What kind of decay causes neptunium-238 to form uranium-238?
From the periodic table we can see that Neptunium has an atomic number of 93 and Uranium has an atomic number of 92. If neptunium-238 has to decay to form uranium-238, the atomic number has to decrease by 1 but the atomic mass remains the same at 238.
This is possible through the process of beta plus decay. In beta plus decay there is a conversion of a proton in the atom into a neutron with a positron and a neutrino also being produced during the process. Positrons and electrons are anti-matter counterparts of each other. Beta plus decay also has an accompanying positron emission.
N-238 --> U-238 + B+