SPECTRUM OF PHOTONS PRODUCED IN THE ABSORPTION OF $\pi^-$ MESONS BY HYDROGEN

About a year ago a report appeared in print1 stating that, in the capture of $\pi^-$ mesons by hydrogen-containing compounds (LiH, CH$_2$), there is observed, contrary to all expectations, a very small yield of $\gamma$ quanta and that, apparently, the probability is very small for the $\pi^-$ meson to be captured specifically by a hydrogen atom. Recently a more detailed work2 devoted to this same question has appeared; in this work, as absorber for the mesons, pure hydrogen was used in the form of a gas compressed to a pressure of 190 atm and cooled by liquid nitrogen (the density of this gas was 0.018 g/cm$^3$).

A tungsten target about 0.1 cm thick was bombarded with protons of energy 330 MeV. At a distance of 6.3 cm from the target there was a vessel with hydrogen, of volume 600 cm$^3$. This vessel was surrounded by a thin-walled shell filled with liquid nitrogen. The total thickness traversed by the mesons produced in the tungsten target before entering the hydrogen was 4 g/cm$^2$. The photons produced in the absorption of mesons by hydrogen were passed through two lead collimators (arranged to prevent interference), went through an opening in a concrete wall, and struck a tantalum plate, where they produced positron–electron pairs. The spectrum of the $\gamma$ quanta was studied with the aid of a pair spectrometer. As such a spectrometer, two pairs of proportional counters were used, operated in quadruple coincidence, with simultaneous entry of an electron and a positron into each pair of counters, separated by a magnetic field. Between the two counters of each pair, absorbers of various thicknesses were placed in order to estimate the energies of the electron and the positron.

Fig. 1. Spectrum of photons produced in the absorption of $\pi^-$ mesons by hydrogen.

It was noted that the counting rate agrees qualitatively with the assumption that all $\pi^-$ mesons stopped in hydrogen ultimately give $\gamma$ quanta. Thus the yield of $\pi^-$ decay proved insignificant. The authors considered the possibility of production of $\gamma$ quanta in some other way, not connected with mesons, and rejected such possibilities.

Figure 1 gives the spectrum of the observed $\gamma$ quanta. Replacement of hydrogen by other materials, for example carbon or helium, led to a complete destruction of the observed effect of photon emission. It is especially interesting that the replacement of hydrogen by hydrogen-containing compounds (LiH and CH$_2$) led to the same result. The probability of final capture of a $\pi^-$ meson on the $K$ orbit of hydrogen with subsequent capture by a proton proved, in the case of CH$_2$, to be less than $10^{-3}$ and, in the case of LiH, less than $3 \cdot 10^{-3}$. The explanation of this fact proposed by the authors is that, although in the case of such molecules as CH$_2$ and LiH, more—

Most of the $\pi^-$ mesons are initially captured into the higher Bohr orbits of the hydrogen atom, but then the neutral $\pi^- \mathrm{H}$ system diffuses through the lattice and collides with C or Li atoms. In this case there is a high probability that the $\pi^-$ meson will be captured by a Li or C nucleus, with the subsequent formation of a star instead of a $\gamma$ quantum.

The case of pure hydrogen is special, for the reaction $\pi^- + \mathrm{H}^+ = n$ is possible only in the presence of other nucleons; upon absorption by a free proton an additional particle with integer spin must be emitted, if one assumes that the $\pi^-$ meson also has integer spin. Such a particle may be a photon, or, if this is energetically possible, a neutral $\pi^0$ meson. The absorption processes have been discussed in detail in theoretical papers$^{3,4}$, where it was shown that the sum of the times of slowing down, capture, and transition of the meson to the $K$ orbit is small compared with the time of $\pi$-$\mu$ decay.

On going over to the discussion of the photon spectrum obtained as a result of the experiments, the authors believe that near the energy 130 MeV the emitted $\gamma$ quanta are not monochromatic and that photons with energies of about 130 MeV do not belong to the “tail” of the distribution extending from 70 MeV, but constitute a separate group associated with a special process.

The authors consider three processes of absorption of $\pi^-$ mesons by hydrogen:

1) $\pi^- + \mathrm{H} \to n\ (9\ \mathrm{MeV}) + \gamma\ (132\ \mathrm{MeV}),$
2) $\pi^- + \mathrm{H} \to n + 2\gamma,$
3) $\pi^- + \mathrm{H} \to n + Q + \pi^\circ \to 2\gamma.$

The authors regard process 2) as improbable, and they interpret the results obtained as a competition between processes 1) and 3).

Assuming the existence of process 3), the authors draw several conclusions concerning the neutral $\pi^\circ$ meson. Thus, if the half-width of the peak in the spectrum of photons arising from the decay of $\pi^\circ$ mesons, determined by the Doppler shift $\delta = p/M_{\pi^\circ}$ (where $p$ is the momentum of the $\pi^\circ$ meson and of the neutron), is known from experiment ($\delta < 0.21$), then one can determine the upper limit of the kinetic energy of the $\pi^\circ$ and of the neutron and the bounds of the mass difference between the negative and neutral meson:
$1.3\ \mathrm{MeV} < \Delta M_{\pi^- - \pi^\circ} < 4.7\ \mathrm{MeV}.$ Furthermore, since the $\pi^-$ meson is captured from an orbit with zero angular momentum, and the kinetic energy of the $\pi^\circ$ meson is so small that it can be emitted only in the form of an $S$ wave, it may be concluded that the $\pi^-$ and $\pi^\circ$ mesons have the same parity. Finally, the authors point out that the spin of the $\pi^\circ$ meson cannot be equal to one, since it decays with the formation of two $\gamma$ quanta. It should be noted that this proposition, in its most general form, for any particle, was first proved by L. D. Landau,$^{5}$ which the authors do not mention.

G. I.

CITED LITERATURE

1. W. K. H. Panofsky and H. F. Jork, Phys. Rev. 78, 89 (1950).
2. W. K. H. Panofsky, L. Aamodt and H. F. Jork, Phys. Rev. 78, 825 (1950).
3. A. S. Wightman, Phys. Rev. 77, 521 (1950).
4. R. E. Marshak and A. S. Wightman, Phys. Rev. 76, 114 (1949).
5. L. D. Landau, Doklady AN SSSR 60, 207 (1948).