Radioactivity

RADIOACTIVITY. - Atoms, according to the suggestive model proposed by Rutherford, are made up of a nucleus and satellite electrons (e.). And the nuclei are in turn formed of protons (p.) and neutrons (n.), particles almost equivalent from the standpoint of mass, but with a positive elementary charge the former and electrically neutral the latter.

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

(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 ones are known and about forty unstable or radioactive ones. Those substances are called radioactive which are capable of spontaneously emitting radiations (corpuscles or electromagnetic waves) revealed by certain of their effects, such as the blackening of photographic emulsions, the fluorescence of certain substances, the ionization of air and gases.

The discovery of radioactive phenomena by H. Becquerel in 1896 recalls the discovery of X-rays by Roentgen. Here too it is by chance that Becquerel finds that uranium salts (U) left on a closed box of photographic plates manage to impress them even through the cardboard of the box. Later Pierre and Marie Curie, in order to reveal the r. of a substance and measure its intensity, use an electroscope and deduce the radioactive intensity of the substance from the rapidity with which it discharges the electroscope. The Curies thus succeed in establishing that the effects are proportional to the quantity of U present and subsequently succeed in isolating from pitchblende two new elements far more radioactive, which were called the one 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 a million times more radioactive than U, that is, one mg of Ra discharges the electroscope in the same time as 1 kg of U.

The initiative in studies on r. passes subsequently 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 deviation by electric or magnetic fields: it was called γ radiation and proved to be made up of photons of great energy, that is, of electromagnetic waves with a wavelength equal to that of X-rays. The other two radiations were called α and β. The sense and the value of the deviation of the α rays in an electric field and in a 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 mass unit m. u. represents 1/16 of the mass of the O atom). The α therefore prove to be particles with Z=2 and M=4 and are therefore identified with nuclei of helium 4He³. The β radiations are also corpuscles and are identified with the e. as regards charge (which is a negative elementary charge) and as regards mass (1/1840 of m. u.), which is in many cases negligible. Since these α and β radiations are of nuclear origin, it is easy to understand the following laws due to Soddy and Fajans on radioactive disintegrations: 1) by the emission of an α a nucleus is transformed into the nucleus of another element with atomic number Z decreased by 2 and mass number M decreased by 4; 2) by 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 immediately evident given that a 2α⁴ is equivalent to 2p+2n; for example for ⁸⁸Ra²²⁶ we have:

⁸⁸Ra²²⁶ longrightarrow ²α⁴ + ⁸⁶Em²²²

For the nuclear origin of the β one must admit 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 variation of 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 three following isotopes are radioactive: potassium (Z=19), samarium (Z=62), lutetium (Z=71); ¹⁹K²⁰, ⁶²Sm²¹⁰, ¹⁷Lu¹¹⁰; K, Sm, Lu are three elements each with two or more stable isotopes; the other radioactive nuclei (about forty) all belong to the heaviest elements, and precisely to thallium (81), lead (82), bismuth (83), which also have stable isotopes, and to polonium (84), emanation (86), radium (88), actinium (89), thorium (90), protactinium (91), uranium (92), which have only radioactive isotopes. These radioactive nuclei can be grouped into three radioactive "families" which have as their progenitors the three nuclei U²³⁵, U²³², Th²³², from which the others are obtained by successive β and α disintegrations. Let us list one of these families, that of ₂₀U²³⁶ (in parentheses the type of r. by which one passes from one nucleus to the other):

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 last descendant a stable isotope of Pb which for U²³¹ is respectively Pb²⁰⁷ and Pb²⁰⁸.

It may be observed that the nuclei of one and the same family must have mass numbers that differ by a multiple of α, inasmuch as the variations of M occur only as a consequence of the emission of an α². Besides the above three radioactive families, there could therefore exist a fourth, descended from a progenitor with M=233 or else 237, which however is not found in nature. The radioactive nuclei of it have been manufactured artificially by Seaborg at the laboratory of Berkeley in California.

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

One may draw this analogy: if the average life of men is 50 years, the average probability of death of a man in a year is 1/50. One cannot however assert either that a given individual will die precisely at the end of his 50 years, nor that out of 50 men present on Jan. 1 one and only one will have died by the following Dec. 31; but by extending the observations to 50,000 men the prediction of 1000 deaths in a year can be made with a probable error of only 3%; and on 50 million men the prediction of 1 million deaths per year has a probable error reduced to 0.1%.

Recalling that Avogadro's number (number of atoms contained in M gr of substance) is 6.10²², one obtains that 1 gr 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 average life (Eng. "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 (Eng. "half life") represents instead the time necessary for the disintegration of one half of a given radioactive substance; it can be demonstrated that T is approximately equal to 60/100 of the average life. We give some characteristic data of some 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. Above all, in order of time, there were used first the α particles of natural radioactive substances (Rutherford), then p., d., α accelerated by special machines (linear accelerators, cyclotrons, etc.) and finally the n. of which intense beams can today be obtained by means of atomic piles. By bombarding nuclei with neutrons, hundreds of new radioactive nuclei have been obtained which very rarely exhibit α r., often β r., and moreover other forms of r. which consist in the emission of p., d., n., positive electrons.

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

For the bibl. cf. NUCLEO ATOMICO. Tito Franzini