monitor’s surface? 4. Before beginning the experiment, determine the spacing between the monitor and the source and how you plan to position the shielding materials. Remember, the monitor and source locations cannot change. Explore what distance between the source and monitor will work for your thickest shielding so that you get reasonable count rates. Reasonable count rate means that with your thickest shielding the count rate should be at least twice background. You will need to collect data for 8 to 10 different shielding thicknesses. Once you have found a setup for the maximum thickness, record the specific thicknesses in a data table below. 5. Make a graph of the data by plotting thickness on the horizontal axis and activity (Corrected CPM) on the vertical axis. Label your axes. You will need to pick the best scale for both axes so the data fits nicely on the graph. Does the graph resemble any mathematical function you know of, for example a straight line (linear), parabolic, exponential or other? 6. How can you determine if there is a specific mathematical relationship between the counts detected and the separation? The type of curve shown is likely to be due to either an exponential type relationship ( C C0 e r / r0 ), where C0 and r0 are both constants), or by a power-law relationship ( C C0 /r p ), where C0 and p are both constants). Test if the exponential relationship is appropriate. Create a semi-log graph (double-click on the vertical axis, select the scale tab, select “logarithmic scale” and then close the dialog box). If the on a line. You can then determine the exponential scale is appropriate, the data should lie the constants C0 and r0 . appropriate trend-line in Excel anddetermine 7. If the relationship was not linear on the semi-log graph, then a power law may be appropriate. Test this by making a log-log graph. Convert the horizontal axis to a logarithmic scale (double-click on the horizontal axis and repeat what you did in the previous step). If the data now appear to be on a line, your data follows a power law relationship and you can now determine the appropriate trend-line in Excel to find the power p.