The $K$-series x-rays from the $\pi$-mesonic Be, B, C, N, and O atoms have been studied. In each case measurements have been made of the $2p-1s$ quantum energy, and of the fraction of stopped pions which give rise to $K$-series x-rays (the "$K$-series yield"). The yields are: Be, 18.8±1.7%; B, 13.6±1.5%; C, 9.5±1.0%; N, 4.5±0.5%; O, 3.4±0.4%. The decrease in yield with increasing $Z$ is attributed to nuclear absorption of the pion from the $2p$ state. The $2p-1s$ quantum energies are: Be, 43±3.5 kev; B, 68±3.5 kev; C, 92±7.5 kev; N, 132±2.5 kev; O, 163.8±2.7 kev. The expected energy in the case of oxygen, if only electromagnetic interactions are important, is 176.0 kev; the difference is presumably due to a pion-nucleus interaction which is repulsive in the $1s$ state.

• Received 28 April 1955

DOI:https://doi.org/10.1103/PhysRev.99.897