Physics 476LW Modern Physics Laboratory
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1. 5 E Where E is the Full Width of the peak at Half of the Maximum count level FWHM measured in number of channels and E is the channel number at the centroid i e the mid point of the photopeak In Fig 3 the photopeak is in channel 280 and its FWHM 32 channels From Eq 5 the resolution is calculated to be 11 5 It is standard practice to specify the quality of the Nal T1 detector by stating its measured percent resolution on the 662 keV photopeak from a Cs radioactive source The resolution of a scintillation detector depends upon several factors Among them are 1 number of photons per scintillation event number of photons that strike the photocathode number of photoelectrons released per photon hitting the photocathode number of photoelectrons that strike the first dynode multiplication factor of the photomultiplier tube peo aet trode a In general the greater each one of the above quantities is the better i e smaller the resolution Factors 1 and 2 refer to properties of the scintillator while factors 3 3 and 5 refer to the phototube Mathematically we say that R a B JE 6 where R is the resolution a and b are constants referring to the scintillator and phototube respectively and Eg is the g ray energy Note that R varies inversely as 4 E and thus the higher Eg is the smaller R is Procedure 1 Using the same amplifier and analyzer gain settings as before measure the g ray spectra of Co2. EFTE TT Pulser Litt LZ LALL i 10 Amplifier ORTEC 572 Resolution at 1 33 meV 1 9 keV FWHM 3 6 keV FWTM Peak to Compton Ratio 37 1 1 Channel 138 eV pap pe E Detector ORTEC Coaxial HPGe 10 compared to spectrum of Nal T 3 x 3 in a oO Mele md A Se Pa el ae 1500 2000 Channel Number Detector tut 3500 Fig 7 1 A Portion of aCo Spectrum Illustrating the Energy Resolutions and Peak to Compton Ratlos for a Coaxlal HPGe Detector Compared to a Nal TI Figure 1 Comparison of peaks from HPGe and Nal TI detectors From ORTEC publication Experiment 7 High Resolution Gamma Ray Spectroscopy 2 The Experiment 2 1 The Apparatus Integrated Nal T1 Scintillator Photomultiplier Tube and PMT Base with Stand Lead shielding Multi Channel Analyzer MCA Windows computer with PCI card MAESTRO 32 software A set of radioactive samples Department of Physics May 30 2013 University of Missouri Kansas City 1 of 6 ESCAPE OF y ONE K X RAY BETA ABSORBER _ GREMSSTRAHLUNG r ANNIHILATION COMPTON d 4 RADIATION SCATTERING PHOTON r REFLECTOR ANNIHILATION RADIATION UV PHOTONS Pa SHIELDING Produced from Local Excitet Pb SHIELDING States Following lomusbon PHOTOCA THODE Pb X RAY S PHOTORMUL TIPLIER PHOTOELECTRON JE reteed trom Cothonte Anoor Figure 2 Diagram of Na
3. and Mn Tabulate these results along with those obtained for Co and Cs 2 Using the equation 5 which defines resolution calculate the resolution for each of the g ray energies and put them in your table 3 Plot the resolution R vs 1 VE and on a second graph R vs Eg Determine the parameters alpha and beta from the plot of R vs 1 4 E and equation 6 Discuss the plots that you obtained References and Acknowledgements Sections marked with an are from the ORTEC Experiments in Nuclear Science Experiment 3 Gamma Ray Spectroscopy Using Na TI Detectors Department of Physics May 30 2013 6 of 6 University of Missouri Kansas City
4. Physics 476LW Modern Physics Laboratory Gamma Ray Spectroscopy Introduction The purpose of this experiment is to acquaint the student with some of the basic techniques used for measuring g rays It is based on the use of a thallium activated sodium iodide detector Nal Tl Much of this manual is adapted from the ORTEC documents Experiment 3 Gamma Ray Spectroscopy Using Nal Tl and MAESTRO 32 MCA Emulator Software User s Manual There are a number of types of detectors for gamma radiation and they are used in a variety of areas of physics For example g ray detectors that were used for studying g rays produced by electron positron annihilation near black holes detected g rays produced by lightning on Earth g ray production by lightning had been unknown and the serendipitous measurements opened up a variety of questions that continue to be studied The premium solid state g ray detector is high purity germanium HPGe Although it produces excellent results it is very expensive and a high level of expertise is need use it In this lab we use a Nal TI detector which is a scintillation type detector One of the weaknesses of Nal TI is that it does not have the energy resolution of HPGe as can be seen by comparing the width of the peaks in Figure 1 Therefore one part of our experiment will be to determine the energy resolution of our Nal T1 detector Counts Per Channel T Co Spectrum T 1 17 MeV v Nata t Me
