A photon nearing a nucleus and under its intense electric field splits up into a pair of particles -- an electron and a positron.
Revered Members,
What is the source of electric field? Is it the stationary positively charged nucleus or the electrons orbiting around the nucleus?

 

A photon nearing a nucleus and under its intense electric field splits up into a pair of particles -- an electron and a positron.
Revered Members,
What is the source of electric field? Is it the stationary positively charged nucleus or the electrons orbiting around the nucleus?

The main source of electric field is the positively charged nucleus.

The electrons are farther away and have diffuse distribution. Near the nucleus, the orbiting electrons tend to cancel each other's field which provides a "shielding" effect. The nucleus has all protons concentrated in a very small volume. As you get near that nucleus, the electric field from the protons becomes large and is very much larger than the field due to the electrons.

 

Thing is, they balance over the volume of the entire atom. At least that is my understanding.

 

they balance

What are you referring to as "they"?

 

The electrons and protons in an atom. When they are not balanced the atom is ionized, which leads to all sorts of interesting special effects, usually (but not always) a plasma.

 

If you are approaching the nucleus, presumably you are well within the electron 'orbitals'. The fields due to the electrons will essentially cancel out, or be much reduced, within the region of the nucleus. Thus leaving only the very strong field of the protons at close range.

 

The process is called pair 'production'

You should also study 'Bremsstrahlung'
which is (nearly) the reverse process.

go well