SCINTILLATION DETECTORS & GAMMA SPECTROSCOPY: AN INTRODUCTION OBJECTIVE The primary objective of this experiment is to use an NaI(Tl) detector, photomultiplier tube and multichannel analyzer software system to obtain the major (primary) gamma peaks of various elements, and to compare the acquired values with theoretical predictions. INTRODUCTION Background Scintillation is the fluorescence (in approximately the visible spectrum) of a particular material (scintillator) due to interaction with gamma rays. The phenomenon of scintillation is well known and was used as early as the 19 th century. Rutherford used ZnS(Ag) crystals to study alpha-scattering; they used unaided vision to detect separate scintillation events, and therefore were not able to study distinctions between variations in light intensity. Research on the luminescence output of various crystals was studied extensively in the 20 th century; new techniques and instruments, such as the photomultiplier tube and electronic signal generation, were used to increase the sensitivity and efficiency of detection. In 1948, Robert Hofstadter and his graduate student demonstrated that a sodium iodide crystal doped with thallium, NaI(Tl), produced better luminescence compared to all other doped crystals studied. Furthermore, Hofstadter s results showed that a linearity existed between the height of a pulse at a given energy and the probability of gamma emission at that energy. This discovery opened the gates to an efficient method to perform nuclear-gamma spectroscopy on gamma-emitting isotopes. NaI(Tl) Scintillation Detectors Lower energy beta particles cannot penetrate most scintillators due to the nature of interaction required for scintillation to occur; hence, scintillation detectors are limited to interaction with gamma radiation only. Gamma rays interact with a scintillator crystal via the photoelectric effect, compton scattering and pair production. The ejected photoelectron travels within the crystal and converts its energy into light photons by colliding with several other electrons. The energy carried by these light photons is directly proportional to the energy value of the incident gamma photon. A fraction of the exiting light photons encounter a photomultiplier tube (PMT) that is connected to the scintillator via silicon grease in order to minimise refraction and reflection. The PMT contains a photocathode in which the incident light photons eject photoelectrons; the efficiency of this phenomenon is 20% for an incident gamma energy centered at 415nm. The photoelectrons are then accelerated, in high voltage, towards the first of ten dynodes before being collected into the anode of the PMT (Hence, called a 10 stage PMT). Although the dynodes create a significant multiplication factor for each photoelectron, the charge created is relatively weak. Hence, a preamplifier is used to strengthen the raw signal before feeding it to 1
the multi channel analyser (MCA) system. Figure 1 below displays a graphical representation of the Scintillator/PMT/MCA setup: FIGURE 1: Schematic representation of the primary processes in the scintillator and PMT apparatus The resolution of this detection system is dependent on collecting as many light photons created by the gamma ray as possible. Light photons emitted in the scintillator crystal propagate in all possible angles; hence, an Al 2 O 3 /Teflon high efficiency reflector is placed around the scintillator to reflect the generated light towards the PMT region. The scintillator is enclosed in an aluminum shield to prevent external magnetic fields from decreasing the ability of electrons in the crystal to generate light photons. See Figure 2 below for a schematic diagram of the scintillation detector setup: FIGURE 2: Schematic diagram of the NaI(Tl) scintillation detector and source holder from Spectrum Techniques 2
Multichannel Analyzer (MCA) The pre-amplified voltage pulses arriving at the MCA are then amplified again and converted by and Analog-to-Digital (ADC) process. The digitized signal is then sorted into one of 1024 bins, where 0 represents a voltage pulse less than a hundredth of a volt and 1023 represents a pulse larger than 8V. Pulses between 0V and 8V are proportionately placed in an integer channel number between 0 and 1023. Hence, what is finally displayed on the computer screen is a histogram of the number of gamma counts as a function of the channel number. The channel number can be converted into energy units by using a suitable radioisotope that outputs known characteristic gamma energies. By selecting a minimum of 2 peaks corresponding to the expected gamma energies, software can then determine the channelenergy ratio and convert the x-axis into an energy spectrum. This calibration allows for the scintillation detector setup to be an efficient and accurate nuclear gamma spectroscopic device. Decay Schemes Figures 3 to 6 are schematic diagrams of various radioactive isotopes that will be used for this lab. Take note that in all figures, decays of the parent nucleus to the excited daughter state occur via beta minus (β " ) decay, beta plus (β $ ) decay or electron capture (EC). Hence, gamma emission is generated from the transition of the various daughter nuclei excited states to the ground state. FIGURE 3: Decay Scheme of 22 Na 3
FIGURE 4: Decay Scheme of 65 Zn FIGURE 5: Decay Scheme of 60 Co 4
FIGURE 6: Decay Scheme of 137 Cs EQUIPMENT The UCS30 MCA-based NaI(Tl) scintillator spectroscopy system; please see figure 7 for further details Associated software that allows for operation of UCS30 and data collection 137 Cs, 60 Co, 65 Zn and 22 Na radioactive sources that must be signed out and returned. FIGURE 7: THE UCS30 Scintillation System Left: the Scintillator and PMT are mounted on an aluminum shield that sits on a grating holder for radioisotopes. Right: the MCA device 5
