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As we know, in a positron decay, by the involvement of weak forces, a proton converts into an neutron, emitting a neutrino and a positron, all of this happens inside the nucleus.

  1. Why doesn't the emitted positron, while coming out of the nucleus, and passing through the electron cloud annhiliate an electron?

  2. Now, the new nucleus has a proton less and a neutron more. Will the atom still remain electrically neutral? If yes, then the total protons must be equal to the electrons, since one proton is reduced in the interaction, one electron also must reduce, where does this electron go?

  3. Also, my Professor told me that some nuclei who aren't "sufficiently unstable" cannot undergo positron decay. Instead they undergo K-electron capture. What is the necessary condition for positron decay? And why does an electron get "captured' by the nucleus? Is there a force/interaction responsible for this?

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Why doesn't the emitted positron, while coming out of the nucleus, and passing through the electron cloud annhiliate an electron?

It might, if it happened to scatter on one, but the probability is low, because the electrons are in orbitals around the nucleus, countable, and there is a lot of empty space.

Now, the new nucleus has a proton less and a neutron more. Will the atom still remain electrically neutral? If yes, then the total protons must be equal to the electrons, since one proton is reduced in the interaction, one electron also must reduce, where does this electron go?

The atom will be a negative ion,and depending on the material, (gas, solid , liquid) it will lose an electron to the surroundings.

Also, my Professor told me that some nuclei who aren't "sufficiently unstable" cannot undergo positron decay.

"Sufficiently unstable" means there are lower energy levels for the nucleus, but the energy is not enough to generate a positron. Thus

Instead they undergo K-electron capture. What is the necessary condition for positron decay?

Enough transition between states energy to generate the mass of the positron.

And why does an electron get "captured' by the nucleus? Is there a force/interaction responsible for this?

If you look at the orbitals of electrons, the S ( zero angular momentum) orbitals have a probability of being in the nucleus. Depending on the potentials, their wavefunction will overlap with the wavefunction of a quark within the nuclei, and the weak interaction will allow the capture to happen.

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  • $\begingroup$ Positron decay and electron capture are 2 different modes of beta decay. The one example is 40K, that is one of few known isotopes undergoing all 3 modes of beta decay: e-, e+ and e- capture. en.wikipedia.org/wiki/Isotopes_of_potassium?wprov=sfla1 And it has the longest half life among positron emitters. $\endgroup$ – Poutnik May 9 at 6:42
  • $\begingroup$ So, Why does the electron get absorbed by the nucleus ? What is responsible for this? $\endgroup$ – user226375 May 9 at 7:21
  • $\begingroup$ @anna v , read this en.m.wikipedia.org/wiki/Electron_capture $\endgroup$ – user226375 May 9 at 7:23
  • $\begingroup$ "Electron capture is the primary decay mode for isotopes with a relative superabundance of protons in the nucleus, but with insufficient energy difference between the isotope and its prospective daughter (the isobar with one less positive charge) for the nuclide to decay by emitting a positron. Electron capture is always an alternative decay mode for radioactive isotopes that do not have sufficient energy to decay by positron emission. Electron capture is sometimes included as a type of beta decay,[1] because the basic nuclear process, mediated by the weak force, is the same."- Wikipedia $\endgroup$ – user226375 May 9 at 7:25
  • $\begingroup$ OK, I think you are correct. . I will edit . $\endgroup$ – anna v May 9 at 8:30

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