When a low energy positron collides with a low energy electron, there is an emittance of two gamma rays in the opposite direction. This phenomenon, also called positron annihilation, results in two gamma rays at 180° and with 0.511 MeV of energy moving in opposite directions…
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The gamma rays released can be detected and measured. A source of positrons and a detector for gamma radiation is required to study positron annihilation. The positron annihilation experiment involves the use of two detectors placed at an angle anywhere between 160-200°. 22Na and 60Co are used in the experiment. The scintillation detectors are used not only for the detection but also measurement of gamma radiation. Electronic counters are used for investigating the annihilation events. The observations made regarding the annihilation events in case of 22Na were found to be in agreement with theoretical considerations. The angular distribution of gamma rays from a 60Co source was also investigated. This part of the experiment provided rather convoluted information. The gamma rays resulting from annihilation events were found to be more spread rather than peaked. Such a behavior can be attributed to the changes in the angular momentum of the 60Co nucleus as it progresses through its unstable, excited state. Observations made in this experiment are in agreement with theoretical observations. The experiment demonstrates back-to-back emission of annihilation photons, peaking at 180°. The coincidence events for 22Na were found to be more reliable than for 60Co.
Collision of a low energy positron by an electron of low energy results in their annihilation, causing the production of gamma ray photons. They are responsible for carrying away the momentum and energy of the pair. e? + e+ > ? + ? These photon rays do not have enough mass and energy to result in the production of heavier particles. The process of positron annihilation satisfies certain laws of conservation such as the conservation of electric charge, conservation of total energy, and conservation of momentum (both linear and angular). The conservation of linear momentum and energy does not accommodate the creation of one photon but rather two gamma rays. The two gamma rays ensuing from the positron annihilation move in the opposite direction. The energy possessed by these two gamma rays is approximately 0.511 MeV (Mega electron Volts). Figure 1: Feynman diagram of positron annihilation (Booklet, 2012) The two gamma rays are created because there is no momentum in the system during annihilation as both the electron and the positron come to rest for a short moment. The momentum of the system cannot be conserved if only one proton is created in the process. The collective amount of 1.022 MeV energy of the two gamma rays that are moving in opposite directions satisfies the conservation of the momentum and energy. The positron used in the annihilation process is often obtained from the decay of a proton into a neutron, resulting in the release of a positron and a neutrino. Figure 2: Emission of positron and its annihilation (PET, 2007) The gamma rays released in the process of positron annihilation are both detectable and measurable. Time coincidence counting is a technique that is employed for studying radioactive materials. With this technique, radioactive materials can be detected and identified. The disintegration rates of the radioactive materials can also be calibrated with this technique. The absolute activity of the materials can be measured by counting the radiation events occurring in the radioactive material. The radiation could comprise of beta and gamma rays and can be easily measured using the technique. During the decay of an unstable nucleus, several photons may be emitted in a cascade. 60Co nucleus is one such example. In 22Na, the decay product, which is a positron, annihilates in the source itself. To induce positron annihilation or study it, there is a need for a positron emitter. Various materials have been used
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