Absorption of Gamma Rays - Ryerson Department of Physics

N1-1Absorption of Gamma RaysThe object of this experiment is to measure the absorption coefficient for gamma rayspassing through lead, copper, and aluminum. The materials will be compared as to their abilityto block the rays.IntroductionThree types of radiation are observed to be emitted by naturally radioactive materials:alpha particles, beta particles, and gamma rays. Gamma rays are created by processes that occurin radioactive nuclei. The gamma rays to be used in this experiment originate in two radioactivenuclides: 137 Cs and 60 Co. The energy of the gamma rays used in this experiment are,respectively, 0.662 MeV from 137 Cs and 1.17 MeV and 1.33 MeV from the 60 Co source. Thecorresponding wavelengths are: 0.0019 nm, 0.00106 nm, and 0.00093 nm. The energies of the60 Co gamma photons are in the order of a million times greater than those of visible light!Gamma rays are very penetrating. Given their high energy and lack of electric charge,this is hardly surprising. When they do interact with matter they do so in one of three possibleprocesses. The first possibility is the photoelectric process in which the gamma rays interactwith electrons initially bound to an atom to eject an electron from the atom. The secondpossibility is the Compton process in which they make collisions with the free electrons in anabsorbing material. The third possibility can only occur if the gammas have energies over 1.022MeV. In this process the gamma interacts in the electromagnetic field of an atomic nucleus tomaterialize itself as an electron-positron pair.TheoryWhen a beam of gamma radiation passes through an absorbing material the number ofphotons absorbed (dn) in a thickness of the material (dx) is proportional to the number of photons(n) arriving at that layer of material. This is expressed in the following equation:dn = −μndx(1)where the proportionality constant, µ, is called the linear absorption coefficient. Equation (1) isa differential equation that can be solved to obtain the following exponential equation:n−μx= n 0e(2)where n 0 is the number of gamma rays per unit of time incident on the layer of absorber, n is thenumber of gammas getting through the absorber in the same time interval, and x is the thicknessof the material in centimetres.

N1-2The linear absorption coefficient will be seen to be a function of the material used toblock the gamma rays. In particular, it strongly depends on the material density. Thisdependence is revealed by dividing the linear absorption coefficient by the material density. Thenumber obtained by this division is called the mass absorption coefficient and should haveroughly the same value for all materials. If the linear absorption coefficient is in units of 1/cm,the mass absorption coefficient will have units of cm 2 /g.ApparatusThe detector is a Geiger-Muller tube. Radiation incident upon the tube enters through athin (and very delicate) mica window and into a gas filled cylinder that contains two terminalswith a high potential difference between them: one of the terminals is the outside of the cylinder(or cathode) which is negatively charged, and the other terminal is a wire that runs down thecentre of the tube (the anode) which is positively charged. Radiation moving through the gasfilledcylinder will ionise the gas. When these ions move towards either the cathode or theanode, the electric circuit is closed and a signal is sent to the scaler. The scaler is simply anadding machine that counts the number of times that a particle of radiation is detected.Figure 1 Diagram of the scaler-timer unit

N1-3Precautions1. The detector is located at the top of the opening located in the lower right hand side of theapparatus. The mica window is very thin and fragile. Do not touch it under any circumstances.2. Treat the radioactive sources with respect. They are quite weak as radioactive sourcesgo, but nevertheless, they are radioactive. Handle them only with the tongs provided. Do notput one in your pocket. Do not attempt to break their plastic containers. Keep them behind leadshielding as much as possible. Return them to the lab instructor as soon as you are finished withthem.3. One of the materials you will be using in this experiment is lead. Lead is a toxicsubstance, especially if ingested, so be sure to wash your hands immediately after the lab.Procedure1. Setting the voltage: Turn on the scaler unit. Push the H.V. (high voltage) button once.The LED screen will now show you the voltage of the anode. Using the UP and DOWN buttonsadjust the anode voltage to 360 Volts. Push the H.V. button again to have the LED screen putback on count mode (i.e. the light above the H.V. button is off).2. Setting the timing interval: You will be counting the gamma ray incidents in one minuteintervals. Push the button marked TIME. The LED screen will now show the time (in seconds)that the timer is set for. Adjust this time until it reads 60 seconds. Push the TIME button to goback into the COUNT mode (i.e. light above the TIME button is off).3. Sign out a 137 Cs source from your lab demonstrator (make sure you record the source aswell as the time you signed it out and the time of its return when you are done with it). Place thesource in its plastic tray at the bottom of the detector cavity. Place the plastic sheet with a holein the centre of it four or five levels away from the 137 Cs source. This plastic sheet will act as aholder for the various blockers that you will be using in the lab, and it should be located as closeto the detector as possible.4. The scaler is now ready to start counting the incident radiation. To start counting simplypress the COUNT button. The scaler will count for the amount of time the timing interval wasset for (60 seconds in this case) and the number of radiation events will be displayed on the LEDscreen. When the scaler is done counting, the light above the COUNT button will go out, andthe number of radiation events will be displayed on the LED screen. Take three one-minutecounts of the amount of radiation emitted by the 137 Cs source without any blocking (n 0 ).5. Place one sheet of lead on the plastic holder above the source and take three counts with

N1-4the blocker. Repeat this for multiple layers of lead up to four sheets. Record the thickness ofeach lead sheet using a micrometer.6. Repeat step 5. for copper and aluminum. Note: for each of these materials n 0 will besame as the value recorded in step 3.7. Return the 137 Cs source to your lab demonstrator and sign out a 60 Co source. Repeatsteps 3. and 4. for the 60 Co (i.e. lead only).8. Return the 60 Co source to your lab demonstrator. Remove all the blockers from thedetector cavity and take three one-minute counts for the background radiation.9. Turn down the high voltage to zero and turn off the scaler timer.AnalysisTake the average of each set of three counts per minute. Subtract the background average fromeach average to correct for background radiation. Plot all the results on a single graph page,plotting the natural logarithm of the average counts/min against the absorber thickness incentimetres. Draw the best straight line through the points for each material. The slopes of theselines are the linear absorption coefficients. Look up the densities of the absorbing materials used.Divide each of their linear absorption coefficients by the appropriate density in g/cm 3 . The resultis called the mass absorption coefficient. Compare the results for each absorber and comment.Also compare the results for lead using the different sources and comment.