Reports of the Academy of Sciences of the USSR

1. Volume 151, No. 4

PHYSICS

O. V. LOZHKIN, N. A. PERFILOV, Yu. P. YAKOVLEV

FEATURES OF THE FORMATION OF $\mathrm{Li}_3^8$ IN THE INTERACTION OF 660 MeV PROTONS WITH $\mathrm{C}_6^{12}$ NUCLEI

(Presented by Academician B. P. Konstantinov on 12 III 1963)

The study of the process of formation of fast multiply charged particles in the disintegration of light nuclei by high-energy protons is acquiring special interest in connection with the development of the theory of direct nuclear reactions. In this connection, in addition to information on the mechanism by which such processes proceed, one may hope to obtain from experimental data certain information on the structure of the target nucleus in its ground and excited states and, possibly, on reactions with unstable nuclei playing the role of intermediate particles. Moreover, investigation of the indicated reactions may prove essential for clarifying a number of questions connected with the mechanism of fragmentation in heavy nuclei. Indeed, one may suppose that, during the development of a nuclear-cascade process in a nucleus, fast nucleons, in interacting with clusters in the diffuse region of the nucleus, along with possible quasi-elastic scattering, produce peculiar nuclear reactions in which fragments are also formed.

In the investigation carried out by us, the task was to study the angular distribution and energy spectra of $\mathrm{Li}_3^8$ fragments emitted at various angles to the beam of incident protons in the disintegration of $\mathrm{C}_6^{12}$ nuclei; for this purpose a vacuum chamber was used. Thin films (polystyrene 1.24 mg/cm$^2$ and polyethylene 4 mg/cm$^2$) were irradiated in the chamber with a proton flux of $10^{13}$ cm$^{-2}$ on the extracted beam of the phasotron of the Joint Institute for Nuclear Research (proton energy 660 MeV). The $\mathrm{Li}_3^8$ fragments were recorded with a low-sensitivity P-90 nuclear emulsion at angles from 20 to 160° to the proton beam.

As a result of processing the experimental material, range spectra were obtained for particles producing T-shaped tracks (which, in principle, may be produced by $\mathrm{Li}_3^8$, $\mathrm{Li}_3^9$, $\mathrm{B}_5^8$ nuclei) at angles of 20, 47, 90, and 137°. Analysis of the ionization characteristics of the T-shaped tracks, carried out by the scale method ($^1$), showed that almost all the observed T-shaped tracks (with ranges greater than 35 μ) are due to the $\mathrm{Li}_3^8$ nucleus (a very small fraction of the T-shaped tracks is produced by the $\mathrm{Li}_3^9$ nucleus). In Fig. 1 the experimental range and energy distributions of $\mathrm{Li}_3^8$ are presented. The latter were reduced to zero target thickness by recalculating the ranges by the method described in work ($^2$), using methods of best agreement ($^3$).

The angular distribution of $\mathrm{Li}_3^8$ fragments is shown in Fig. 2 for the entire energy spectrum.

The fact noted above of the absence of the $\mathrm{B}_5^8$ isobar is of great significance for interpreting the data obtained, in connection with the possibility, discussed in the literature ($^4$), of quasi-elastic knockout of large groupings of nucleons from light nuclei. From this point of view, the absence of the $\mathrm{B}_5^8$ isobar means a substantial asymmetry of the structure of the $\mathrm{C}_6^{12}$ nucleus with respect to the mirror clusters $\mathrm{B}_5^8$ and $\mathrm{Li}_3^8$.

On the other hand, against the assumption of quasi-elastic knockout of clusters by the incident proton there argues the insignificant change in the most probable energy of \(\mathrm{Li}_3^8\) when the observation angle is changed and when going over to a higher energy of the incident particles. This is clearly seen from a comparison of Fig. 1 and Fig. 3 from work \((^5)\), in which the formation of \(\mathrm{Li}_3^8\) from \(\mathrm{C}^{12}\) at a proton energy of \(2.2\) Bev was studied.

Fig. 1. Range distributions (1) and energy spectra of \(\mathrm{Li}_3^8\) (2) from \(\mathrm{C}^{12}\) at different angles relative to the incident protons

One may also note the small change in the angular distribution of \(\mathrm{Li}_3^8\) when going over to a proton energy of \(2.2\) Bev (unfortunately, the data for such a comparison from work \((^5)\) can be obtained only for the ratios of the yields of \(\mathrm{Li}_3^8\) at angles \(30^\circ/150^\circ\) and \(55^\circ/125^\circ\)).

From the point of view of a cascade mechanism of interaction of fast protons with carbon nuclei, the formation of \(\mathrm{Li}_3^8\) occurs as a result of 2–3-fold

collisions of the incident proton with the nucleons of the \(C^{12}\) nucleus and the subsequent decay of the excited residual nuclei \(Be^{10}_{4}\), \(Be^{9}_{4}\), etc. Calculations of such a cascade process, carried out by P. I. Fedotov\(^6\) by the Monte Carlo method, show that the residual nuclei acquire a comparatively small momentum (an average longitudinal momentum of about \(130\ \mathrm{MeV}/c\)) and an excitation energy of about \(30\ \mathrm{MeV}\).

From the spectra of \(Li^{8}_{3}\) shown in Fig. 1 one can estimate the average longitudinal momentum in the forward direction. It turns out to be approximately \(280\ \mathrm{MeV}/c\), which considerably exceeds the value obtained in the calculation mentioned above. According to the observed distributions, the maximum momentum of \(Li^{8}_{3}\) proves to be about \(900\ \mathrm{MeV}/c\).

Fig. 2. Angular distributions of \(Li^{8}_{3}\) for the entire energy spectrum \(N(\theta)\) (in arbitrary units)

It seems natural to assume that the process proceeds in such a way that the softest part of the \(Li^{8}_{3}\) spectrum owes its origin to reactions \((N, 2N)\) on an intermediate particle of the \(Be^{9}_{4}\) type and to the decay of residual nuclei formed in similar reactions on the \(C^{12}\) nucleus itself. At the same time, the large momentum transfers could be explained in processes involving \(\pi\)-mesons (with their absorption as intermediate particles by nucleon associations, or in reactions with the formation of \(\pi\)-mesons in the interaction of the incident proton with a grouping of nucleons).

In conclusion, the authors express their deep gratitude to Prof. V. P. Dzhelepov for supporting this work; to R. G. Vasil'ev, V. N. Kuz'min, E. S. Rozhkova, and R. M. Yakovlev for help in carrying out the experiment; and to P. A. Gorichev for discussing a number of questions touched upon in this article.

Received
1 III 1963

CITED LITERATURE

1. O. V. Lozhkin, A. A. Rimsky-Korsakov, Pribory i tekhn. eksperimenta, No. 5 (1960).
2. V. I. Ostroumov, Yu. P. Yakovlev, ZhETF, 35, 1358 (1958).
3. K. Lantsosh, Practical Methods of Applied Analysis, Moscow, 1961, pp. 312–432.
4. M. M. Makarov, N. A. Perfilov, DAN, 138, 579 (1961).
5. S. Katkoff, Phys. Rev., 114, 105 (1959).
6. P. I. Fedotov, Author’s abstract of dissertation, Leningrad Physico-Technical Institute, Academy of Sciences of the USSR, 1961.