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them in mica. The presence of neutrons was established from the ionization produced in a counter by the recoil nuclei. The experiments showed that the isotopes Li\(^6\) and Li\(^7\) behave quite differently in nuclear reactions.
Li\(^7\), bombarded by protons, gives \(\alpha\)-particles with a range of \(8.4\) cm, first detected by Cockcroft and Walton. They correspond to the following nuclear reaction:
\[ {}_{3}\mathrm{Li}^{7}+{}_{1}\mathrm{H}^{1}\to{}_{4}\mathrm{Be}^{8}\to{}_{2}\mathrm{He}^{4}+{}_{2}\mathrm{He}^{4}. \]
Some layers of Li\(^7\) gave only the above-mentioned \(\alpha\)-particles; for others a small number of \(\alpha\)-particles with another range, characteristic of Li\(^6\), was observed.
When Li\(^6\) was bombarded by protons, the authors observed \(\alpha\)-particles with a range of \(11.5\) mm and (not more than 1% of the total number) with a range of \(8.4\) mm. The latter are caused by admixtures of Li\(^7\). The corresponding nuclear reaction is:
\[ {}_{3}\mathrm{Li}^{6}+{}_{1}\mathrm{H}^{1}\to{}_{4}\mathrm{Be}^{7}\to{}_{2}\mathrm{He}^{4}+{}_{2}\mathrm{He}^{3}. \]
\({}_{2}\mathrm{He}^{3}\) is an \(\alpha\)-particle with a range of \(11.5\) mm, \({}_{2}\mathrm{He}^{4}\)—an \(\alpha\)-particle with a range of \(8\) mm, which, under the conditions of the experiment, the authors could not observe.
When Li\(^6\) was bombarded by deuterons, \(\alpha\)-particles with a range of \(13.2\) cm were observed. The nuclear reaction, in all probability, is the following:
\[ {}_{3}\mathrm{Li}^{6}+{}_{1}\mathrm{H}^{2}\to{}_{4}\mathrm{Be}^{8}\to{}_{2}\mathrm{He}^{4}+{}_{2}\mathrm{He}^{4}. \]
In addition, there are two groups of protons with ranges of 14 and 30 cm. In the case of Li\(^7\), bombarded by deuterons, the emitted \(\alpha\)-particles have a continuous spectrum with ranges from 1 to 8 cm. This case was considered in one of Rutherford’s earlier works with collaborators. They came to the conclusion that in this case there is a reaction with the emission of three particles according to the formula:
\[ {}_{3}\mathrm{Li}^{7}+{}_{1}\mathrm{H}^{2}\to{}_{4}\mathrm{Be}^{9}\to{}_{2}\mathrm{He}^{4}+{}_{2}\mathrm{He}^{4}+{}_{0}\mathrm{n}^{1}. \]
Indeed, when Li\(^7\) was bombarded by deuterons, the presence of neutrons was established in a noticeably larger amount in comparison with Li\(^6\) and Fe bombarded by deuterons.
Literature
- Proc. Roy. Soc. 146, 922, 1934.
- Proc. Roy. Soc. 141, 259, 1933.
L. Groshev
POSITRON EMISSION OF RADIOACTIVE ELEMENTS
Until now three kinds of radiation emitted by radioactive elements have been known: \(\alpha\)-, \(\beta\)- and \(\gamma\)-rays. With the discovery of positrons, naturally, the question arose whether radioactive elements also emit positrons alongside the above-mentioned radiations. This question was answered positively in two recent works devoted to the study, by a known method, of the production of positrons when heavy elements are irradiated by \(\gamma\)-rays of sufficiently high energy.
As early as 1933 Thibaud (Thibaud, Compt. Rend. 197, 915, 1933) established that a thin-walled glass ampoule filled with radium emanation emits a large number of positrons, whose appearance was difficult to attribute to the action of \(\gamma\)-radiation on the thin glass walls, which likewise consist of light elements. Similar results were obtained with an ampoule containing radiothorium. To explain the origin of these positrons the author assumed that they are emitted by the atoms of the radioactive element itself.
