Measurement ofKx rays from muonic helium formed in a low-density target in an intense pulsed muon beam

An experiment was constructed to measure the contribution of vacuum polarization [in quantum electrodynamics (QED)] to the 3d-3p energy difference in muonic helium. Major features of the experimental apparatus included design and construction of a pulsed beam of stopping muons with the highest instantaneous intensity available, a laser system producing multijoule pulses from doubly isotopic ${\mathrm{CO}}_2$ gas, a cavity to trap the laser light in 3 atm of helium, and Si(Li) detectors to measure the helium x rays. The number of stimulated x rays accumulated was not sufficient to permit a statistically significant QED measurement, but the results demonstrate the feasibility of laser spectroscopy in a pulsed muon beam. The intensities of separate K x-ray lines in muonic helium were measured at 1 and 3 atm for comparison with recent theories of pressure effects on the muon cascade.