hi tiny-tim !
Yes I think so, then it is the energy and momentum conservation that places a restriction
on the positron to have the same energy as electron , right ?
I was a little bit confused by the "... in the filed of nucleus" part of the question, then
this filed of a nucleus does not change much (except maybe the needed energy).
So, the needed energy for pair creation is E=2*5442=10.88keV