Question

why bita decay is possible as we know that the nucleus is positive charged

Answer 1

In beta minus decay a neutron in the nucleus spontaneously becomes a proton and an electron and and anti-neutrino .
\[n _{0}^{1} \rightarrow p _{1}^{1}, ^{+} + e^- + anti \nu _{e}\]
This is the result of a down quark inside the nucleon switching to become up.
The neutral neutron always had this potential to split it self into two charged state