Kaonic Helium Research Articles

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 ${K}^{\ensuremath{-}}$ 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\ifmmode\pm\else\textpm\fi{}0.4 keV and was identified to be the ${L}_{\ensuremath{\alpha}}$ transition of kaonic helium. There was also some evidence for a second line in the spectrum at 35.4\ifmmode\pm\else\textpm\fi{}0.9 keV, assumed to be the ${K}_{\ensuremath{\alpha}}$, with an intensity of (49\ifmmode\pm\else\textpm\fi{}22)% relative to the ${L}_{\ensuremath{\alpha}}$. The absolute yield of the ${L}_{\ensuremath{\alpha}}$ transition was determined to be (85\ifmmode\pm\else\textpm\fi{}25)%. This directly establishes that most ${K}^{\ensuremath{-}}$ 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 ${L}_{\ensuremath{\alpha}}$ yield and the cascade time consistent with the experimental values, and was used to predict a nuclear-capture schedule for ${K}^{\ensuremath{-}}$ in helium. From the line assumed to be the ${K}_{\ensuremath{\alpha}}$, it was possible to deduce an upper limit for the complex-energy shift to the $1S$ level from the $\overline{K}\ensuremath{-}\ensuremath{\alpha}$ interaction, and a value for the absorption rate from the $2P$ level.