Absorption of Gamma Rays

Transcription

1 Introduction Absorption of Gamma Rays In this experiment, the absorption coefficient of gamma rays passing through several materials is studied. The materials will be compared to one another on their efficacy on blocking gamma rays. Gamma rays are created by processes that occur in radioactive nuclei. In this lab, the two sources of gamma rays will come from the radioisotopes 137 Cs and 60 Co. The dose received from the sources used in this lab, at arms length away, and for the duration of the lab, are well below the 1mSv per year limit for the public outlined by Canadian Radiation Protection Regulations. Although the activity of the sources in this lab are quite low, they are still radioactive and radiation precautions must be taken. Minimizing exposure time and maximizing distance from the source is important. A few safety things to note: There a strict sign-in sign-out policy. Please follow this policy. Do not attempt to break their plastic containers. Handle them only with the tongs provided. The sources should either be in the sample loading area or in the lead safe. No sources should be left on the tables. Make sure you do not inadvertantly take them. This will be bad for both you and the department. (It s happened...) Lead is a toxic metal and is also used in this lab. Be sure to wash your hands immediately after the lab and ABSOLUTELY NO FOOD DURING THIS LAB. Gamma rays are very penetrating due to their high energy and lack of electric charge. However, when they do interact with matter they do so in one of three possible processes. The first possibility is the photoelectric process in which the gamma rays interact with electrons initially bound to an atom to eject an electron from the atom. The second possibility is the Compton process in which they make collisions with the free electrons in an absorbing material. The third possibility can only occur if the gammas have energies over MeV. In this process the gamma ray interacts in the electromagnetic field of an atomic nucleus to materialize itself as an electron-positron pair. Regardless of which process takes place, the gamma ray is considered absorbed if it has interacted with a material (and ceases to be a gamma ray). In general, we can express the absorption of the number of gamma rays n through a medium of thickness x in differential form as: Page 1 of 5

2 dn = µn (1) dx Here, µ is defined as the linear absorption (attenuation) coefficient as it is an indication of how easy (or hard) gammar rays n are absorbed into the material. Using this differential, the number of unabsorbed rays is derived to be: Apparatus n = n 0 e µx (2) Spectech ST-160 Nuclear Lab Station Lead Plates (4) Aluminium Plates (4) Copper Plates (4) Micrometer 60 Co Radiation Source 137 Cs Radiation Source Source Holder Absorber Holder Radiation Source Log Book Spectech ST-160 Nuclear Lab Station Page 2 of 5

3 Spectech ST-160 The Spectech ST-160 station allows for you to detect gamma rays via a Geiger-Muller tube located at the top of the sample loading area in the lower right. Please do not touch the tube as the mica window is very thin and fragile. The top interface has a digital display along with several buttons with indicator lights above them. Here is an outline of how each works. 1.) COUNT - When you press this button, the indicator light with turn on and will begin counting the number of gamma rays for a period of time defined by the TIME setting. When the time has elapsed, the indicator light will turn off. The count will be displayed on the digital display. 2.) STOP - Hitting this button will interrupt any COUNT action. 3.) H.V. [High Voltage] - This button allows you to set the voltage across the Geiger- Muller tube using the UP and DOWN buttons. Make sure that COUNT is off when you adjust this button. 4.) TIME - This button allows you to set the duration (in seconds) which the Geiger- Muller tube detects gamma rays using the UP and DOWN buttons. Make sure that COUNT is off when you adjust this button. Pre-Lab Questions Please complete the following questions prior to coming to lab. At the beginning of lab, you will be given a short quiz which is heavily based on one (or more) of these questions. 1.) Read through the entire lab writeup before beginning. 2.) Why are gamma rays much more penetrating compared with other types of radiation? 3.) Derive Eqn. 2 from Eqn. 1. Include a comment in the derivation as to why there is a negative sign in Eqn ) The mass absorption coefficient is defined as µ where ρ is the density of the material. ρ What does this property mean and why is it useful? Hint: Consider a situation where you have to design a system that absorbs a specific amount of gamma rays. 5.) What would you expect the linear coefficient of absorption values of Aluminium, Copper and Lead to compare to one another. Would you expect the same relation between the mass absorption coefficients? Page 3 of 5

4 Procedure It is highly recommended to record your data in a spreadsheet file as you will need to do several calculations that are tedious to do by hand. 1.) Turn on the station (red button on the back) and set H.V. to 360 V, and TIME to 60 s. 2.) Sign out a 137 Cs source from your TA and make sure you record the source as well as the time you signed it out. 3.) Place the source in a source holder (plastic tray) and insert it on the bottom level of the sample loading area. 4.) Place the absorber holder (plastic sheet with hole in centre) at the top of the sample loading area. It should be located as close to the detector as possible. You may however need to position it down one level depending on how many absorbers you are currently using. 5.) Measure the number of radiation events (over 60 s) by hitting COUNT. Record this number and repeat 2 more times. Note: with TIME and COUNT on, the display will show elapsed time and not the number of radiation events. To fix this make sure TIME is off. 6.) Select a lead sheet, and measure and record the thickness using a micrometer. 7.) Place the sheet of lead on the absorber holder and repeat the measurement. Repeat this for increasing layers of lead, up to 4, making sure to record each thickness. 8.) Repeat this whole process using copper and aluminum. Note: you won t need to remeasure the unblocked count again 9.) Return the 137 Cs source and record it in the log book. Sign out a 60 Co source. 10.) Repeat the measurement using lead only. Note: this time you will need to remeasure the unblocked count as you ve changed your source 11.) Return the 60 Co source and record it in the log book. 12.) Lastly, measure the background radiation by having an empty sample loading area. Be sure to take the measurement 3 times. 13.) Set H.V. to zero and turn off the station. Page 4 of 5

5 Analysis 1.) Determine the average of each of your measurements as well as uncertaintly. Here since we ve taken 3 measurements, you can determine uncertainty statiscially. See Introduction to Measurement Uncertainty on our website for help. 2.) Next, correct for background radiation by subtracting that value from all other measurements. Be sure to propagate uncertainty. 3.) In a single plot, graph the natural logarithm of the average counts/min against the absorber thickness in centimetres for each material used with the 137 Cs source. Be sure to label each set of data properly. 4.) Add error bars to your data by once again propagating uncertainty. 5.) Apply a linear fit and determine the linear absorption coefficients for each material. Does it fit within your calculated error? 6.) From Eqn. 2, Derive the linear equation used in the plots identifying each component. 7.) Look up the densities of the absorbing materials used. 8.) Divide each of their linear absorption coefficients by the appropriate density in g/cm 3. The result is called the mass absorption coefficient. Compare the results for each absorber. Are they as you expected? 9.) Generate a plot comparing the results for lead using 137 Cs and 60 Co. Comment on the differences. Last Few Steps 1.) Save any data/plots (in any format) with an easily identifiable name. 2.) Submit your data file to your group submission folder on D2L. 3.) Once this is complete and are certain that the data is saved, restart the computer when all experiments are completed. 4.) Tidy up your work station by returning the sources and tidying the metal plates used ensuring the station is ready for your fellow students in other sections. 5.) Wash your hands after you have completed all these tasks. Page 5 of 5