PROCEDURE Part 1: Setup and Software 1. Obtain the following pieces of apparatus (refer to Figure 7): a. Grating holder/aluminum shield b. NaI(Tl) scintillator/ PMT piece, kept in the shield c. MCA analyzer box d. Two BCN-like cables, one with yellow tape on end e. Power source connector f. USB connector 2. Connect the high voltage cable (with yellow tape) from the HV terminal on the PMT to the high voltage terminal at the back of the MCA box. 3. Connect the second cable from the other terminal on the PMT to the input terminal at the back of the MCA box. 4. Connect the USB cable from the USB terminal at the back of the MCA box to the computer. 5. Connect the Power cable from the power terminal at the back of the MCA box to a power outlet on the lab bench. 6. Switch the MCA box on. 7. On the desktop, open Launchpad and open UCS30; this is the software program used to operate the MCA Analyser. Open MATLAB to prepare for data collection. Refer to UCS Voltage and Gain Calibration video to get familiar with the program. NOTE 1: Open the UCS30 program after you switch the box on; if done in reverse, the program may not connect properly. Check top of screen to verify successful connection. Part 2: Calibration of Voltage and Gain using 137 Cs 1. Sign out a 137 Cs source (please follow signing procedures), and insert the tray onto the topmost rack under the scintillation detector. 2. On the main screen, locate Auto Calibrate which is found under Settings-Energy Calibration. 3. After the calibration process is completed, note the value of the following parameters: a. High Voltage b. Course gain c. Fine gain 4. If your scintillation detector is: a. Black (and taller), set the high voltage value 50V lower b. Grey (and smaller), set the high voltage value 20V lower 5. Take out the 137 Cs source, keep it on the side (but far away from the detector). 6
Part 3: Calibration of Channels & Energies using 22 Na 6. Sign out a 22 Na source (please follow signing procedures). 7. Erase the previous graph by clicking on the button resembling an X. 8. On the main screen, confirm that the Y log scale button is on to set a logarithmic y-axis. 9. On the main screen, locate the Live Time field, input 600 (i.e. 10 mins) and press enter; live time refers to the time that the detector can actually register signals, hence, will be equal to or be longer than real time. 10. Place your 22 Na source into the topmost grating under the detector, and click the Start button; you should immediately see a spectrum emerging. 11. Once data collection is completed, run a 2-point calibration, under Settings-Energy Calibration. 12. Under units, confirm that the name is kev, and click Set. 13. To set channels and energy values, click on the top of the first major peak; the channel number will automatically populate; you must manually enter the energy value corresponding to that peak. Refer to UCS Finding Channel number video. 14. Before pressing Set, record the channel number in a new variable, Channel, on MATLAB, as well as the corresponding energy in a new variable Energy. 15. Click Set. 16. Repeat the steps 12 and 13 for the second major peak. 17. Click Set. Calibration of channels and energies is complete. 18. Print screen this graph for your report. Part 4: Energy/Channel Values of of 60 Co 19. Return the 22 Na source and sign out a 60 Co source (please follow signing procedures). 20. Erase the previous graph by clicking on the button resembling an X. 21. Confirm that all Voltage and Gain values are the same, and that the input on the Live Time field is 600 - click the Start button. 22. After the acquisition process is completed, Print screen this graph for your report. NOTE: The following procedure is only used to acquire the channel number of the major peaks. Hence, after acquiring the channel number, cancel the calibration. 23. Click on 2-point calibration, under Settings-Energy Calibration. 24. Under units, confirm that the name is kev, and click Set. 25. Click on the first major peak, and record the channel number displayed in MATLAB, as well as the corresponding energy value. 26. Cancel the calibration. 27. Repeat steps 23-25 for the second peak. **NOTE: Complete Analysis Questions 1-6 Before Proceeding Further** 7
Part 5: Energy/Channel Values of 137 Cs 1. Return the 60 Co source and insert the 137 Cs source (please follow signing procedures). 2. Erase the previous graph by clicking on the button resembling an X. 3. Confirm that all Voltage and Gain values are the same, and that the input on the Live Time field is 600 - click the Start button. 4. After the acquisition process is completed, Print screen this graph for your report. 5. Click on 2-point calibration, under Settings-Energy Calibration. 6. Under units, confirm that the name is kev, and click Set. 7. Click on the top of the major peak, and record the channel number displayed, as well as the corresponding energy value. 8. Cancel the calibration. Part 6: Background 1. Return the 137 Cs source. 2. Erase the previous graph by clicking on the button resembling an X. 3. Confirm that all Voltage and Gain values are the same, and that the input on the Live Time field is 600 - click the Start button. 4. After the acquisition process is completed, Print screen this graph for your report. DATA ANALYSIS 1. Display the following: a. High Voltage used b. Course gain used c. Fine gain used 2. Display the print screens of: a. 22 Na b. 60 Co 3. Provide the experimental values of the major peaks associated with 60 Co, the theoretical values associated with those peaks, and the experimental error percentage between the values. 4. Create a table in your report that displays the energy values and the associated channel values for the points recorded in the arrays channel and energy for 22 Na and 60 Co (Should be 4 sets of data). 5. On MATLAB, create a script that does the following: a. Plot the gamma energy values (y-axis) versus the channel numbers (x-axis) the plot should be a dot or cross plot, not a line plot. b. Label and title your graph c. Include the script and graph in your report d. Find the basic fitting tool on the graph menu: i. Plot a straight line to the data, and obtain the equation ii. Save the graph showing the equation in your report 8
**NOTE: Complete Procedure Part 5 & 6 Before Proceeding Further** 6. Using the linear fit equation, predict the channel number where the major peak of 137 Cs will take place. 7. Display the print screens of: a. 137 Cs b. Background 8. Determine the difference in channel numbers between the experimental observation and that calculated from the linear fit equation (Q6). 9. Determine the percent difference in energy value between the experimental and theoretical 10. Looking at the background energy spectrum: a. What is it s approximate energy value (peak)? b. What do you think is responsible for generating this energy peak? 11. Provide at least two errors pertaining to the experiment, and explain how these errors can be eliminated or improved upon. 9