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How was neptunium first prepared in 1940?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — actinoid element Modern version of the periodic table of the elements (printable).

Neptunium was first positively produced and identified in 1940 by American physicists Edwin Mattison McMillan and Philip Hauge Abelson at the University of California at Berkeley.12

The preparation involved exposing uranium oxide to neutrons generated by a cyclotron target. The process began with the uranium-238 isotope capturing a neutron to become uranium-239. This uranium-239 nucleus then underwent negative beta-particle decay, a process in which a neutron within the nucleus transformed into a proton by emitting a high-velocity electron (a beta particle).12

Because the beta particle carries a negative charge, the loss of this particle increased the positive charge of the nucleus by one, resulting in an atomic number of 93 while the mass number remained 239. This newly formed nucleus was identified as an isotope of the element neptunium, marking the first time a transuranium element had been artificially produced and confirmed.123