Gamma-Gamma Correlations in Na-22 David Sergio

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

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 1.275 MeV gamma-ray

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

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.

A Typical MCA Spectrum (Na-22)‏

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.

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°

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

A Typical MCA Spectrum (Na-22)‏

Electron-positron pair production The gamma ray is converted into an electron-positron pair For photon energies greater than 1.022 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 1.022 MeV below the photopeak. These also may produce an annihilation peak at 511 keV

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.

Coincidence Setup

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°

Results

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

References Amptek “GAMMA-RAD Scintillation Detector” <http://www.amptek.com/grad.html> Middlebury College “Gamma Ray Spectroscopy” <http://cat.middlebury.edu/~PHManual/gamma.html> Melissinos & Napolitano Experiments in Modern Physics 2nd Ed.