K-Mesonic X Rays andK−Absorption in Liquid Helium

An experiment is described in which $K$-mesonic x rays were observed in ${\mathrm{He}}^4$. The experiment was performed in an unseparated beam by selecting $K^{-}$ stopping in a liquid-helium target with a system of fast counters, and observing the x rays in a gas proportional counter gated by the stopping signal. An x-ray line was observed at 6.5±0.4 keV and was identified to be the $L_{\alpha }$ transition of kaonic helium. There was also some evidence for a second line in the spectrum at 35.4±0.9 keV, assumed to be the $K_{\alpha }$, with an intensity of (49±22)% relative to the $L_{\alpha }$. The absolute yield of the $L_{\alpha }$ transition was determined to be (85±25)%. This directly establishes that most $K^{-}$ are absorbed from low-lying states in helium. A simple model was constructed for the de-excitation of kaonic helium and was used to calculate the atomic cascade. With reasonable assumptions about the initial population of states, this calculation gave values for both the $L_{\alpha }$ yield and the cascade time consistent with the experimental values, and was used to predict a nuclear-capture schedule for $K^{-}$ in helium. From the line assumed to be the $K_{\alpha }$, it was possible to deduce an upper limit for the complex-energy shift to the $1S$ level from the $\overline{K}-\alpha$ interaction, and a value for the absorption rate from the $2P$ level.