Measurements have been made of the isotope shift in the muonic $K$ x rays for the pairs of isotopes ${\mathrm{O}}^{16}$-${\mathrm{O}}^{18}$, ${\mathrm{Ca}}^{40}$-${\mathrm{Ca}}^{44}$, ${\mathrm{Sn}}^{116}$-${\mathrm{Sn}}^{124}$, and ${\mathrm{Gd}}^{155}$-${\mathrm{Gd}}^{160}$. The measurements were limited in resolution by the conventional NaI detectors used: no fine or hyperfine structure was resolved. The results confirm the indications from optical isotope-shift measurements that the expansion in charge distribution of a nucleus on addition of neutrons is less than the expansion in the density distribution, and indicate that this trend is even more marked in the light nuclei examined.

• Received 3 August 1965

DOI:https://doi.org/10.1103/PhysRev.141.48