From the Current Literature

Photograph of the Beta Decay of a Light Meson in a Wilson Chamber

In the July issue of Physical Review[^1] a Wilson-chamber photograph was published in which, in the opinion of the authors, the beta decay of a light meson was recorded. In the Wilson chamber in which the photograph was obtained, there was at the top a horizontal lead plate 1 cm thick; at the bottom, two lead plates each 0.9 cm thick; between them were likewise placed horizontally eight layers of aluminum foil 0.020 cm thick each.

Between the upper lead plate and the first layer of aluminum there is visible an almost horizontal, thick track ending in the aluminum. As the authors state, it can be shown that the point where the track ends is well illuminated, so that what we have here is indeed the end of a track, and not the departure of the particle from the field of view. An estimate of the density of ionization and of the corresponding range shows that the particle to which this track belongs can be neither an electron (since under the conditions of the experiment the latter would not have traversed even two centimeters in the gas) nor a proton (since its range in aluminum would have had to exceed the thickness of the foil, whereas the particle did not emerge from the aluminum). The authors therefore consider that the track belongs to a meson with a mass of about \(200\,m_e\) (\(m_e\) is the electron mass).

From the point at which the meson stopped, a downward-directed track of a much less ionizing particle emerges. It apparently ends in the upper lead plate at the bottom of the chamber. Since a proton traversing less than 0.9 cm in lead must ionize more than twice as strongly as is observed in the photograph, the secondary particle is either an electron or an even lighter meson (\(\pi \to \mu\) transformation). However, in all cases hitherto known of transformations of some mesons into others[^2], the reaction products possessed energies of about 4 MeV and ionized considerably more strongly than the particle to which the secondary track belongs. The authors therefore consider it more probable that the secondary particle is an electron, and we observe here the beta decay of a light meson. If this assumption is accepted, the energy of the decay electron can be estimated from the mean angle of its deflection in the aluminum layers[^3]. The electron energy is found to be \(15 \pm 3\) MeV.

V. Averbakh

Cited Literature

1. E. C. Fowler, R. L. Cool and J. C. Street, Phys. Rev. 74, 101 (1948).
2. C. M. G. Lattes, G. P. S. Occhialini and C. F. Powell, Nature 160, 486 (1947).
3. B. Rossi and K. Greisen, Rev. Mod. Phys. 13, 240 (1941).