RADIOACTIVITY. - Atoms, according to the suggestive model proposed by Rutherford, are constituted by a nucleus and by satellite electrons (e.). The nuclei in turn are formed by protons (p.) and neutrons (n.), particles almost equivalent from the point of view of mass, but with an elementary positive charge in the former and electrically neutral in the latter.
The content of p. and n. characterizes a nucleus. The number Z of the 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 and about forty unstable or radioactive ones are known. Radioactive are those substances that are capable of spontaneously emitting radiations (corpuscles or electromagnetic waves) revealed by certain of their effects, such as the fogging of photographic emulsions, the fluorescence of certain substances, and 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 was by chance that Becquerel found that uranium (U) salts left on a closed box of photographic plates would fog them even through the cardboard of the box. Later, Pierre and Marie Curie, to reveal the radioactivity of a substance and measure its intensity, used an electroscope and deduced the radioactive intensity of the substance from the speed with which it discharged the electroscope. The Curies thus succeeded in establishing that the effects are proportional to the amount of U present and subsequently succeeded in isolating from pitchblende two new elements much more radioactive, which were called 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, 1 mg of Ra discharges the electroscope in the same time as 1 kg of U.
The initiative in studies on radioactivity then 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 deviation by electric or magnetic fields: this was called γ radiation and was found to consist of photons of great energy, that is, of electromagnetic waves of wavelength equal to that of X-rays. The other two radiations were called α and β. The direction and value of the deviation of the 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 the O atom). The α particles are therefore particles with Z=2 and M=4 and are identified as helium nuclei 3He4. The β radiations are also corpuscles and are identified with the e. as charge (which is an elementary negative charge) and as mass (1/1840 of m.u.), which in many cases is 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) with the emission of an α particle, 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) with the emission of a β particle, a nucleus is transformed into that of another element with Z increased by 1 and M unchanged. The first of these laws is immediately evident since an α² is equivalent to 2p+2n; for example, for 84Ra226 we have:
\[
84Ra^{226} \rightarrow 2X^4 + 86Em^{222}
\]
As for the nuclear origin of β, it must be admitted 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 mass of the nucleus is negligible (M, the sum of p. and n., remains unchanged), but its positive charge increases by one elementary unit.
Radioactive are the nuclei of the following three isotopes of potassium (Z=19), samarium (Z=62), lutetium (Z=71): 14K40, 42Sm148, 17Lu176; K, Sm, Lu are three elements each with two or more stable isotopes; the other radioactive nuclei (about forty) all belong to the heavier elements, namely thallium (81), lead (82), bismuth (83), which also have stable isotopes, and polonium (84), emanation (86), radium (88), actinium (89), thorium (90), protoactinium (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, U238, Th232, from which the others are obtained by successive β and α disintegrations. Let us list one of these families, that of the ...
... (in parentheses the type of radiation with which one passes from one nucleus to another):
Greater efforts (up to 20,000 km.) and with ever higher antennas (up to more than 300 meters) to which naturally had to be associated similar powers of radiation. But the practical results, although already notable, never corresponded to expectations, because in transmissions over very great distances there were still mysterious interferences that prevented the regularity of radio services.
It was again Marconi who shed some light on the serious issue. As early as 1902, experimenting, he had observed that no differences in intensity were found in the signals received by day and by night when they came from distances of less than 800 km., but that by day the signals from distances greater than 1,000 km. were completely missing, while at night they arrived normally even from distances of 2,500 km. and were still decipherable from distances of 3,500 km.
It was therefore sunlight that caused the disturbances. At the same time he had observed that over a distance of 244 km. nighttime signals could be transmitted with an antenna of 12 m., while to obtain daytime signals of equal intensity an antenna of as much as 185 m. was required. Finally, noting that often a ship could not communicate with a nearby station, while it regularly communicated with a distant station, he discovered the existence of what are now called dead zones.
And it was precisely on these experimental bases, applying the Galilean criterion, that one could arrive at an at least approximate understanding of what happens in the other layers of the atmosphere. Some of these, at a height of about 100 km., under the action of ultraviolet rays from the sun and cosmic rays, become strongly ionized and conductive; they therefore strongly absorb, like any other conductor, long waves, while they absorb little and therefore reflect shorter waves. The mysterious facts were thus clarified, but while most technicians thought that before the natural fact signaled nothing more could be done than to take it into account in assuming the commitments of transmitting telegrams, Marconi took the most decisive step for the future of radio transmissions: he returned, that is, to experimenting with short-wave transmissions, which he and the technicians had neglected, and shortly was certain that only with them could all difficulties be resolved. Communicating the new discovery he literally said: "I have been mistaken, and with me all others who have followed me. I shall nevertheless be the first to retrace my steps, abandoning long waves for short waves, on which the future of radio will be founded."
### III. MEDIUM WAVES AND RADIOTELEPHONY
Before proceeding to give some details on the characteristics of short-wave transmissions, it is well to consider briefly the technique of medium waves, with which all ordinary radio broadcasting was first accomplished. First of all, it should be noted that with medium waves we are no longer dealing with telegraphy—that is, the simple transmission of conventional signals—but with telephony, that is, the transmission of sounds, which is a far more difficult task. Yet the already highly advanced technique quickly overcame these difficulties.
