FROM THE CURRENT LITERATURE

ON THE COLLISION OF FAST NUCLEONS WITH NUCLEI

It has been noted earlier\(^{1,2}\) that the collision of fast nucleons with energy \(10^{12}\) eV with complex nuclei cannot be regarded as a series of successive independent collisions. In the work of E. L. Feinberg\(^{3}\) (see also\(^{2}\)) the process of collision of a nucleon with a nucleus at lower energies was considered, and it was found that even at such energies the picture of successive collisions is by no means always valid.

Despite the sufficient persuasiveness of the theoretical arguments against an intranuclear cascade, it is nevertheless advisable to turn to experimental data in order to test experimentally the hypothesis of successive collisions. This is all the more important because abroad this hypothesis is widely circulated\(^{4}\). As was indicated\(^{2}\), the decisive criterion for this hypothesis at sufficiently high energies (\(\sim 10^{11}\) eV) is the comparison of the multiplicity of meson production on nuclei with different atomic numbers. Unfortunately, until recently there had been no sufficiently convincing experiments in this direction. Recently, however, new data have appeared on showers of high energy on various elements, which testify significantly more convincingly against such a hypothesis.

Thus, Kaplan and others\(^{5}\), who used for this purpose the so-called emulsion chamber, i.e. a stack of emulsions interleaved with a substance in which showers are produced, found that at shower energy \(E_0 \sim 5 \cdot 10^{12}\) eV*) the average multiplicity in lead is \(n_{\text{Pb}} \sim 24\), and in brass \(n_{\text{brass}} \sim 18\). It was noted earlier\(^{2}\) that at high energies the model of an intranuclear cascade implies, even neglecting the interaction of secondary mesons with nucleons, the ratio

\[ \frac{n_{\text{Pb}}}{n_{\text{brass}}} \sim 2^{\frac{A_{\text{Pb}}^{1/3}-A_{\text{brass}}^{1/3}}{1.5}} \sim 3, \]

which cannot in any way be reconciled with the experimental ratio \(\frac{n_{\text{Pb}}}{n_{\text{brass}}} \sim 1.3\). On the contrary, this ratio agrees excellently with the dependence of multiplicity on atomic number obtained on the basis of the model of collision of a nucleon with a cylindrical tube of structureless nuclear matter.

*) The energy was estimated very roughly from the number of tracks of relativistic particles in accordance with the formula \(n \sim \left(\frac{E_0}{Mc^2}\right)^{1/4}\); \(M\) is the mass of the nucleon.

Application of this model to the Fermi—Landau theory gives \(A^{1/4}\). Such a dependence for the case of collisions with lead and brass nuclei gives, for the quantity

\[ \frac{n_{\mathrm{Pb}}}{n_{\mathrm{brass}}} \]

a value of \(\sim 1.3\).

Some data are also available concerning the character of the interaction of nucleons with nuclei at lower energies. Thus, with the aid of a controlled Wilson chamber, showers formed in lead and aluminum were compared\(^6\). A characteristic feature of the apparatus used in this work is the very “soft” character of the control of the counter group, which introduces practically no discrimination in the registration of showers. The authors of this work found that the ratio of the frequencies of registration of showers from lead and aluminum is 1.56 for showers with five relativistic particles and 1.0 for showers with six particles. At the same time, application of the model of intranuclear cascade processes should have given a considerably larger value. Thus, according to one of the concrete variants of the calculation in accordance with such a model (Heitler—Janossy\(^7\)), these ratios proved to be 2.4 and 4.6.

In another group of works, carried out by Soviet authors\(^8\), the ratio for absorption mean free paths and for the interaction of nucleons with energy \(\sim 10^{10}\) eV was compared for various elements. It was found that this ratio is approximately the same for graphite and lead (\(\sim 3\)). This result is very difficult to interpret from the standpoint of an intranuclear process, since it would mean that one lead nucleus would have to absorb like three graphite nuclei, and, consequently, the ratios indicated above would have to depend sharply on the atomic weight of the elements.

Thus, the new experimental data testify still more convincingly against the model of successive intranuclear collisions of fast nucleons.

I. R. and D. Ch.

REFERENCES CITED

1. L. D. Landau, Izv. AN SSSR, ser. fiz. 17, 51 (1953).
2. I. L. Rozental, D. S. Chernavskii, UFN 52, 185 (1954).
3. E. L. Feinberg, ZhETF 28, No. 2 (1955).
4. I. Messel, Progress in Cosmic Ray Physics 2 (1954).
5. M. E. Kaplon, D. M. Ritson, W. D. Walkes, Phys. Rev. 90, 716 (1953).
6. R. L. Sen Gupta, K. K. Koy, M. M. Biswas, Zeits. f. Phys. 136, 654 (1954).
7. W. Heitler, L. Janossy, Proc. Phys. Soc. 62, 374, 669 (1949).
8. S. A. Azimov, N. A. Dobrotin, A. L. Lyubimov, K. P. Ryzhkova, Izv. AN SSSR, ser. fiz. 17, 80 (1953).