Precision Measurement of XRay Fine Structure; Effects of Nuclear Size and Quantum Electrodynamics
Abstract
Schawlow and Townes have made a theoretical calculation of the perturbing effect of the finite size of the nucleus on the L_{II}L_{III} xray doublet splitting in heavy elements. Combined with approximate calculations by Christy and Keller of the unperturbed splitting for the case of a point nucleus, comparison of this theory with such experimental values of the splitting as were then available led to an anomalously large value of nuclear radius, R=r_{0}A^{13} with r_{0}=2.1×10^{13} cm. Schawlow and Townes offered the suggestion to account for this that quantum electrodynamic effects probably modify the fine structure in much the same way as an oversize nucleus. The present investigation was undertaken to improve on the precision of the xray measurements yielding the L_{II}L_{III} fine structure splitting and to incorporate into a new comparison between theory and experiment the recent vacuum polarization correction of Wichmann and Kroll. The measurements of the L_{II}L_{III} splitting for W, Pt, Bi, Th, U, and Pu are based on twocrystal spectrometer determinations of the Bragg angles of the Lα_{2} and Lβ_{1} xray lines of these elements. Techniques of measurement and corrections for vertical divergence and crystal diffraction pattern asymmetry leading to a relative precision (relative standard deviation) in the splitting of about 50 parts per million are described. A comparison is made with the data used by Schawlow and Townes, and a discrepancy is found in several earlier wavelength values which may account partly for the large value of r_{0} obtained by them. A comparison of the theoretical to the present experimental values of the splitting, assuming no quantum electrodynamic effects, yields a value of r_{0}=1.08×10^{13} cm. When corrections are made for vacuum polarization and a nuclear radius of r_{0}=1.2×10^{13} cm, a comparison with experiment shows that a discrepancy remains which is then used to evaluate an empirical correction term. The sign, magnitude, and Z dependence of this term suggest that the remaining discrepancy might arise principally from the Lamb shift effect.
 Publication:

Physical Review
 Pub Date:
 May 1957
 DOI:
 10.1103/PhysRev.106.501
 Bibcode:
 1957PhRv..106..501S