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NEUTRONS KNOCKED OUT BY γ-QUANTA FROM BERYLLIUM NUCLEI
When a γ-quantum of sufficiently high energy is absorbed, an atomic nucleus may undergo a certain transformation; in this process one of its constituent particles—an α-particle, a proton, or a neutron—may be ejected from the nucleus. One case of such nuclear transformation was discovered by Chadwick and Goldhaber¹. Using an ionization chamber connected through a linear amplifier to an oscillograph, they observed the appearance of protons when nuclei of heavy hydrogen, ThC″, were irradiated with γ-rays.
Szilard and Chalmers² developed another method for detecting nuclear transformations accompanied by the ejection of neutrons. They made use of the fact that neutrons, upon entering matter, produce in it an artificial radioactivity with a quite definite half-life characteristic of each element. Using this method, the authors established that beryllium nuclei, when irradiated with γ-quanta from radium, emit neutrons. Their experiments were carried out as follows. 150 mg of radium was sealed in a vessel whose walls stopped all α-particles from the source falling upon them. Around the vessel was placed 25 g of beryllium, and all this in turn was surrounded by 100 cm³ of ethyl iodide, which served as an indicator of the presence of neutron emission from beryllium subjected to the action of Ra γ-rays (we note that the new radioactive element arising from iodine under the influence of neutrons is an isotope of the original element). After exposure, from the ethyl iodide, by appropriate treatment, a precipitate of iodine silver was obtained, which showed artificial radioactivity with a half-life characteristic for iodine of 30 minutes (in agreement with Fermi’s data). The active precipitate gave about 200 counts per minute in a Geiger–Müller counter, whereas in the absence of beryllium no more than 12 counts per minute were observed. The authors note that the effect observed in iodine is sufficiently large that it can be detected even without chemical separation of the radioactive element produced.
Literature
- Chadwick and Goldhaber, Nature 134, 237, 1934.
- Szilard and Chalmers, Nature 134, 494, 1934.
L. Groshev
NUCLEAR REACTIONS ON SEPARATED LITHIUM ISOTOPES
In the study of nuclear reactions occurring in matter under the influence of a flux of fast, artificially produced particles (protons, deuterons), one usually encounters the difficulty of deciding to which isotope of the element under study the observed decay products should be assigned. A consequence of this is that different interpretations are sometimes given to one and the same process.
The most radical way out of such a situation would be the study of nuclear reactions on separated isotopes, especially since the reactions for the individual isotopes of one and the same element are very different. This is precisely the path followed in their recent work by Oliphant, Shire, and Crowther¹. They applied a mass-spectroscopic method for separating the lithium isotopes Li⁶ and Li⁷. The main disadvantage of this method is that, when it is applied here, the currents are extremely small and rarely exceed \(10^{-8}\) A, whereas to obtain one quarter of a microgram of Li⁷ it would be necessary to pass a current of 1 μA for one hour. However, in the particular case of lithium, as Oliphant and Rutherford² had shown earlier, the probability of destruction of lithium nuclei by protons and deuterons is so great that, in order to study the decay products, it is sufficient to have only a monomolecular layer of lithium.
To separate the lithium isotopes, Oliphant, Shire, and Crowther built two special mass spectrographs with crossed electric and magnetic ...
and magnetic fields. In both cases the lithium ions were obtained from a preliminarily activated platinum wire coated with a layer of \(^{3}\mathrm{Li}_{2}\mathrm{CO}_{3}\), \(\mathrm{Al}_{2}\mathrm{O}_{3}\), \(3\mathrm{Si}_{2}\mathrm{O}\), from which an ion current of several hundred microamperes could be drawn for a number of hours. The separated isotopes were collected on a cooled liquid-nitrogen metal plate and fixed on it in the form of LiCl by admitting HCl vapor into the vessel.
The first mass spectrograph, constructed by the authors, is a quadrangular box, the side walls of which, made of soft iron, are the poles of an electromagnet, while the other two serve as deflecting plates, between which an electrostatic field is created. The inner walls of the box form a channel measuring \(2 \times 20\) mm. Through it, from the source, passes a beam of lithium ions (800 V). The length of the deflecting plates is 10 cm, that of the magnetic poles 14 cm; moreover, over a length of 4 cm near the collector there is only a magnetic field, which strongly deflects the ion beam from the rectilinear path and thereby frees it from the atomic beam. Separation of the isotopes is achieved by choosing the magnitude of the magnetic and electric fields (4000 gauss, 600 V/cm), which make it possible for only one isotope to leave the channel; the other isotope, long before the end of the channel, is deflected from the rectilinear path and is trapped in it.
Fig. 1.
Sharp outlines of the spot at the places where the ions strike indicate uniformity in the ion velocities and the purity of the separated isotopes.
With the aid of the first mass spectrograph the authors succeeded in obtaining isolated isotopes in an amount of approximately \(5 \cdot 10^{-8}\) g, passing an ion current of \(\mathrm{Li}^{7}\) of \(2.5 \cdot 10^{-6}\) A for 5 min and \(0.25 \cdot 10^{-6}\) A of \(\mathrm{Li}^{6}\) for 50 min.
Figure 1 shows the second mass spectrograph used by the authors for separating Li isotopes. \(PP\) are the deflecting plates. The dashed line indicates the position of the magnetic field. \(F\) is the ion source. \(E_{1}, E_{2}, E_{3}\) are a series of auxiliary electrodes. By applying the appropriate accelerating electric fields between \(E_{1}, E_{2}, E_{3}\), the ion beam is focused in the plane of the collector \(CC\). The magnetic and electric fields in the mass spectrograph were chosen so that ions of one isotope were focused on one plate \(C\), ions of the other on the other plate (the distance between the plates \(CC\) was 15 mm).
In this apparatus the authors used for \(\mathrm{Li}^{7}\) a current of \(5 \cdot 10^{-6}\) A, and for \(\mathrm{Li}^{6}\), \(0.4 \cdot 10^{-6}\) A, and were able to obtain isotopes in amounts of several tenths of a microgram. Larger quantities of isolated isotopes could not be collected because of the destruction in the Li layer produced by the incident Li ions themselves. The authors promise to eliminate this effect by reducing the velocity of the ions before they reach the collector.
The separated isotopes, deposited on a copper or nickel plate, were bombarded by the authors with protons and deuterons (160 kV). The range of the particles arising from the disintegration of the nuclei was measured by absorption
them in mica. The presence of neutrons was established from the ionization produced in a counter by 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; in others, however, a small number of \(\alpha\)-particles with another range was observed, characteristic of Li\(^6\).
When bombarding Li\(^6\) with 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 impurities 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, while \({}_{2}\mathrm{He}^{4}\) is 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 with deuterons, \(\alpha\)-particles with a range of 13.2 cm were observed. The nuclear reaction is, in all probability, 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 with deuterons, the emitted \(\alpha\)-particles have a continuous spectrum with ranges from 1 to 8 cm. This case was examined 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 with deuterons, the presence of neutrons was established in a noticeably larger quantity than in the case of Li\(^6\) and Fe bombarded with 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, the question naturally arose whether radioactive elements emit positrons as well as the above-mentioned radiations. This question was answered affirmatively in a series of recent works devoted to the study of the known method of producing positrons by irradiating heavy elements with \(\gamma\)-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, the appearance of which was difficult to attribute to the action of the radiation on the thin glass walls, consisting likewise 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), studying with the aid of