FROM CURRENT LITERATURE

INVESTIGATION OF THE SPECTRUM OF ELECTRONS PRODUCED IN THE DECAY OF STOPPED MESONS USING A WILSON CHAMBER

During 1948–1949 a number of papers were published in periodicals on the study of the spectrum of electrons produced in the decay of mesons constituting the main part of the hard component of cosmic radiation. The results of these investigations lead to the conclusion that the spectrum of decay electrons is continuous, and does not consist of a single monochromatic line, as had earlier been supposed, when sufficiently accurate experimental data were not yet available and the most widespread opinion was the assumption of the decay of the meson into two particles—an electron and a neutrino. The nonmonochromatic character of the spectrum of decay electrons was convincingly shown in the work of G. B. Zhdanov and A. A. Khaidarov^1 (see also UFN 37, 254, 1949), in which decay electrons were selected by the method of delayed coincidences, and then their average ionization energy loss in matter was measured. In addition, several photographs obtained with Wilson chambers are known, in which decay events of stopped mesons were recorded, the energy of the electron in a number of cases proving to be considerably less than 50 MeV (see, for example,^2).

Fig. 1.

Quite recently Leighton, Anderson, and Seriff^3 made a detailed investigation of the spectrum of decay electrons using a controlled Wilson chamber. In all, in this work, carried out at sea level, 15,000 photographs were taken, of which 75 photographs recorded traces of mesons stopped in the plate, walls, and gas of the chamber together with the traces of the decay electrons. A schematic drawing of the apparatus used by the authors is presented in Fig. 1. The triggering of the chamber was caused

coincidences of discharges in counters \(C_1\) and \(C_3\), with counter \(C_2\) located inside the chamber. The group of counters \(C_3\) was included in the anticoincidence branch of the radio circuit, so that a particle passing through all three groups of counters was not recorded by the chamber. In addition to counter \(C_2\), a carbon plate \(P\) (for slowing down mesons), \(2.0\ \mathrm{g/cm^2}\) thick, was introduced into the chamber.

The energy of the electrons was determined from the radius of curvature of the trajectory in a magnetic field (field strength \(7250\) gauss). Since almost all the mesons whose decay was recorded by the chamber stopped either in plate \(P\) or in the walls of counter \(C_3\) and of the chamber (only one case of a meson stopping and decaying in gas was observed), in calculating the electron energy it was necessary to introduce a correction for energy loss in matter. The introduction of such a correction requires knowledge of the thickness of the layer of matter from which the decay electron emerged; this can be determined if the momentum and mass of the meson stopped in the matter are known. The authors

Fig. 2.

assumed that all the mesons they observed had a mass equal to \(220\,m_e\) (\(m_e\) is the electron mass). Under this condition the correction sometimes reached \(11\ \mathrm{MeV}\), but in most cases it did not exceed \(5\ \mathrm{MeV}\). On average, the error in determining the electron energy was \(1\)—\(2\ \mathrm{MeV}\). As for the assumption of equality of the masses of the observed stopped mesons, it appears quite natural, since it is well known that about \(90\%\) of all mesons entering the hard component of cosmic radiation have a mass of about \(200\,m_e\). It should be noted that in a number of cases the authors of the paper under review were able to measure the mass of the meson (from the loss of momentum in the plate), and whenever these measurements were possible, values from \(175\,m_e\) to \(300\,m_e\) were obtained for the mass of the meson (deviations from the mass \(220\,m_e\) did not exceed the measurement errors). Concluding the discussion of questions related to the experimental method, let us add that all the experiments were carried out with a falling Wilson chamber, i.e. a chamber which, after the particle has passed through its volume, falls out of the interpolar gap of the magnet, and the track is photographed “in flight,” during the fall. The use of a falling chamber makes it possible to reduce the interpolar gap of the magnet, to increase the degree of homogeneity of the magnetic field, and to increase the exposure time during photography, which in ordinary chambers is limited by the fall time of the track droplets.

The differential spectrum of decay electrons obtained by the authors is shown in Fig. 2. As can be seen from the figure, the spectrum is continuous, with an upper limit of \(55 \pm 1\ \mathrm{MeV}\). The shape of the spectrum agrees well

is consistent with the assumption of the decay of the meson into an electron and two neutrinos, if it is assumed⁴ that the meson has spin \(\frac{1}{2}\). From the values of the upper boundary of the spectrum and from the assumption concerning the mass of the observed mesons⁵ \((216 \pm 4\,m_e)\), it follows that the sum of the masses of the two neutrinos must be less than \(30\,m_e\). For the mean energy of the spectrum, the value \(34\) MeV is obtained, in good agreement with the data of experiments carried out by the method of delayed coincidences¹.

Essentially new in the present work, in comparison with earlier published investigations, is the fact that the differential spectrum of decay electrons was obtained directly. Too-large statistical errors do not yet allow a more detailed comparison of the experimental data of Leighton, Anderson, and Seriff with the theoretical calculations⁴ performed under the assumption of various types of interaction Hamiltonian for the case of the decay of a meson of spin \(\frac{1}{2}\) into an electron and two neutrinos.

Obtaining more accurate experimental data is a most urgent task for further experiments.

Sh.

CITED LITERATURE

1. G. B. Zhdanov and A. A. Khaidarov, DAN 65, 287 (1949).
2. N. G. Birger, DAN 61, 243 (1948).
3. R. B. Leighton, C. D. Anderson and A. J. Seriff, Phys. Rev. 75, 1432 (1949).
4. J. Tiomono, J. A. Wheeler, Rev. Mod. Phys. 21, 144 (1949).
5. A. S. Bishop, Phys. Rev. 75, 1468A (1949).