5. d gamma ray can be determined by solving the energy and momentum conservation equations for the collision The solution for these equations in terms of the scattered gamma ray energy can be written as E oe D 2 1 l cos0 mc where Ey is the reduced energy of the scattered g ray y in MeV O is the scattering angle for the scattered g ray relative to the incident g ray E is the energy of the incident g ray y in MeV moc 0 511 MeV is the equivalent energy of the rest mass mo of the electron and c is the speed of light Department of Physics May 30 2013 4 of 6 University of Missouri Kansas City For a head on collision the gamma ray is scattered backwards along its initial trajectory and 0 180 For this condition the backscattered gamma ray energy becomes E Bae TS 2 Y Where the convenient approximation mocy 2 has been used Note for the units to work the E in the denominator is written in terms of MeV but units are not included with the numerical value If this backscatter event happens in the detector the maximum energy transferred to the recoiling electron will be E E E 4 y Thus the maximum energy that can be recorded in the spectrum for a gamma ray that interacts in the detector by Compton scattering is given by equation 4 This defines the energy of the Compton edge in Figures 3 and 4 For an initial gamma ray energy of 1MeV equations 5 and 6 predict that the Compton ed
6. ge will occur at 0 80 MeV and the energy of the backscattered gamma ray will be 0 20 MeV Because the gamma ray photon can be scattered through any angle from 0 to 180 and the scattered photon can escape the detector the energy deposited in the detector can vary from the maximum at the Compton edge through all values down to zero This is the genesis of the Compton continuum in Figures 3 and 4 Find a backscatter peak in this spectrum It s energy plus the Compton edge energy equals the energy of the main photopeak Note that there is a small but non zero probability that the Compton scattered photon will be subsequently absorbed in the crystal by the photoelectric process This two step interaction will generate a pulse that falls in the full energy peak Compton scattering from an entirely different location causes the backscatter peak in Figures 3 and 4 Consider a gamma ray emitted by the radioactive source in a direction heading away from the detector This gamma ray can encounter material in the neighborhood of the radioactive source and undergo Compton scattering If the scattering angle is 180 the scattered gamma ray travels back towards the detector with an energy defined by equation 5 If this lower energy gamma ray interacts in the scintillator by the photoelectric effect it will contribute to a photopeak at the lower energy Typically this backscatter peak will be of low intensity if there is minimal material behind the radioactive sou
7. l T1 g ray scintillation detector system 2 2 Startup Procedures The apparatus will be set up for this experiment Complete the following steps Log onto the computer using your university username and password Start the MAESTRO 32 software Under tab Acquire select option MCB Properties Select tab high voltage and select On The high voltage is 800 V 2 3 Spectra Collection Calibration The first task that must be performed is calibration The detector consists of 1024 channels of a multichannel analyzer MCA system The system has a linear relation between the height of the pulse coming from the gamma ray detector and the energy deposited in the detector by a g ray We will calibrate the system to determine the slope and y intercept of the calibration function Calibration procedure e Under Acquire select the Clear option e Place the Cs sample on the blue sample holder and slide it into the top of the sample chamber rack Make sure the lead absorber is in the slot of the rack just below the sample Close the door on the sample chamber Department of Physics May 30 2013 2 of 6 University of Missouri Kansas City Click the Go icon and collect counts until there are several hundred counts in the channels e Click the Stop icon and save the data in an SPE file on the desktop with a name that you will be able to recognize later e Remove the Cs and replace it with Co e Under Acquire select the C
8. lear option e Close the door and collect a sample for the Co e Save again The energy data given for Cs and Co and the channel number of these peaks are three data points from which you can determine the coefficients of the calibration equation E m channel number b 1 where E is the g ray energy in MeV m is the slope of the calibration line and b is the intercept on the energy axis Item Energy MeV Channel Number 1 Cs 0 662 2 Co 1 17 3 i 1 33 Table 1 Determine m and b and plot the calibration curve T T T T T T T T 350 T T T T T T T PHOTOPEAK Event Energy MeV Channel No E CHANNEL 280 1 17 MeV Peak Photopeak 0 662 280 300 Compton Edge 0 478 180 lt Backscatter 0 184 100 b a 1 33 MeV Peak 250 Resolution 2 x 100 250 E Compton Edge q for 1 33 MeV 22x 100 P 4 BACKSCATTER 3 280 F 200 4 11 5 e 2 3 p H 5 BACKSCATTER FWHM 32 CHANNELS S so on BIE 3 iso a comproneoce 100 g Pr wol al tarn Y so Q d Event Energy MeV Channel No Photopesk 1 33 560 J bod a Photopeak 1 17 496 Backscatter 0210 80 ail 1 1 1 t 3 0 40 80 120 160 200 240 280 320 360 o 1 1 L 1 1 1 1 Channel Number g a 160 20 e 00 seo sso annel Numi Figure 3 Cs peak Figure 4 Co peaks EXPERIMENT 3 2 Energy Analysis of an Unknown Gamma Source 3 2 1 Purpose This experiment uses the calibrated system
