1. Gamma-Gamma Correlations in Na-22
David Sergio

2. Outline Na-22 decay scheme Detection of Na-22 Decay
Spatial and temporal decay correlations in Na-22
The experiment and apparatus
Results and applications

3. Na-22 Decay Scheme Na-22 decays via β+ decay
The emitted positron quickly slows and annihilates with an electron, producing two 511 keV gamma-rays traveling in opposite directions.
The transition of Ne-22 to its ground state produces a MeV gamma-ray

4. Detection of Na-22 Decay
NaI(Tl) scintillators are used to detect photons
An MCA (Multichannel Analyzer) is used to get a spectrum of Na-22
An MCA accepts an input signal and bins it according to voltage, across 500 or 2000 channels, depending on the MCA
SCAs (Single Channel Analyzer) are used to “window” pulse heights of a specific voltage
An SCA accepts an input signal and outputs either a 1 or 0, depending on whether the input pulse is between an E and E+ΔE

5. Detection of Na-22 Decay
The probability of a scintillator interaction will depend the energy of the incident photon
Na-22 gamma rays will experience the photoelectric effect, Compton Scattering and pair production. 511 keV gamma rays will experience the photoelectric effect and Compton Scattering.

6. A Typical MCA Spectrum (Na-22)‏

7. Photoelectric effect
The gamma ray is completely absorbed, having all its energy transfered to an electron.
The electron's energy is then absorbed into the scintillator, causing it to “scintillate,” to emit light which is detected by the PMT.

8. Compton Scattering
Only some of the gamma ray energy is transferred to an electron
The photon scatters at θ = 0 to 180°. At 180° the electron has the greatest recoil energy (the photon has the most lost energy) This marks the “Compton Edge,” which has an energy of the original photon (at the photopeak) minus ΔE.
As energy decreases, the spectrum is said to have a “plateau” of 0° < θ < 180°

9. Compton Scattering
At low angle scattering, the photon strikes the apparatus itself and scatters at a large angle, and becomes absorbed. This is known as the backscatter peak.
The energy of the backscatter peak + the energy of the Compton Edge = the photopeak energy.
Because the energy of the backscatter peak = the energy of the 180° scattered photon, and the energy of the Compton Edge equals the energy of the incident photon minus the energy of the 180° scattered photon

10. A Typical MCA Spectrum (Na-22)‏

11. Electron-positron pair production
The gamma ray is converted into an electron-positron pair
For photon energies greater than MeV
These electrons and positrons will either escape the scintillator, or become absorbed. In the first case, no energy is lost, otherwise the “escape peak” will be 511 keV or MeV below the photopeak.
These also may produce an annihilation peak at 511 keV

12. Temporal and spatial correlations of gamma-gamma annihilation photons
Positrons, which annihilate with electrons in the source, will produce gamma rays traveling in opposite directions.
To test this prediction, a coincidence circuit can be used to test for a strong temporal correlation between two detections.

13. Coincidence Setup

14. The Experiment The SCAs were calibrated with known Na-22 peak energies
The SCAs were then windowed to allow only this energy (in this case at the 511 keV annihilation peak)‏
Measurements of the number of counts observed were taken at angles of 90° to 180°

15. Results

16. Applications
Gamma ray spectroscopy has wide applications, including radiation safety and security and general research
The resolution of these detectors is good for simple purposes, however more demanding applications may require solid state detectors, which are much more expensive, but have much higher resolution
One definition of resolution is FWHM / E, in the case of NaI Scintillators, it is about 10%
Solid state detectors use a semiconductor material layered in a pn junction mode to produce a “reverse biased diode,” through which small current will flow in the presence of ionizing radiation

17. References Amptek “GAMMA-RAD Scintillation Detector”
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Middlebury College
“Gamma Ray Spectroscopy”
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Melissinos & Napolitano
Experiments in Modern Physics 2nd Ed.