Nuclear Fine Structure in theμ-Mesonic Atom

The $2p-1s$ transition of a $\mu$-mesonic atom is split not only by the spin-orbit energy but also by the electric quadrupole interaction between meson and nucleus. If a high-$Z$ nucleus has excited states at several hundred kilovolts or less this interaction can be expected to mix nuclear states, so that for example an even-even nucleus with ground state spin zero can produce a quadrupole fine structure. General formulas are given for the calculation of the line patterns; these are worked out for W, ${\mathrm{U}}^{238}$, and ${\mathrm{Ta}}^{181}$. The resulting fine structure is spread over a region of 300-500 kev. The details of the positions and intensities of the lines depend on the quadrupole moments of the excited nuclear states as well as on other parameters which can in principle be measured in other types of experiments; the effect therefore offers a method for investigating these excited moments. A certain fraction of the mesonic gamma rays can be followed by a nuclear radiation; this fraction can be as high as 0.50.