I was reading about the mass defect or $Q$-value in Beta Decay and the book that I was following said this, It iscalled beta minus decay as negatively charged beta particles are emitted. The rest mass energy of the initial constituents is $$U_i = [ m(X_{z}^a) -Zm_e]c^2$$

and that of the final constituents is $$U_r= [mY_{Z+1}^A-(Z+l)m_e]c^2.$$

The kinetic energy available to the product particles is,

$$Q= Ui- Ur= [m(X_{z}^a - mY_{Z+1}^A] c^2 .$$ Now I am not able to understand why was Mass of electrons subtracted in Ui as well as Uf. Any explanation would be appreciated. All the above formulas are referenced from Concepts Of Physics-HC Verma

  • $\begingroup$ The second equation of Ur is written incorrect or is incomplete, supposedly. You must read this topic thoroughly and learn that the correct equation of Ur, that which you've not written, is what really answers your question of why is, or should I actually say, why ' isn't ' the rest mass energy of the electron is not explicitly subtracted in this equation. $\endgroup$ Feb 13, 2021 at 12:07
  • $\begingroup$ @BlackThunder Is it still a question? If so, could you give a link (or reference) where are these formulas from? $\endgroup$
    – fanyul
    Feb 15, 2021 at 18:33


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