Radioactivity

RADIOACTIVITY. – Atoms, according to the suggestive model proposed by Rutherford, consist of a nucleus and satellite electrons (e.). The nuclei, in turn, are formed by protons (p.) and neutrons (n.), particles nearly equivalent from the standpoint of mass, but the former having a positive elementary charge and the latter being electrically neutral.

The content of p. and n. characterizes a nucleus. The number Z of p. is called the atomic number of the nucleus

oppose arbitrary concentrations of retained foundations, or undue interference by tax officials to suppress others. These local colleges were attached to the Section of the Opera dei Congressi, and not a few cases were won before the tribunals and Courts, or through appeals to the Council of State. R. T. held this presidency for as long as the Opera dei Congressi lasted (1904), to the great advantage of pious institutions; in matters of

(or of the atom to which the nucleus belongs). The total number M of nucleons (p+n) is defined as the mass number of the nucleus. The n. will therefore be M-Z. Among the nuclei existing in nature, 230 stable and about forty unstable or radioactive nuclei are known. Substances capable of spontaneously emitting radiation (corpuscles or electromagnetic waves), detected through some of their effects, such as the blackening of photographic emulsions, the fluorescence of certain substances, and the ionization of air and gases, are called radioactive.

The discovery of radioactive phenomena by H. Becquerel in 1896 recalls the discovery of X-rays by Roentgen. Here too it was by chance that Becquerel discovered that uranium (U) salts left on a closed box of photographic plates were able to impress them even through the cardboard of the box. Later, Pierre and Marie Curie, in order to detect the r. of a substance and measure its intensity, used an electroscope and inferred the substance’s radioactive intensity from the speed with which it discharged the electroscope. The Curies thus succeeded in establishing that the effects were proportional to the quantity of U present and subsequently succeeded in isolating from pitchblende two new elements that were much more radioactive, one of which was called polonium (Po), in honor of the country of origin of Marie Curie, and the other radium (Ra), as if to say the most typically radioactive element. Ra is one million times more radioactive than U, that is, 1 mg of Ra discharges the electroscope in the same time as 1 kg of U.

The initiative for research on r. subsequently passed to the Cambridge school directed by Rutherford, who was able to establish that radioactive substances emit three types of radiation, one of which, like light and X-rays, undergoes no deflection from electric or magnetic fields: it was called γ radiation and proved to consist of high-energy photons, that is, electromagnetic waves with a wavelength equal to that of X-rays. The other two radiations were called α and β. The direction and magnitude of the deflection of α rays in an electric and magnetic field proved that they are corpuscles with a positive charge equal to twice that of the e., and with a mass of 4 m. u. (the unit of mass m. u. represents 1/16 of the mass of an atom of O). The α particles therefore have Z=2 and M=4 and are thus identified with helium nuclei 4He³. β radiation also consists of corpuscles and is identified with the e., both in charge (which is a negative elementary charge) and in mass (1/1840 of m. u.), which in many cases is negligible. Since these α and β radiations are of nuclear origin, the following laws concerning radioactive disintegrations, due to Soddy and Fajans, are readily understood: 1) through the emission of an α, a nucleus is transformed into the nucleus of another element whose atomic number Z is reduced by 2 and whose mass number M is reduced by 4; 2) through the emission of a β particle, a nucleus is transformed into that of another element having Z increased by 1 and M unchanged. The first of these laws is entirely evident, since a 2α⁴ is equivalent to 2p+2n; for example, for ⁸⁸Ra²²⁶ one has:

⁸⁸Ra²²⁶ longrightarrow ²α⁴ + ⁸⁶Em²²²

To account for the nuclear origin of β particles, one must assume that in the nucleus an n. splits into p. and e.; the p. remains in the nucleus, in place of the n., while the e. is expelled. The change in the mass of the nucleus is negligible (M, the sum of the p. and n., remains unchanged), but its positive charge increases by one elementary unit.

