From the Current Literature

Nuclear Disintegrations under the Action of Mesotrons

The problem of so-called “stars”—multiple nuclear disintegrations—is one of the most pressing in cosmic-ray physics. In particular, “stars” produced by slow mesotrons are of considerable interest, since the study of them makes it possible to estimate the mesotron mass (the energy necessary for the formation of a “star” is drawn from the mesotron’s own mass). Such disintegrations were investigated by Academician P. I. Lukirskii and N. A. Perfilov.^3 In two recent papers by the same authors^1,2 new data are reported, obtained by them by the method of thick-layer photographic plates.

Fig. 1.

In the first paper^1 a control experiment is described, intended to verify whether most of the disintegrations are indeed caused by slow mesotrons. The experiment (carried out in a high-altitude laboratory on Alagez, since a large number of “stars” was required) consisted in concentrating slow mesotrons by an electric field with subsequent measurement of the number of “stars” formed. For this purpose a thick-layer photographic plate was placed in a spherical condenser with a central plane electrode, to which a potential of \(+5000\) V was applied (the outer electrode was grounded). It is clear that, if the disintegrations are caused mainly by slow mesotrons, then the number of “stars” on the plate in the condenser should increase noticeably in comparison with control plates located under exactly the same conditions, but outside the field. It turned out that the total number of disintegrations in the field was 119 as against 59 in the absence of a field. Such a sharp difference, in the opinion of the authors, plainly cannot be explained by statistical fluctuation, but is of a regular character, confirming the original assumption about the nature of the “stars.” From the energy balance of the reaction the mesotron mass was estimated, proving to be equal to \(196.8\,m_e\) (\(m_e\) is the electron mass), in agreement with the earlier value.

In the second paper^2 experiments are described that demonstrate the presence of mesotrons with masses greater than \(200\,m_e\). Let us first consider Fig. 1, on

in which the tracks of four particles are depicted. Analysis of the density of the developed emulsion grains leads the authors to the conclusion that this disintegration was caused by a mesotron (to which the long track belongs). The remaining tracks belong to a proton and two \(\alpha\)-particles. The authors believe that this photograph shows the reaction of the disintegration of a carbon nucleus by a negative mesotron

\[ \mu^- + {}_6\mathrm{C}^{12} \to 2\,{}_2\mathrm{He}^{4} + {}_1\mathrm{H}^{1} + 3n_0^{1} \]

(\(\mu\)—the mass of the mesotron). The law of conservation of energy gives (under the assumption of the smallness of the kinetic energy of the mesotron upon capture)

\[ \mu^- = 96\,m_e + \sum_{i=1}^{3} E_i, \]

where \(E_i\) is the energy of the \(i\)-th neutron.

Obviously,

\[ \sum_{i=1}^{3} E_i \]

is minimal if all three neutrons fly out in one direction. This value \(\sum_i E_i\) was calculated with the aid of the law of conservation of momentum from the known momenta of the charged particles and the angles between their trajectories. As a result one obtains

\[ \mu^- \geq 292m_e. \]

Fig. 2.

Another interesting case of disintegration is presented in Fig. 2. Here we have a symmetric emission of twelve charged particles. This symmetry compels one to suppose that the reaction proceeds at the expense of the proper mass of the incident particle, and not at the expense of its kinetic energy, since otherwise a grouping of the fragment trajectories in the direction of motion of the primary particle would have been more probable. In this picture, evidently,

there is a splitting of a silver or bromine nucleus. The mass of the annihilating negative particle was calculated for both cases, with the following alternative assumptions: a) neutrons are emitted simultaneously with the charged particles; b) neutrons are not emitted, and an unstable isotope is obtained, which subsequently undergoes beta decay.

The results of the calculation are given in the table (p. 220). Thus, the authors conclude, there are negative mesotrons with masses considerably exceeding the generally accepted value of \(200\,m_e\). This result confirms and supplements the well-known investigations of Academician A. I. Alikhanov and A. I. Alikhanyan, carried out by another method. (See, for example, \(^{4}\).)

V. Averbakh

CITED LITERATURE

1. P. I. Lukirskii and N. A. Perfilov, DAN, 61, 257 (1948).
2. P. I. Lukirskii and N. A. Perfilov, DAN, 61, 259 (1948).
3. P. I. Lukirskii and N. A. Perfilov, DAN, 54, 219 (1946).
4. A. I. Alikhanov and A. I. Alikhanyan, Journ. of Phys., 11, 97 (1947).