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Gamma Spectroscopy

Gamma Spectroscopy. Chris Clark. Miguel Cruz. Outline. Introduction of Concepts Nuclear States Types of Decay Interaction with Matter Experimental Realization Experimental Setup Detector Operation Multichannel Analyzer Results and Analysis Calibration Curve Resolution Curve

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Gamma Spectroscopy

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  1. Gamma Spectroscopy Chris Clark Miguel Cruz

  2. Outline Introduction of Concepts Nuclear States Types of Decay Interaction with Matter Experimental Realization Experimental Setup Detector Operation Multichannel Analyzer Results and Analysis Calibration Curve Resolution Curve Conclusions Spectral Features What’s Missing Unknown Sample

  3. Nuclear States • Nucleons occupy shells, analogous to electron shells, with associated quantum states. • The energies of states are based on the spin, angular momentum, and geometry of the nucleus. • Nuclei tend towards the lowest energy configuration using decay transformations.

  4. Types of Decay Observable Decay Occurred, but not observable

  5. Interaction with Matter • Photoelectric Effect • A photon is fully absorbed by an electron. Momentum is conserved between the electron and its atom. • Compton Scattering • A photon is partially absorbed by an electron. Momentum conserved between electron and photon. • Pair Production • A photon with energy greater than 1.02 MeV forms an electron positron pair in the influence of the electromagnetic field of an atom. Momentum is conserved between the electron and positron.

  6. Photoelectric Effect The photoelectric effect is dominant in the low energy end of the spectrum.

  7. Compton Scattering • It is crucial to remember that both energy and momentum must be conserved in this interaction. • With just energy conservation, it is still possible for the electron to absorb all of the photon’s energy • But with momentum conservation added, the amount of energy absorbed becomes a function of scattering angle • When =180, this energy is a maximum, which we write as EC EC(E) = E/(1+mec2/2E)

  8. Pair Production • Only occurs when in interaction with matter because on its own there is some frame of reference in which the relativistic Doppler shift reduces its frequency to below the frequency corresponding to 1.02MeV of energy. • The positron that is created will only annihilate with an electron when its velocity has been reduced to nearly the velocity of the electron. • Upon annihilation, two gamma rays are emitted, each with energy of 511KeV, and both in the plane created by the trajectories of the positron and electron

  9. Experimental Setup • Lead Oven (for noise reduction) • Bicron 1.75 m2/2 NaI(TI) detector • ADIT Photomultiplier Tube (PMT) • Ortec Linear Amplifier • LeCroy Multi Channel Analyzer (MCA) • Data Acquisition Pascal Program

  10. Detector Operation • Radiation is released by the source and enters the detector where it undergoes the previously described interactions with the scintillator crystal. • Electrons in the scintillating crystal become excited and then de-excite releasing near-visible photons. • The photons strike a photocathode where photoelectrons are released. • The photoelectrons are then multiplied and accelerated by a photomultiplier tube (PMT) which causes a charge pulse that is linearly related to the energy deposited. • The pulse of charge is then linearly converted to a voltage pulse which is measured by a multi-channel analyzer (MCA). Note - All the energy collected by the detector is summed over an interval of about a microsecond.

  11. Multichannel Analyzer • A multichannel analyzer is a data acquisition instrument that counts pulses of voltage and stores them in channels corresponding to the maximum amplitude of the pulse • The counts are displayed as a histogram through an output to an oscilloscope or computer • It is important that the channel assignments are linearly related to the pulse heights so that the channel will be linear with deposited energy

  12. Calibration E(c) = 2.1665*c-193 • A calibration curve is used to convert the channels • of the multichannel analyzer into actual energies • Using the fact that the channels are linear with • energy and knowing the energy values of certain definite peaks we may find a linear fit.

