4 f → 3 d Pionic Transition in Ba 138 , Ce 140 , and Ce 142

The $4f\ensuremath{\rightarrow}3d$ pionic x rays in ${\mathrm{Ba}}^{138}$, ${\mathrm{Ce}}^{140}$, and ${\mathrm{Ce}}^{142}$ have been observed in order to measure the effect of the nuclear interaction on a pion in a $3d$ atomic level. The results are used to look for differences in the pion interaction with these nuclei. Within experimental error, we could not detect an isotope shift in the ${\mathrm{Ce}}^{142}$-${\mathrm{Ce}}^{140}$ results. The nuclear isotone shift in the x-ray energy and width for ${\mathrm{Ce}}^{140}$-${\mathrm{Ba}}^{138}$ of 1.59 \ifmmode\pm\else\textpm\fi{} 0.30 and 1.3 \ifmmode\pm\else\textpm\fi{} 1.3 keV agrees with the values of 1.31 and 2.0 keV predicted from an optical-model calculation.