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a mass of about . Since a proton and an electron had a combined mass of , this implied the neutron as a proton-electron composite had a binding energy of about , which sounded reasonable, although it was hard to understand how a particle with so little binding energy could be stable. Estimating such a small mass difference required challenging precise measurements, however, and several conflicting results were obtained in 1933-4. By bombarding boron with alpha particles, Frédéric and Irène Joliot-Curie obtained

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