CROSS SECTIONS OF REACTIONS INDUCED BY NEUTRONS WITH AN ENERGY OF 14.5 MeV

The study of the cross sections of reactions \((n\alpha)\), \((np)\), and \((n, 2n)\), occurring in various elements under the action of fast neutrons, is of considerable interest from the standpoint of testing the existing theory of nuclear reactions. In earlier work \(^{1,2,3}\) the neutron spectrum extended to low energies, which made comparison of the obtained results with theory insufficiently definite.

In the paper under review \(^{4}\), the reaction \(\mathrm{T}^3(dn)\mathrm{He}^4\) was used to obtain neutrons. The neutrons produced when a tritium target is bombarded with deuterons of energy \(E = 100\) keV have an energy of 14.5 MeV. The total neutron yield was about \(10^8\) neutrons per second and was measured with a \(\mathrm{BF}_3\) counter surrounded by paraffin. At the same time, to control the angular proportional counter registered the \(\alpha\)-particles emitted from the target at a precisely fixed solid angle.

Samples of elements or their compounds, for the most part in the form of pressed powders, were placed at a distance of 3 cm from the target. The activity produced as a result of irradiation was measured with an end-window counter. To determine the efficiency of the recording apparatus, radioactive impurities (\(\mathrm{Ca}^{45}\), \(\mathrm{Co}^{60}\), \(\mathrm{Tl}^{204}\), etc.), whose activity was known to an accuracy of 3%, were added to the samples. By means of this calibration, the dependence of the recording efficiency on the maximum energy of the electrons of \(\beta\)-decay was obtained. A similar calibration was also carried out for \(\gamma\)-rays.

Cross sections were measured for 38 \((np)\) reactions, 25 \((n\alpha)\) reactions, and 34 \((n, 2n)\) reactions giving known radioactive products with half-lives from 7.3 seconds to 32 days. The experimental data obtained were compared with cross sections calculated according to the statistical theory of nuclear reactions. The values of the formation cross sections for the compound nucleus needed for the calculation were obtained by interpolation of the results of work with 14-MeV neutrons. The ratio of the observed cross section to the calculated one in the case of emission of charged particles proved to depend substantially on the atomic weight \(A\) of the element. For the heaviest nuclei it reaches 20,000. Such a considerable discrepancy cannot be eliminated by varying the parameters in the expressions for the level density of the residual nucleus and the barrier penetrability.

It is possible, however, that all the neutron energy is transferred to an individual proton or \(\alpha\)-particle in the nucleus, followed by the emission of these particles without transfer of energy to other particles. Calculations \(^{5}\) carried out for the \((np)\) reaction show that, if such an effect occurs in \(5\text{–}10\%\) of cases, it fully explains the observed magnitude of the cross section.

In the \((n, 2n)\) reaction, the value of the cross-section ratio remains close to unity. The level density in nuclei with odd \(A\) is almost three times greater than the level density of the corresponding even-even nuclei.

B. R.

CITED LITERATURE

1. J. V. Jelley and E. B. Paul, Proc. Phys. Soc. A63, 112 (1949).
2. H. Wäffler, Helv. Phys. Acta 23, 239 (1951).
3. B. L. Cohen, Phys. Rev. 81, 184 (1951).
4. E. B. Paul and R. L. Clarke, Can. Journ. Phys. 31, 267 (1953).
   J. M. Blatt and V. J. Weisskopf, Theoretical Nuclear Physics, NY, 1952.
5. H. Mc. Manus and W. T. Sharp, Phys. Rev. 87, 188 (1952).