Mesonic X-Rays from Capture of Cosmic-RayμMesons in a Chemical Compound

Measurements have been made of the x-rays emitted by mesonic carbon and oxygen atoms formed by the capture of negative cosmic ray $\mu$ mesons. A 4-liter tank of liquid scintillator, dioxane, ${\mathrm{C}}_4$ ${\mathrm{O}}_2$ ${\mathrm{H}}_8$ (plus 10 g/l of $p$-terphenyl) served as the absorber and as the detector for the stopping of cosmic-ray $\mu$ mesons. The mesons were identified by the method of delayed coincidences. The mesonic x-rays from the dioxane were detected by a scintillation spectrometer. Spectrometer pulses in coincidence with the stopping meson were lengthened and their pulse height recorded on a recording milliammeter. The time distribution of the delayed coincidences was recorded over the delay interval 0.5 to 8.0 μsec. The measured $2p-1s$ transition energies of 82 kev and 145 kev, for carbon and oxygen respectively, agree with the simple Bohr orbit calculations within the precision of the measurement. The observed ratio of captures by oxygen and carbon (0.33±0.04) does not agree with the $Z$-dependence predicted by Fermi and Teller for mesonic atom formation by chemical compounds (0.67) nor with the simple stopping power calculation (0.50); however, in view of the thick absorber (14 cm depth of dioxane), the above experimental ratio is uncertain because of Compton degradation of the oxygen $2p-1s$ radiation which shifts some events from the oxygen to the carbon peak. The observed distribution of delays is consistent with the assignment of the observed radiation to $\mu$-meson capture.