From Current Literature

On the Mechanism of Capture of Negative μ-Mesons

A number of experiments have shown that the capture of slow negative μ-mesons by nuclei is not accompanied by the emission of any charged particles or high-energy γ-quanta.1–4 It is therefore of interest to determine whether neutrons are emitted in such captures. In the studies5–8 carried out to solve this question, a system of telescopic counters was used, selecting cosmic-ray particles stopping inside a layer of lead. If the capture of μ-mesons by nuclei is accompanied by neutron emission, delayed coincidences should have been recorded between the operations of the telescope counters and boron counters, surrounded by paraffin, used for slowing down the neutrons. It turned out that the expected delayed coincidences do indeed occur.

These studies, however, were carried out under conditions that were not sufficiently clean, in the sense that in some of them (5 and 6) μ-mesons were not separated from slow π-mesons, while in others (7 and 8) they were not separated from π-mesons and from comparatively energetic protons capable of generating nuclear disintegrations. It therefore cannot be asserted with certainty that the detected neutrons are connected specifically with the capture of negative μ-mesons. Nor is the possibility noted in 5 excluded, namely that neutrons are formed in processes of fission of lead nuclei under the action of captured μ-mesons, i.e. in processes that do not occur for lighter nuclei. Thus, it should be considered that at present the question of neutron formation in the capture of negative μ-mesons remains open, at least for nuclei that are not too heavy.

Below we present considerations showing that in the majority of cases the capture of μ-mesons is in fact accompanied by the emission of at least one neutron. As a result of the capture, some nucleus with a negative meson forms another nucleus with atomic number smaller by one and with the former atomic weight, if no neutron is emitted; smaller by one, if one neutron is emitted; and so on. Some of the nuclei-products under consideration may turn out to be stable, while others may be unstable, in particular β-active.

All the basic elements that enter into the composition of photographic emulsion and that are capable, to a greater or lesser degree, of capturing μ-mesons (i.e. Ag, Br, O, N, C) possess, as is easy to verify, the property that the emission, as a result of meson capture, of one or two neutrons leads to the formation of stable nuclei. On the other hand, upon capture,

not accompanied by the emission of neutrons, unstable β-active nuclei are formed, emitting electrons with energies of several MeV, while the corresponding half-lives in many cases are relatively short (no more than several hours).

The latter applies to Ag¹⁰⁹, Br⁸¹, O¹⁶, and C¹², which constitute 50–60% of the total weight of the emulsion. Even if one takes into account that the capture of μ-mesons by oxygen and carbon nuclei occurs comparatively rarely, one should still expect that the stopping of negative mesons in the emulsion should in 35–40% of cases be accompanied by the appearance of an electron of relatively low energy (assuming that capture of a μ-meson does not lead to the emission of neutrons).

What has been said is not in agreement with the results obtained with the aid of photographic emulsions sensitive to relativistic particles. Consider, for example, work ⁹, in which 25 negative μ-mesons stopping inside the emulsion were recorded, with the stopping being accompanied by the emission of an electron in only four cases (not counting very slow electrons, the so-called Auger electrons, which have no bearing on the question under discussion). The four cases indicated can be fully explained by the decay of the stopped negative μ-mesons (expected number: about ~5, see ¹⁰). It follows, therefore, that one may conclude that there are no β-electrons, of which about 10 should have been observed if unstable nuclei arose as a result of μ-meson capture. An analogous conclusion may also be obtained from an analysis of the results of other works (for example, ¹⁰ and ¹¹). From the fact of the stability of the residual nuclei it follows, in turn, that capture of a negative μ-meson is in most cases accompanied by the emission of at least one neutron.*)

In conclusion we note that what has been said applies in part also to the capture of negative π-mesons, namely to those cases of capture which are not accompanied by the formation of “stars.”

M. I. Podgoretskii

CITED LITERATURE

1. W. I. Chang, Rev. Mod. Phys. 21, 166 (1949).
2. H. G. Voorhies, J. C. Street, Phys. Rev. 76, 1100 (1949).
3. R. D. Sard, A. J. Althaus, Phys. Rev. 74, 1364 (1948).
4. O. Piccioni, Phys. Rev. 74, 1754 (1948).
5. G. Groetzinger, G. W. McClure, Phys. Rev. 74, 341 (1948).
6. G. W. McClure, G. Groetzinger, Phys. Rev. 75, 340 (1949).
7. R. D. Sard, A. W. B. Ittner, A. M. Conforto, M. F. Crouch, Phys. Rev. 74, 97 (1948).
8. R. D. Sard, A. M. Conforto, M. F. Crouch, Phys. Rev. 76, 1134 (1949).
9. C. Franzinetti, Phil. Mag. 41, 86 (1950).
10. M. Cosyns, C. Dilworth, G. Occhialini, M. Schoenberg, Proc. Phys. Soc. A, 62, 801 (1949).
11. J. H. Davies, W. O. Lock, H. Muirhead, Phil. Mag. 40, 1250 (1949).
12. F. B. Harrison, J. W. Kauffel, G. T. Reynolds, Phys. Rev. 83, 680 (1951).

*) Note added in proof. An analogous conclusion follows, apparently, also from work ¹², published after the writing of the present review.