Transcript Document
Chris Clark Miguel Cruz
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
•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.
Transformation
Spontaneous Fission Neutron Emission Alpha Emission Positron Emission Electron Capture Beta Emission Gamma Emission Isomeric Transition Internal Conversion SF n
Symbol
+ EC
-
IT IC,e-
Daughter Z
Various Z Z-2 Z-1 Z-1 Z+1 Z Z Z Observable Decay Occurred, but not observable
• 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.
The photoelectric effect is dominant in the low energy end of the spectrum.
• It is crucial to remember that both
energy and momentum must be conserved
• When
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 =180
, this energy is a maximum , which we write as E C E C (E) = E/(1+m e c 2 /2E)
• 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 emitted, each with energy of 511KeV, and both in the plane created by the are trajectories of the positron and electron
• • • • • •
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
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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 released. where photoelectrons are 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.
• 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
E(c) = 2.1665*c-193
• A calibration curve is used to
convert the channels of the multichannel analyzer definite peaks we may find a into actual energies
• Using the fact that the channels are linear with
energy and knowing the energy values of certain linear fit .
• 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
the functional form algorithm that fits Gaussian curves to each photopeak and returns the FWHM
• Then we performed a power regression to determine
Definition: Resolution(E) = FWHM/E Typical Result: Resolution(E) = A*E -0.5
R(E) = 1565*E^(-0.8) Calculated resolutions Power Regression Fit
• 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
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 Decreasing Authenticity
• Primary radiation would be seen in
artifact of the presence of matter any case
• Secondary radiation from matter is just an
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
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.
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 another electron ejected fills the vacancy . The electron is never detected, but when , a characteristic X-ray is emitted.
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.
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 conservation of momentum.
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
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 another electron backscattering body.
and excite electrons, causing fluorescence when fills the vacancy noticeably higher using the lead . This is the only energy where the counts are
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.
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.
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.
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 .
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 .
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.
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 .
• 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 8 • hyperphysics.phy-astr.gsu.edu
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• www.scionixusa.com/response.html
Edition
• www.cas.muohio.edu/~marcumsd/p293/lab9/lab9.htm • electron5.phys.utk.edu/gamma/instumentation.htm [sic]