  13. Resolution • Resolution measures how often events are incorrectly counted in neighboring channels • Resolution at energy E is defined to be the FWHM of a photopeak of energy E, divided by E • By determining the resolution at a set of peaks at different energies, resolution can be expressed as a function of energy • To do this, we created an algorithm that fits Gaussian curves to each photopeak and returns the FWHM • Then we performed a power regression to determine the functional form Definition: Resolution(E) = FWHM/E Typical Result: Resolution(E) = A*E-0.5

  14. Resolution R(E) = 1565*E^(-0.8) Calculated resolutions Power Regression Fit

  15. Decay Scheme • Decay scheme diagrams trace radionuclides through the intermediate isomers on the way to a stable configuration • Slanted lines represent decays that change the radionuclide • Vertical lines represent nuclear deexcitation • Our experiment has the ability to directly detect only the gamma transitions

  16. General Spectrum

  17. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Spectral Features • Decreasing Authenticity • Primary radiation would be seen in any case • Secondary radiation from matter is just an • artifact of the presence of matter around any • measurement system • Escape from detector is just an artifact of the inability of detectors to retain all the energy • Detector anomalies are purely limitations of the detector and do not represent physics

  18. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Cs-137 Cesium beta decays into an excited nuclear state of Barium, which then deexcites via gamma emmission, producing this peak.

  19. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Cs-137 Internal Conversion occurs when the multipole electric fields of the excited Barium nucleus couple to the orbital electrons, causing one to be ejected. The electron is never detected, but when another electron fills the vacancy, a characteristic X-ray is emitted.

  20. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Cs-137 As a result of the maximum electron energy, there is a minimum energy of the scattered photon, which corresponds to the Backscatter Peak.

  21. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Na-22 For isotopes like Sodium-22 that decay via positron emission, a peak at 511 KeV is observed as a result of the annihilation of the positron with electrons creating two gamma rays with energy of 511 KeV. Only one of the gamma rays is detected because they radiate in opposite directions because of conservation of momentum.

  22. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Na-22 Gamma rays emitted by the radioisotope can strike the lead shielding and excite electrons, causing fluorescence when another electron fills the vacancy. This is the only energy where the counts are noticeably higher using the lead backscattering body.

  23. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Na-22 If Compton Scattering occurs in the detector, all of the energy will still be summed unless if the scattered photon escapes, which leaves only the kinetic energy of the electron, which has a maximum corresponding to the Compton Edge. The Compton Continuum is the range of energies from 0 to the Compton Edge.

  24. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Co-60 If Pair Production occurs in the detector, then all of the energy will be absorbed unless one or two of the photons escape from the detector. If one escapes, then it will contribute to a peak at E-511 KeV, and if two escape, it will contribute to a peak at E-1.02MeV.

  25. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Co-60 Due to limitations in the NaI detector and equipment, pulses do not always register in the exact channel that would be expected.

  26. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Co-60 Since the detector collects all energy that is deposited within about a microsecond, whenever two or more events occur within this timeframe, they will cause counts for the sum of the energies.

  27. Primary Radiation • Nuclear Deexcitation •  Photopeak • Internal Conversion • Photopeak • Secondary Radiation from Matter • Compton Scattering •  Backscatter Peak • Annihilation • 511 KeV peak Fluorescence 74.96 KeV peak • Escape from Detector • Compton Scattering •  Compton Continuum • Pair Production • Escape Peaks • Detector Anomalies • Poor Resolution •  Peak Spreading • Simultaneous Detections •  Summing Na-22 Since the detector collects all energy that is deposited within about a microsecond, whenever two or more events occur within this timeframe, they will cause counts for the sum of the energies.

  28. What's Missing? Photoelectric Effect Photoelectric Effect is not observed because if it happens outside the detector, then it is not detected, and if it happens inside the detector then it is always detected. Pair Production outside of detector One might expect to see a 511KeV peak from any radioisotope that emits radiation over 1.02MeV. However, because of conservation of momentum, the original gamma ray would have to be traveling roughly toward the detector, and there is not much matter in this path to stimulate pair production.

  29. The Unknown Our measurements of the unknown sample returned little more than the background. Fortunately, the unknown sample was labeled C-14. It turns out that Carbon-14 decays to a Nitrogen-14 via Beta emission, so our apparatus is not sensitive to its radiation.

  30. Resources • Adrian Melissinos Experiments in Modern Physics • Kai Siegbahn Beta and Gamma Ray Spectroscopy • C.E. Crouthamel Applied Gamma-Ray Spectroscopy • Charles Lederer Table of Isotopes 8th Edition • www.cas.muohio.edu/~marcumsd/p293/lab9/lab9.htm • hyperphysics.phy-astr.gsu.edu • electron5.phys.utk.edu/gamma/instumentation.htm [sic] • www.scionixusa.com/response.html

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