X-RAY RADIATION OF AN ATOM WITH A MESONIC ORBIT

The possibility of the formation of a “mesonic” atom, consisting of a positive nucleus and a negatively charged $\pi^-$ or $\mu$-meson in one of the “Bohr” orbits, has been considered theoretically by many authors, and some experimenters have even obtained indirect evidence for the existence of such mesonic orbits.

In the paper under review$^{2}$ the existence of mesonic orbits for the nuclei C, O, and Be ($Z<10$) is proved by a direct method. For such nuclei the nuclear radius is small in comparison with the radius of the lowest Bohr orbit for a meson, and the energy levels can readily be calculated. Such calculations, carried out by the author, give for the energies released in the transition $2p \to 1s$ values equal (for a $\pi^-$-meson with mass $276\,me$) to 100, 178, and 44 kev for the atoms C, O, and Be, respectively. Such X-ray radiation may be expected when negatively charged $\pi^-$-mesons are stopped in targets made of these elements. The experiment was as follows.

The magnetic field of the cyclotron focused $\pi^-$-mesons with an energy of $40 \pm 3$ Mev onto targets C, O (water), or Be. Before striking the target the $\pi^-$-mesons passed through three liquid scintillation counters. Near the target there was placed a crystalline scintillation counter NaJ (Tl), which registered the X-ray radiation of the target. Pulses from this counter that coincided in time with pulses from the three counters through which the $\pi^-$-mesons entering the target passed were fed to a 24-channel pulse-height analyzer. The pulse-height distribution obtained in the case of the carbon target is shown in the figure. The expected half-width of the line for X-ray photons with energy 100 kev is shown in the figure by a horizontal line. The measurements were made with an aluminum or lead absorber placed between the carbon target and the X-ray counter. The transmission of the first absorber was 0.79, of the second 0.02. The figure clearly shows a line corresponding to radiation with an energy of about 100 kev. This line practi-

practically disappears when the aluminum absorber is replaced by a lead one. Thus, in this work direct proof has been obtained of the occurrence of X-ray radiation corresponding to the \(2p \to 1s\) transition for negatively charged \(\pi^-\)-mesons. For the elements O (water) and Be, the corresponding radiation was also observed. In the case of C and O it was possible to measure the yield of X-ray quanta per absorbed \(\pi^-\)-meson. This value is \(0.13 \pm 0.03\) for C and \(0.21 \pm 0.07\) for O. Calculations show, however, that the yield for O should amount to 0.6 of the yield for C, if only one assumes that the probability of absorption of a \(\pi^-\)-meson by the nucleus from the \(2p\) level is the same for the two nuclei. As we see, in reality the yield of X-ray quanta for O is greater than for C and, consequently, the experimental data contradict this assumption. The authors point out that, possibly, in the case of the O nucleus, special selection rules act for the absorption of \(\pi^-\)-mesons by the O nucleus, connected with the structure of the intranuclear nucleon shells. To test this assumption, careful measurements of the dependence of the quantum yield on the atomic number of the elements \(Z\) will be required. It is quite possible that such measurements may provide new information on the structure of intranuclear shells. The existence of a connection between the probability of meson absorption by the nucleus and the structure of intranuclear nucleon shells is also indicated by a recent measurement of this dependence for heavy nuclei. This work is considered in detail in the following note.

A. V.

CITED LITERATURE

1. Chang, Rev. Mod. Phys., 21, 166 (1949); Panofsky, Aamodt and York, Phys. Rev., 78, 825 (1950).
2. Camas, McGuire, Platt, Schulte, Phys. Rev., 88, 134 (1952).