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What is Annihilation Peak – Spectrum – Definition

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An annihilation peak may be observed for high energy gamma rays (greater than 1.02 MeV), where electron-positron pair production is a possible interaction. Radiation Dosimetry

Analysis of gamma spectra is very interesting, since it has a structure and workers must distinguish between true pulses to be analyzed and accompanying pulses from different sources of radiation. We will show the structure of the gamma spectrum on the example of cobalt-60 measured by NaI(Tl) scintillation detector and by the HPGe detector. The HPGe detector allows the separation of many closely spaced gamma lines, which is very beneficial for measuring multi-gamma emitting radioactive sources.

cobalt-60 decay scheme

Cobalt-60  is an artificial radioactive isotope of cobalt with a half-life of 5.2747 years. It is synthetically produced by neutron activation of cobalt-59 in nuclear reactors. Cobalt-60 is a common calibration source found in many laboratories. The gamma spectrum has two significant peaks, one at 1173.2 keV and another at 1332.5 keV. Good scintillation detectors should have adequate resolution to separate the two peaks. For HPGe detectors, these peaks are perfectly separated.

As can be seen from the figure, there are two gamma ray photopeaks. Both detectors also show response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.

HPGe Detector spectrum
Figure: Caption: Comparison of NaI(Tl) and HPGe spectra for cobalt-60. Source: Radioisotopes and Radiation Methodology I, II. Soo Hyun Byun, Lecture Notes. McMaster University, Canada.

 

Annihilation Peak

For high energy gamma rays (greater than 1.02 MeV), electron-positron pair production is a possible interaction. But a positron may then annihilate in the detector or in the surrounding material. If both 0.51 MeV annihilation photons are absorbed in the crystal, a secondary peak 1.02 MeV below the photopeak can be found. If one annihilation photon is absorbed and the second exits the crystal, then 0.51 MeV is removed from the photopeak, resulting in a secondary peak 0.51 MeV below the photopeak. The probability is greater that one annihilation photon will be absorbed.

References:

Radiation Protection:

  1. Knoll, Glenn F., Radiation Detection and Measurement 4th Edition, Wiley, 8/2010. ISBN-13: 978-0470131480.
  2. Stabin, Michael G., Radiation Protection and Dosimetry: An Introduction to Health Physics, Springer, 10/2010. ISBN-13: 978-1441923912.
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Nuclear and Reactor Physics:

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  9. Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.

See also:

Gamma Spectroscopy

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