Electromagnetic Forces and X-Rays: Principles and Generation
Electromagnetic Waves
When studying the material world and analyzing the forces on it, we can establish four very different types of forces: gravitational, electromagnetic, and weak nuclear. The gravitational force appears to us, except in sensitive specific experiences, when there are large concentrations of matter. These are important forces on a global and supraplanetary scale. Our most common experience of them is our own weight and that of bodies around us. This weight is not simply the attraction of the planet. This force allows us to define the property “mass” of material particles. Since it is always positive and cumulative, this property is often identified with the quantity of matter. The force “weight” that we associate with the mass of each body is the attraction it exerts on our planet. This allows us to consider that the body is in the gravitational field of a body with mass (Earth). If we experience a force at a point in space, we determine that at that point in space there is a gravitational field.
Electromagnetic forces are evident, in their simplest form, using magnets or artificial dielectric experiences (plastic rubbed with wool, etc.). Their existence allows us to define the property “charge” of particles. This force acting between particles is exponentially greater than the gravitational force (the force of electrostatic repulsion of two protons is 1036 times greater than the gravitational attraction). This force is responsible for the cohesion of materials and chemical reactions. Almost all physical phenomena we study are based on these forces. The amount of positive charge existing on any piece of matter could lead to severe manifestations of force. We know this is not happening because materials exhibit electrical charges in a careful balance between opposite charges. The only events we observe are derived from very slight local imbalances. If a static body with a non-zero electric charge experiences a force at a point in space, we determine that at that point in space there is an electric field. The forces between magnets, which we call magnetic forces, act on moving charges. There is a different property of matter that is justified. The observation of forces on static material pieces (magnets) is attributable to the internal motion of charged particles in the specific structure of matter in these products. If a body with non-zero electric charge and velocity experiences a force proportional to the magnitude and perpendicular to the direction of the velocity at a point in space, we determine that at that point in space there exists a magnetic field.
X-rays (RX) were discovered by chance by the German physicist Roentgen in 1895, when he was studying luminescence produced by cathode rays. They were expressed as invisible radiation, capable of penetrating materials opaque to all other known radiation and able to blacken a photographic plate. Unable to explain either their origin or their nature, he called them X-rays. It would have to wait until 1912 to show their wave nature, diffracted by crystal lattices (Von Laue). They occur whenever a sufficiently fast electron beam collides with matter. The large acceleration, which is the collision of a fast electron with a larger particle, results in the emission of much of the electron’s energy in the form of electromagnetic waves (OEM) for such frequency energy according to Planck’s law.
In nature, RX production may occur during a storm or from radioactive substances; these natural sources are completely inadequate for any practical application in medicine or any other field. As a byproduct, they also appear in small amounts in commonly used devices including electron guns, such as television screens or computer monitors. RX output can be generated using electronic cannon tubes arranged in various devices. The simplest and most common is the Coolidge tube or vacuum tube, which houses an electron emitter filament (for the Edison effect), a cathode, and an anode or anticathode of heavy metal (W, Pt, Os, etc.). Under the action of several tens of thousands of volts, the electrons are accelerated and collide with the anticathode, producing RX. Other types of particle accelerators may be employed for their production, for example, for radiotherapy. The interaction of electrons with atoms of the anode results in a braking of these electrons. The kinetic energy lost by electrons is manifested in various forms. In particular, a significant fraction of this energy is more or less directly converted into heat, which forces the anode to dissipate it, usually through a cooling system. But another fraction, which interests us here, is radiated from the anode in the form of RX. The relative importance of the energy radiated, and their efficiency, varies widely depending on various factors, including the metal used in the anode. In general, for the tubes as described, it does not exceed a few percent. Even being a heavy metal, for an electron of the anode material, the interaction is diffuse, so the probability of a “frontal” collision with the atom, giving all its energy (E0), is very low. If the voltage is gradually increased, the accelerative spectrum shifts to higher frequencies, while increasing the total representation area of the radiated energy. But from certain voltage values, different sets of lines of a discrete spectrum appear successively and sharply, overlapping the continuous spectrum. This discrete spectrum is different for each metal used in the anode. It is the characteristic spectrum. If the kinetic energy of the incident electrons is sufficient, i.e., greater than or equal to the energy that binds an electron from the inner layers of the metal atom, it can interact with it and move the atom. An electron from a higher level will then take its next energy level, emitting the excess energy as a photon of a very precise frequency. In heavy atoms, the energy level differences in the inner layers reach typical RX values, and they occur in the discrete spectrum. In addition to direct particle accelerators as described (direct application of a potential difference as in the Coolidge tube), more complex devices have been developed that can provide charged particles with considerably higher energies (linear accelerators, synchrotrons, etc.). Consequently, the radiation produced by this collision process can reach equivalent energy values. Therefore, an upper limit for the energy that defines this radiation cannot be set, but it is most frequent to speak of RX in relation to radiation of several tens or even hundreds of keV.