Francis Aston - English Physicist
(1877-1945)
1919
Aston invents the Mass Spectrometer, a device capable of separating isotopes of an element and also accurately measuring isotope masses. Aston initially separated neon into Ne-20 and Ne-22. Over the years, he also found that the mass of an isotope's nucleus never equaled the sum of the masses of its protons and neutrons. Aston reasoned that part of the mass of the protons and neutrons was converted into energy, which could be calculated using E = mc^2. Over the next two decades, Aston would identify 212 of the known stable isoptopes.

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J. J. Thomson had shown in 1897 that charged particles could be deflected by magnetic and electric fields and that the degree of the deflection depends upon the masses and electric charges of the particles. In the mass spectrometer, different isotopes of an element enter the device and are ionized. The ions are then accelerated through a magnetic field which bends the ion paths into a semicircular shape. The radius of this path is dependent upon the mass of the particle (with all other factors such as speed and charge being equal). Thus isotopes of different masses can be separated.

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