RADIAZIONI. — Under the generic name of r. one may designate both the flow of particles or corpuscles and that of electromagnetic waves: that is, there are corpuscular r. and electromagnetic r. Natural radioactive substances emit corpuscular α and β r. and electromagnetic γ waves. All incandescent bodies emit electromagnetic r.; the Hertzian waves and X-rays also belong to this same type of r.
I. PARTICLES
The energy E of the particles depends on their mass m and their velocity v (E = ½ m·v²). The velocity v of the particles cannot exceed the speed c of light; when v is only slightly less than c, the energy is represented by a formula different from the one given. Corpuscular radiation may consist of electrons emitted by radioactive substances or accelerated by suitable potential differences: their energy is measured by the charge multiplied by the accelerating potential V, that is, in eV (electron volts). Ions have a charge equal to or a multiple of that of the electron (e.g., protons and deuterons have charge e; α particles, 2e, etc.); corpuscular radiation consisting of accelerated α particles therefore has twice the energy of electrons accelerated through the same potential difference. The α particles of naturally radioactive substances have energies of several million eV; the particles forming part of cosmic radiation reach energies exceeding billions of eV; with modern accelerating machines (cyclotrons, betatrons, bevatron(s), etc.), particles can be accelerated to energies of millions and billions of eV.II. ELECTROMAGNETIC WAVES
All electromagnetic waves have in common the propagation velocity c, which in a vacuum is approximately 300,000 km/sec. They differ, however, in wavelength λ and frequency ν. Since λν = c, greater wavelengths correspond to lower frequencies, and vice versa. Classified in order of increasing λ, electromagnetic waves include: a) electromagnetic waves of cosmic rays; b) γ rays emitted by radioactive substances; c) X-rays produced in special tubes by the impact of accelerated electrons against matter; d) ultraviolet, visible, and infrared rays emitted by all incandescent bodies; the electromagnetic rays perceptible to the sense of sight are the most important and the most anciently known, since they are used for this primary function, but from the physical point of view they do not differ, apart from their λ and ν, from the other electromagnetic rays; e) Hertzian waves, first obtained by Hertz and now widely used in radiocommunications of every kind.The term “spectrum” is used for the range of λ composing a radiation. An incandescent solid or liquid body emits a continuous spectrum, that is, the rays emitted by it comprise all possible λ. The visible spectrum is the totality of rays perceptible to the human eye. It encompasses all λ from 0.4 to 0.8 micron (1 micron = 1/1000 mm). Visible rays of different λ are perceived by the eye as different sensations of color. Just as infinitely many λ exist between 0.4 and 0.8 micron, so there are infinitely many colors; Newton classified them into 7 conventional regions (red, orange, yellow, green, blue, indigo, violet). Newton himself was able to demonstrate that white sunlight consists of rays of all colors by decomposing the light with a prism.
Electromagnetic rays constitute a form of energy (radiant energy) that propagates at velocity c. When this energy is monitored or measured, it appears concentrated in many small granules of energy called “quanta” or “quantum,” whose value depends on ν, specifically E = h.v, where h is a universal constant (Planck’s constant) equal to 6.6.10⁻²⁷ erg. sec. The energy of the quanta therefore depends on the ν of the radiation to which they belong. This energy is customarily measured in eV; the quanta of visible light have an energy on the order of 1 eV, whereas the quanta of X-rays range from 100,000 eV to 10 million eV (that is, 10 MeV).
In the case of incandescent solid or liquid bodies, the spectral distribution of the emitted radiant energy depends above all on the temperature of the body. This spectral distribution is shown in the figure for the different absolute temperatures T of the emitting body (it may be taken that 5000° K is the T of the filament of a light bulb, 6000° K the T of the surface of the sun, and 600,000° K that of the incandescent globe of an atomic bomb). It should be noted that the maximum of the three curves falls respectively in the infrared, visible, and X-ray regions of the spectrum. More precisely, Wien’s law establishes that the “maximum λ” (λm) is inversely proportional to T, that is, λm = 2900/T (2900 micron degrees = Wien’s constant). For example, in the case of the sun, radiant energy has a maximum at λ = 2900: 6000 = 0.48 micron, corresponding to yellow-green light. The total energy emitted (for all wavelengths) is proportional to the 4th power of T (that is, to T¹). One cm² of the atomic bomb assumed to have a T one hundred times greater than that of the sun emits energy 100° = 100 million times greater than 1 cm² of the sun. Incandescent gaseous bodies emit a discontinuous spectrum, that is, rays belonging to certain λ; for example, sodium vapors emit in the visible spectrum only yellow rays of λ = 0.589 micron, while hydrogen emits 4 visible rays of red, blue, indigo, and violet color (with λ = 0.656; 0.486; 0.434; 0.410 micron).