9. ocess the number of ionized atoms is proportional to the original energy of the photon As the electrons refill the vacancies in the ionized atoms visible light photons are generated This is the source of the scintillation wherein the number of visible photons is proportional to the original energy of the gamma ray Consequently the event populates the photopeak in the spectrum This peak is often called the full energy peak because a two step interaction a Compton scattering followed by a photoelectric interaction also contributes a small number of events to the full energy peak The Compton interaction is a pure kinematic collision between a gamma ray photon and what might be termed a free electron in the Nal TI crystal By this process the incident gamma ray photon gives up only part of its energy to the electron as it bounces off the free electron The recoiling electron loses energy by causing ionization as it travels through the crystal Thus the number of visible photons in the resulting scintillation is proportional to the recoil energy of the Compton electron The amount of energy transferred from the gamma ray photon to the recoiling electron depends on whether the collision is head on or glancing For a head on collision the gamma ray transfers the maximum allowable energy for the Compton interaction Although it involves a photon and an electron the interaction is similar to a billiard ball collision The reduced energy of the scattere
10. of Experiment 3 1 to measure the photopeak energies of an unknown g ray emitter and to identify the unknown isotopes of the elements 3 2 2 Procedure 1 Erase the Co spectrum from the multichannel analyzer MCA but do not change any of the gain calibration settings of the system Department of Physics May 30 2013 3 of 6 University of Missouri Kansas City 2 Obtain an unknown gamma source from the instructor Accumulate a spectrum for the unknown sources for a period of time long enough to clearly identify its photopeak s From the calibration curve determine the energy for each photopeak EXERCISE Use the library services of MAESTRO 32 to identify the unknown isotope EXPERIMENT 3 3 Spectrum Analysis of Co and Cs 3 3 1 Purpose The purpose of this experiment is to explain some of the features other than the photopeaks usually present in a pulse height spectrum These are the Compton edge the Compton continuum and the backscatter peak 3 3 2 Background Information The photopeak is created when a gamma ray photon interacts in the scintillator via the photoelectric effect The photon encounters an electron that is tightly bound to a nucleus The entire energy of the photon is transferred to the electron causing the electron to escape from the atom and the gamma ray disappears in the process As the photoelectron travels through the scintillator it loses its energy by causing additional ionization At the end of the pr
11. rce Usually the backscatter peak is rather broad because of the range of directions that can contribute to the peak For initial gamma ray energy of 1 MeV equation 5 predicts that the backscatter peak will occur at 0 20 MeV Figure 2 illustrates some of the types of interactions that can take place in the Nal Tl detector and the surrounding shielding material a Calculate the energy of the Compton edge for the 0 662 MeV gamma ray from Cs Enter this value in Table 1 From your plot and calibration curve does this calculation agree with your measured value b Calculate the Compton edge energy for the 1 33 MeV gamma ray from Co and enter that result in Table 1 Is that value in agreement with your Co spectrum c From Eq 3 calculate the backscatter peak energies for the gamma rays from Cs and Co Fill in the rest of Table 1 How do your measured energies compare with the theoretical energies from Eq 3 Energy Resolution Department of Physics May 30 2013 5 of 6 University of Missouri Kansas City Purpose The purpose of this part of the experiment is to measure the resolution of the Nal TI detector Relevant Equations The resolution of a spectrometer is a measure of its ability to resolve i e separate two peaks that are fairly close together in energy Fig 3 shows the gamma spectrum that was plotted for the Cs source The resolution of the photopeak is calculated from the following equation R we x 100
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