What is the daughter nucleus produced when 59cu undergoes positron emission?

The daughter nuclide formed when copper undergo positron emission is nickel, with atomic number z= 28.
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⁶³ ₂₉Cu →⁺¹₀ e + ⁶³ ₂₈

In this decay , the a proton in the nucleus of copper changes to a neutron, a positron is emitted, the atomic number reduces by one why the atomic mass remain unchanged.