Chadwick, Blackett, and Occhialini (Chadwick, Blackett, Occhialini, Proc. Roy. Soc. 144, 235, 1934) came to the same conclusions, investigating with the aid of
the Wilson chamber the appearance of positrons when \(\gamma\)-radiation acts on matter. In their experiments the active thorium deposit, deposited on an aluminum wire, was sealed in a thin-walled (0.1 mm) glass tube, through whose walls the greater part of the source’s \(\gamma\)-rays could pass. The tube was placed in a Wilson chamber; in this case 24 positrons were recorded for approximately 2000 negative electrons. One might think that the positrons are a consequence of the action of \(\gamma\)-rays on the aluminum wire; however, replacing the aluminum with platinum changed nothing.
The question of positron emission by radioactive elements was investigated in greater detail in the work of Alikhanov and Kozodaev (Alichanow, Kosodaew, Z. Phys. 90, 249, 1934). They applied to the study of positrons the method of magnetic spectroscopy that had long been developed for \(\beta\)-rays. The positron indicators were two Geiger counters, in which simultaneous discharges were observed, caused by the passage of one and the same positron. They investigated a preparation of radium emanation enclosed in a thin-walled glass ampoule. The preparation gave off a large number of positrons.
A distinctive feature of positrons emitted by a radioactive source is their energy spectrum. Whereas the spectrum of positrons emitted when \(\gamma\)-radiation acts on heavy elements has a series of sharply expressed maxima corresponding to definite lines of the \(\gamma\)-spectrum of the element under study, the spectrum of positrons emitted by the radioactive element itself is continuous, with small maxima, due in all probability to the interaction of \(\gamma\)-radiation with the nuclei of the radioactive element. The continuous positron spectrum is in general similar to the continuous spectrum of \(\beta\)-rays.
To explain positron emission by radioactive elements, two possibilities are available: 1) the positrons are emitted by the nuclei of the element, 2) the emission of positrons is a secondary effect caused by the interaction of \(\alpha\)-, \(\beta\)- and \(\gamma\)-rays with extranuclear electrons. At present the second possibility must be considered the more probable.
L. Groshev
\(\gamma\)-QUANTA OF HIGH ENERGY ARISING IN THE BOMBARDMENT OF LITHIUM AND FLUORINE BY FAST PROTONS1
When fast protons or deuterons act on certain light elements, \(\gamma\)-quanta arise in the latter. To estimate the energy of these quanta, use is usually made of data obtained from analysis of the absorption curves of the given \(\gamma\)-radiation in some substance. However, this method not only has low sensitivity, but also does not make it possible to determine the energy of a \(\gamma\)-quantum unambiguously in the case when the absorption curve has been obtained in only one substance. The reason for this is that, for quanta of high energy, in addition to the usual absorption mechanisms, which weaken with increasing frequency of the \(\gamma\)-quanta (Compton scattering, photoelectric absorption), a new type of absorption—pair production (electron—positron)—plays an ever increasing role; the part of the absorbed energy due to it increases strongly with increasing frequency of the \(\gamma\)-quantum and depends to a great degree on the atomic number of the absorbing element. Fig. 1 gives theoretical curves for the dependence of the absorption coefficient (taking pair production into account) on the energy of the \(\gamma\)-quantum, calculated by Oppenheimer for Pb and Cu. From the curves it is clear how strongly their form changes in going from Cu to Pb. They also show why, from the absorption coefficient for a single element, one cannot judge the energy of a \(\gamma\)-quantum unambiguously. Therefore, in order to determine the energy of \(\gamma\)-radiation by this method, it is necessary to measure simultaneously the absorption coefficients for at least two substances.
Lauritsen et al. determined the absorption coefficients in lead and copper