The nuclei of the following three isotopes are radioactive: potassium (Z=19), samarium (Z=62), and lutetium (Z=71); ¹⁹K²⁰, ⁶²Sm²¹⁰, ¹⁷Lu¹¹⁰; K, Sm, and Lu are three elements, each with two or more stable isotopes; all the other radioactive nuclei (about forty) belong to the heavier elements, specifically thallium (81), lead (82), and bismuth (83), which also have stable isotopes, and polonium (84), emanation (86), radium (88), actinium (89), thorium (90), protactinium (91), and uranium (92), which have only radioactive isotopes. These radioactive nuclei may be grouped into three radioactive “families,” headed by the three nuclei U²³⁵, U²³², and Th²³², from which the others are obtained through successive β and α disintegrations. We list one of

these families, that of ₂₀U²³⁶ (in parentheses, the type of r. by which one nucleus passes into the next):

U²¹⁶(α) Th²³¹(β) Pa²³¹(β) U²³¹(α) Th²³¹(γ) Ra²² (α) Em²²²(α) Po²¹⁴(α) Pb²¹⁴(β) Bi²¹⁴(β) Po²¹⁴(α) Pb²¹⁶(β) Bi²¹⁶(β) Po²¹⁶(β) Pb²⁰⁶.

The three families have as their final descendant a stable isotope of Pb, which for U²³¹ is respectively Pb²⁰⁷ and Pb²⁰⁸.

It may be observed that the nuclei of the same family must have mass numbers differing by a multiple of α, since variations in M occur only as a consequence of the emission of an α². In addition to the three aforementioned radioactive families, there could therefore be a fourth, descended from a progenitor with M=233 or 237, which, however, is not found in nature. Its radioactive nuclei were produced artificially by Seaborg at the Berkeley laboratory in California.

Every radioactive nucleus is characterized by a radioactive constant; this is the fraction of nuclei that disintegrate per unit of time (1 sec); for example, the radioactive constant of Ra²²⁸ is 1.38.10⁻¹¹/sec. This small fraction has meaning only when it refers to a very large number of nuclei, for which the concept of the probability of an event approaches the certainty of a mathematical law.

The following analogy may be drawn: if the average human lifespan is 50 years, the average probability of a man’s dying in one year is 1/50. One cannot, however, assert either that a given individual will die precisely at the end of 50 years, or that of 50 men present on 1 January one and only one will have died by the following 31 December; but by extending the observations to 50,000 men, the prediction of 1,000 deaths in one year can be made with a probable error of only 3%; and for 50 million men, the prediction of 1 million annual deaths has a probable error reduced to 0.1%.

Recalling that Avogadro’s number (the number of atoms contained in M grams of a substance) is 6.10²², one obtains that 1 g of Ra²²⁸ contains 6.10²²: 226 = 2.66.10²¹ nuclei; the disintegrations per second (dps) are 2.66.10²¹.1.38.10⁻¹¹ = 37 billion, with a probable error of only 0.0005%. The unit of r., called the “curie,” is defined precisely as the r. of a substance in which 37.10² dps occur. The mean life (English “means life”) of a radioactive substance, as in the case of human mortality, is the inverse of the probability of disintegration per unit of time, that is, the inverse of the radioactive constant; the period T (English “half life”), on the other hand, represents the time required for half of a given radioactive substance to disintegrate; it can be shown that T is approximately equal to 60/100 of the mean life. We give some characteristic data for certain radioactive substances (a.=years; d.=days):

Artificial radioactive substances can be obtained by bombarding natural nuclei (radioactive or not) with corpuscles or photons of sufficient energy. In chronological order, the particles used first were the α particles of naturally occurring radioactive substances (Rutherford), then p., d., and α particles accelerated by special machines (linear accelerators, cyclotrons, etc.), and finally n., of which intensely powerful beams can now be obtained by means of atomic piles. By bombarding nuclei with neutrons, hundreds of new radioactive nuclei have been obtained; these very rarely exhibit α r., often β r., and also other forms of r. consisting in the emission of p., d., n., and positive electrons.

For the study of r., photographic emulsions are still widely used, with a special technique developed chiefly by Powell. The ionization effects of gases are exploited in detecting instruments, much more sensitive than electroscopes, known as Geiger–Müller counters, cloud chambers or Wilson chambers, and ionization chambers. These instruments make it possible to record the passage even of a single elementary radiation and to measure the energy it possesses.

For the bibliography, cf. NUCLEO ATOMICO. Tito Franzini