OBSERVATIONS OF COSMIC RADIATION USING PHOTOGRAPHIC PLATES SENSITIVE TO ELECTRONS

In the paper under review¹, in contrast to earlier papers from the same and other laboratories², highly sensitive plates were used on which the tracks of relativistic electrons producing minimum ionization can be observed. After a short (8–16-day) exposure at an altitude of 11,000 feet (3300 m) beneath 10 cm of lead, the plates were developed immediately. The authors state that under these conditions it was possible to neglect the phenomenon of “fading” of the latent image. A number of interesting results were obtained.

1) Electrons from the decay of μ-mesons were observed (i.e., the main mesonic component with a mass of about 200 $m_e$) for 100 meson tracks ending in the emulsion. In 60% of the cases, one electron track with minimum ionization and a large range was observed. The remaining 40% of the cases, in which such a track was not observed, can quite well be explained by the capture of some of the negative mesons by silver and bromine nuclei. Nuclear disintegrations in such capture are not observed, which agrees with the results of other studies, but is nevertheless very surprising.

2) In six cases, from the magnitude of multiple Coulomb scattering, it was possible to estimate the energy of the electrons produced by the decay of μ-mesons. The values found were: $15 \pm 3$; $25 \pm 5$; $38 \pm 6$; $42 \pm 2$; $44 \pm 4$; $48 \pm 6$ MeV. The absence of a definite electron energy was noted recently by a number of authors who used an entirely different experimental method³. Unfortunately, the constancy of the μ-meson masses was not checked in this work. Therefore the conclusion that in μ-meson decay we are dealing with three secondary particles (one electron and two neutral ones) is only highly probable, but not absolutely inevitable.

FROM CURRENT LITERATURE

3) The most interesting is the photomicrograph shown in the figure, which the authors interpret as indicating the existence of an unstable meson with mass \(\sim 1000\,m_e\), decaying into three charged mesons of smaller mass. The mass of the primary particle \(k\) was estimated by the method of counting the developed emulsion grains. Graphs were constructed for the number of grains falling within a \(10\mu\) segment as a function of the distance to the end of the track for protons, \(\mu\)-mesons, and the given particle. The curve for particle \(k\) lay somewhat below the curve for protons, whence its mass is equal to \(1080 \pm 160\,m_e\). In discussing the accuracy of the grain-counting method, the authors report that by this method they repeated the determination of the ratio of the mass of the \(\pi\)-meson (a heavy meson capable of producing nuclear disintegrations when captured by a nucleus) to the mass of the \(\mu\)-meson and obtained \(1.33 \pm 0.05\), in agreement with data obtained on the basis of the Berkeley data, in contrast to the inaccurate, previously published result \(1.65 \pm 0.11\). (This erroneous ratio, as is known, led the authors at one time to the conclusion that the second component of the decay of the \(\pi\)-meson into a \(\mu\)-meson is a neutral meson of finite mass. Now this conclusion is not considered more valid.) In the authors’ opinion, the error in the old experiments was due to “fading.” Determination of the mass of the primary particle from the magnitude of multiple scattering gave \(1800 \pm 400\,m_e\). In view of the large statistical error, the authors do not take this determination into account further. It is assumed further that the secondary particle \(t\), which produces a nuclear disintegration at point \(B\), is a \(\pi\)-meson with mass 286. (According to determinations made in Berkeley on photographic plates, such is the mass of mesons causing the main part of nuclear disintegrations.) Its energy and momentum can be accurately determined from the magnitude of its range. A careful—

a detailed study of the geometrical arrangement of the tracks \(t, a\), and \(b\), which arose in the presumed decay of the particle \(h\) at point \(A\) (taking into account deformation of the emulsion layer during photomanipulations). All three tracks lie in one plane, which confirms the supposition of spontaneous decay without the participation of any invisible particles (recoil nucleus, neutral particles). The vector sum of the three momenta must be equal to zero, whence (using the known momentum \(t\)) the momenta of particles \(a\) and \(b\) can be found. In addition, these momenta can be independently estimated by the grain-counting method or by the multiple-scattering method for each possible assumption about their masses.

All three methods of determination give consistent results if it is assumed that particles \(a\) and \(b\) are mesons (\(\mu\) or \(\pi\)—it is impossible to say with certainty, but the latter is more probable).

In conclusion the authors compare their results with other works (Leprince-Ringuet, Rochester and Butler, Alikhanian, Alikhanov and Weisenberg—the authors cite only one of the recent papers by the Soviet authors,—Bradet and Peters—oral communication). The authors consider it possible that they are observing a positive analogue of Leprince-Ringuet’s negative heavy meson (in that author’s case a star with formation of a \(\pi\)-meson was observed). Mesons \(a\) and \(b\), under this assumption, are both positive. The assumption that the decay into three particles proceeds in two stages (for example, via an intermediate neutral heavy meson, like that postulated by Rochester and Butler) is considered by the authors to be improbable, since this would require a lifetime of the neutral meson of the order of \(10^{-14}\) sec. (otherwise the beginnings of the three tracks would not issue from a single point with such great accuracy).

A. Sakharov.

CITED LITERATURE

1. C. F. Powell, P. Brown, U. Camerini, R. Fowler, H. Muirhead, D. M. Ritson, Nature 163, 47, 82 (1949).
2. See the translation of the paper by Powell et al., UFN, XXXIV, issue 3, 370 (1948).
3. G. Zhdanov and Kh. Haidarov, DAN, LXV, No. 3, 287 (1949). For an abstract of analogous works, see UFN, XXXVII, 255 (1949).