From Current Literature

Spectrum of Electrons Produced in Meson Decay

The study of the spectrum of electrons arising in meson decay makes it possible to establish the decay scheme if the meson mass is known. On the other hand, for a known decay scheme the spectrum of decay electrons makes it possible to determine the mass of mesons with great accuracy. Unfortunately, the experimental material accumulated so far is too small to allow definite conclusions to be drawn (see UFN, vol. 34, no. 3, 441 (1948)). The well-known photograph of meson decay in Wilson’s chamber, belonging to Williams and Roberts, in which a meson with a mass of about \(200\,m_e\), energy \(10^8\) eV, and a decay electron with energy \(\varepsilon = 70\) MeV \(\pm 50\%\) were recorded, is regarded as evidence that mesons with a mass of \(200\,m_e\) decay according to the following scheme:

\[ \text{meson}^{\pm} \to \text{electron}^{\pm} + \text{neutrino}. \tag{1} \]

In 1946 Conversi and Piccioni\(^{2}\) estimated the energy of decay electrons by measuring their absorption in iron. They indicated that the value \(\varepsilon = 50\) MeV agrees with their results. This may serve as evidence that the greater part of the mesons at sea level have a mass of about \(200\,m_e\) and undergo \(\beta\)-decay according to scheme (1).

However, in 1948 Anderson and co-workers\(^{3}\), having raised a Wilson chamber and electromagnet to an altitude of \(6\text{–}7\) km by airplane, obtained two new photographs of meson decay, in which for the energies of the decay electrons in both cases one and the same value is obtained, \(\varepsilon = 25\) MeV. These photographs are regarded by the authors as possible confirmation of the following decay scheme:

\[ \text{meson}^{\pm} \to \text{electron}^{\pm} + \text{meson}^{0}. \tag{2} \]

It is assumed here that a meson with a mass of \(200\,m_e\) decays; the neutral meson takes up \(3/4\) of the rest energy of the decaying meson, and the electron receives 25 MeV. We have already pointed out (UFN, vol. 34, no. 3, 442 (1948)) that with greater justification one may consider that in this case there occurs the decay of a meson with a mass of \(100\,m_e\) (energy 50 MeV) into a neutrino and an electron (\(\varepsilon = 25\) MeV). The existence of charged mesons of such a mass was first shown in the work of the cosmic-ray laboratory on Mount Alagez\(^{4}\). These data also exhausted our information about the energy of decay electrons. Three new papers, published in the August and September issues of Physical Review, in which the spectrum of decay electrons was measured, are therefore of considerable interest. In two of them, belonging to Hincks and Pontecorvo\(^{5}\) and Steinberger\(^{6}\), the absorption of decay electrons in carbon is studied. Figure 1 shows the scheme of the experiment

ELECTRON SPECTRUM

Hinks and Pontecorvo. Anticoincidences \((AB-C)\) record mesons absorbed in the moderator. The radio circuit is arranged in such a way that the anticoincidence pulse starts a pulse \(D\), of duration \(4.6\ \mu\text{sec}\), whose leading edge is shifted relative to the leading edge of the \((AB-C)\) pulse by one \(\mu\text{sec}\). Coincidences of discharges in counters \(AB\), with pulse \(C\), will be caused by decay electrons arising in the time interval from 1 to \(5.6\ \mu\text{sec}\) after absorption of the meson in the moderator. Steinberger’s experimental arrangement is in principle analogous to the arrangement described above, and we shall not describe it. The absorption curves obtained in these experiments are given in the graphs of Fig. 2, \(a\) and \(b\), where along the ordinate is plotted the number of recorded decay electrons per hour, and along the abscissa the thickness of the absorber. Let us note that electrons with energies of 25 MeV and 50 MeV have ranges in graphite equal to 15 g per cm\(^2\) and 26 g per cm\(^2\), respectively.

Fig. 1.
(Labels in the diagram: lead; steel; graphite absorber; graphite moderator; \(A\), \(B\), \(C\); scale 0, 5, 10, 15 cm.)

Examining the experimental points on both graphs, we see that in both cases a considerable part of the decay electrons has a range greater than 15 g per cm\(^2\), and that for absorber thicknesses greater than 26 g per cm\(^2\) the number of coincidences becomes practically equal to the background of the measurements. It follows from this that, among the decay electrons, electrons with energy greater than 50 MeV are absent.

Fig. 2.
(In the graphs: ordinate—number of decay electrons per hour; abscissa—absorber, g/cm\(^2\); dashed line—background of measurements; panels \(a\) and \(b\).)

Unfortunately, the curves presented do not directly answer the question of whether decay electrons with lower energy are present in the electron spectrum. This is because electron scattering and the “poor geometry” of both experiments strongly distort the course of the absorption curve. The answer to this question, in all probability, follows from the work of Thompson\(^7\), who succeeded in obtaining 10 new photographs of meson decay in a cloud chamber. Wilson’s chamber, specially designed for recording decay electrons, was set up. In the middle of the chamber there was placed a meson moderator—an aluminum plate 6 mm thick. The mesons decayed most often in this plate (6 cases), as well as in the glass walls (3 cases) and in the gas of the chamber (1 case). The decay energy, measured in these 10 cases, is given in the table:

The author indicates that the last 5 measurements (photographs 6–10) have the greatest reliability. Thus, from Thomson’s measurements it follows that in the spectrum of decay electrons, at sea level, electrons with energies \(40\text{–}50\) MeV predominate. This result appears natural, since from the available measurements of the meson mass made by Fretter^8, and also by Retallack and Brode^9, it is known that at sea level the majority of mesons have a mass of about \(200\,m_e\).

A. V.

References Cited

1. Williams and Roberts, Nature 145, 102, 151 (1940).
2. Conversi and Piccioni, Phys. Rev. 70, 874 (1946).
3. Anderson, Adams, Lloyd, Ran and Saxene, Rev. Mod. Phys. 20, 334 (1948).
4. Alikhanian, Vaisenberg, Kharitonov, Daion, Doklady Akademii Nauk SSSR 9, No. 9, 1515 (1948).
5. Hincks and Pontecorvo, Phys. Rev. 74, 697 (1948).
6. Steinberger, Phys. Rev. 74, 500 (1948).
7. Thompson, Phys. Rev. 74, 490 (1948).
8. Fretter, Phys. Rev. 70, 625 (1946).
9. Retallack, Phys. Rev. 73, 921 (1948).