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HIGH-ENERGY QUANTA
In the radioactive decay of normal atoms, γ-quanta are emitted whose energy does not exceed 2.6 MeV (ThC″). High-energy quanta arise when matter is irradiated with fast particles (α-particles, protons, deuterons). Thus, for example, Lauritsen et al.^1 established that when lithium and fluorine are irradiated with fast protons, γ-quanta with energies up to 12 MeV appear. Still larger quanta were observed by Joliot and Kowarski,^2 who studied γ-radiation appearing in artificial radioactivity produced in matter by neutrons. In this case the radiation already covers the region of soft cosmic rays.
Fermi and his collaborators^3 established that, in artificial radioactivity produced by neutrons in heavy atoms, an isotope of the original element is formed with an atomic number greater by one. One of the possible interpretations of this effect is the idea of capture of the neutron by the nucleus. Such a neutron, entering the nucleus, causes its rearrangement. This rearrangement of the nucleus is accompanied by the emission of γ-quanta, which characterize the energy of attachment of the neutron to the nucleus. Joliot and Kowarski studied precisely these quanta.
They placed a piece of silver in front of the wall of a Wilson chamber with a magnetic field (~1000 gauss) and induced artificial radioactivity in the silver by means of neutrons obtained from a polonium preparation (150 millicuries) with beryllium. In their experiments the presence in the Wilson chamber of slightly curved tracks was established. These tracks are caused by electrons whose energy is greater than 10 MeV.
In view of the rather weak magnetic field, it was possible to establish only rough limits for the energy of these electrons: 20–30 MeV. Several experiments were carried out with a 3-mm lead plate placed across the Wilson chamber. Of the four electrons photographed in this case, some had an energy greater than 15 MeV, the rest about 9 MeV.
Control experiments, carried out in the absence of the silver plate, showed that the cause of the existence of electrons of high energies lies in the silver itself.
Analogous results were obtained with J and P, with the only difference that in this case electrons of high energies were observed even after removal of the neutron source, whereas for Ag the electrons were observed only in the presence of the neutron source.
The authors explain the presence of fast electrons by the existence of high-energy γ-quanta that appear during the rearrangement of the nucleus. The difference in the behavior of Ag, on the one hand, and J and P, on the other, can be explained by the fact that the γ-quanta in the first case arise during the formation of a new radioactive atom, while in the second case they arise during the decay of the new radioactive atom.
From the masses of the neutron and of the initial and final atoms, the authors attempted to calculate the energy liberated during the rearrangement of the nucleus. It is true that for Ag the corresponding data for the calculation are insufficient, but for Se more or less accurate data exist. The calculation carried out for Se gives for the liberated energy a value of \(20 \pm 10\) MeV, which in order of magnitude agrees with the experimental data.
LITERATURE
- Crane, Delsasso, Fowler and Lauritsen, Phys. Rev. 46, 531, 1934.
- Joliot et Kowarski C. R. 200, 824, 1935.
- Amaldi, d’Agostino, Fermi, Rasetti, Segré, Proc. Roy. Soc. A. 146, 483, 1934.
L. Groshev