Charge radius
The rms charge radius is a measure of the size of an atomic nucleus, particularly of a proton or a deuteron. It can be measured by the scattering of electrons by the nucleus and also inferred from the effects of finite nuclear size on electron energy levals as measured in atomic spectra.
Definition
The problem of defining a radius for the atomic nucleus is similar to the problem of atomic radius, in that neither atoms nor their nuclei have definite boundaries. However, the nucleus can be modelled as a sphere of positive charge for the interpretation of electron scattering experiments: because there is no definite boundary to the nucleus, the electrons "see" a range of cross-sections, for which a mean can be taken. The qualification of "rms" (for "root mean square") arises because it is the nuclear cross-section, proportional to the square of the radius, which is important for electron scattering.
History
The first estimate of a nuclear charge radius was made by Hans Geiger and Ernest Marsden in 1909,[1] under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester, UK. The famous experiment involved the scattering of α-particles by gold foil, with some of the particles being scattered through angles of more than 90°, that is coming back to the same side of the foil as the α-source. Rutherford was able to put an upper limit on the radius of the gold nucleus of 34 femtometres.[2]
Modern measurements
References
- ↑ Geiger, H.; Marsden, E. On a Diffuse Reflection of the α-Particles. Proc. Roy. Soc., Ser. A 1909, 82, 495–500. DOI: 10.1098/rspa.1909.0054.
- ↑ Rutherford, E. The Scattering of α and β Particles by Matter and the Structure of the Atom. Phil. Mag., Ser. 6 1911, 21, 669–88. DOI: 10.1080/14786440508637080.
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