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Question
What is gamma decay?
Solution
Gamma rays are emitted by the parent nucleus. As you know, gamma ray is a high energy photons. The daughter nucleus is the same as the parent nucleus as no other particle is emitted, but it has less energy as some energy goes out in the form of the emitted gamma ray.
The electrons in an atom are arranged in different energy levels (orbits) and an electron from a higher orbit can make a transition to the lower orbit emitting a photon in the process. The situation in a nucleus is similar. The nucleons occupy energy levels with different energies. A nucleon can make a transition from a higher energy level to a lower energy level, emitting a photon in the process.
Usually, the nucleons in a nucleus are in the lowest possible energy state. They cannot easily get excited as a large amount of energy (in keVs or MeVs) is required for their excitation. A nucleon however may end up in an excited state as a result of the parent nucleus undergoing alpha or beta decay. Thus, gamma decays usually occur after one of these decays.
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