Let the transmitting station be set to emit electromagnetic waves of a given wavelength λ, for example, 300 m. Naturally, its frequency ν will be such that the product λν equals the speed of light; thus, it will be equal to 1,000,000 periods or alternations per second, so rapid that, unless special devices are used, the frequency ν is the same as that of a continuous flow of electromagnetic energy. The emission of a Morse telegraphic signal corresponds to a simple burst of this radiation, as brief as possible for dots and slightly longer for dashes. To transmit a certain sound, for example, an A-sharp of 435 periods, it is necessary instead that the said electromagnetic flow undergo 435 alternations per second, or, in other words, that the very high electromagnetic frequency be modulated by the acoustic frequency of the sound to be transmitted, which is achieved through appropriate technical devices. Thus, the waves propagating through the ether always have the fundamental frequency ν of the transmitting station, but modulated by the acoustic frequency of the note, and can only be detected by a receiver tuned to frequency ν, producing the corresponding sound in either a telephone receiver or a loudspeaker. All these transformations occur while preserving not only the frequency (the note) but also the timbre of the transmitted sound, making it possible to transmit not only music but also the far more difficult transmission of speech.
### IV. SHORT WAVES
The term "short waves" now refers to waves with wavelengths between 100 and 15 m., while the designations "ultra-short waves" or "microwaves" are reserved for those with wavelengths between 15 and 1 m., and less than 1 m., respectively.
Among the useful properties of short waves, besides their near-insensitivity to the harmful effects of sunlight, and perhaps an even more useful property, their insensitivity to inclement weather, is the invaluable and now widely utilized characteristic of being easily directed—a property upon which modern radio applications are essentially based.
Short waves, even apart from the fact that they are generally produced hertzianally—that is, with a transmitting device not connected to the ground—already have a pronounced tendency to propagate in a straight line, like the much shorter optical waves. This property allows them to be produced, as Marconi noted, in beams, even of small cross-section, which do not tend to spread significantly either during propagation or upon reflection, and which can be directed from the moment of emission toward a predetermined direction. This direction is chosen so that, upon reflection within the aforementioned spherical ionized layer (which is conductive and reflective), the beam strikes exactly the desired location. Naturally, if the distance is very great, for example, when attempting to telegraph to the antipodes, a single reflection will not suffice; in such cases, the direction of departure can be arranged so that the reflected beam strikes a location (generally the sea) from which it is reflected again toward the upper layer, so that after one or more additional reflections, it reaches the intended destination. It is understood that this theoretical procedure must be supported by appropriate technical aids and controls; yet the fact remains that within a relatively few years, it has been possible not only to telegraph but also to telephonically communicate across the antipodes.
It is interesting to note that beneath the reflections of the upper layer, there are large zones of absolute silence, similar to the less absolute ones observed in the case of long waves, which are very useful in avoiding excessive publicity of transmissions. Equally interesting and useful is the fact that the entire transmission process occurs exclusively through successive electronic phenomena and motions, without mechanical devices, which would inevitably introduce delays due to their inertia. It is precisely this absence of delays that has made it possible to achieve in telegraphy a transmission speed even greater than that already very high speed attained by telegraphy over conductors. Economically, there is also the great advantage of enormous energy savings, since short-wave and beam transmissions require only installations of very limited power, which have the added benefit of not interfering with one another.
Ultra-short waves do not present sufficient differences from the preceding ones to warrant special applications. Microwaves, however, though still very different from their optical counterparts, are already of great scientific and technical interest. In particular, they are now in use in radar applications (v. RADAR) and are increasingly employed in various visual applications (v. TELEVISION).
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### V. RADIO AND CATHOLICS
The Church could not remain indifferent to the problems arising from the good or bad use of radio as a means of propaganda, nor to the need to employ radio as an instrument of apostolate.
1. **Radio and the Holy See.** — The Holy See established its first radio station in the Vatican City in 1931, under the guidance of the inventor of radio himself, Guglielmo Marconi, and Pope Pius XI began that year those radio messages which, under his pontificate and especially under that of his successor, would carry the thought of the Church and the exhortations of the Supreme Pastor to every part of the Catholic world with a speed and breadth hitherto unknown, transmitting news, religious celebrations, and instructions in the principal languages.
A decree of the Sacred Penitentiary, *De Benedictione Papali ope radiophonica accepta*, of June 15, 1933 (AAS, 23 [1931], p. 277), sanctions the liturgical validity of the papal blessing transmitted by radio and the possibility of gaining the indulgences attached to it even for radio listeners; indeed, the Latin formula following the blessing includes the phrase *"cunctis Christi fidelibus qui adsunt et qui ubique terrarum ope radiophonica... benedicentis vocem pie devoteque accipiant."* However, a Mass heard over the radio is not considered valid for fulfilling the Sunday precept.
2. **Catholic Initiatives.** — In Europe, broadcasting stations almost always belong to the state or to semi-state entities. Nevertheless, Catholics seek to influence them, sometimes obtaining their own special broadcasts. In many countries, the Mass and the Sunday Gospel, and sometimes even Catholic catechism, are broadcast. Before the Second World War, the transmissions of the K.R.O. (Dutch Catholic Radio) were particularly notable for their direct role in programming, and the German Catholic radio organization was also significant. In Italy, too, the Catholic Radio Center was established in May 1940 with the aim of morally and spiritually influencing, both directly and indirectly, the programs of the national radio.
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