1845 - 1923

Physical, its name is tied up to the discovery of the rays x, for which it received the Nobel prize for the physics in 1901.

Child of a merchant of cloths, entered 1865 to the Polytechnic in Zurich where you/he/she graduated in engineering in 1868, in 1870 you/he/she was assistant of physics to Würzburg, between 1876 and the 1879 teacher without desk of theoretical physics to Strasburgo and in the 1879 titular teacher to Giessen.

In 1888 happens to Kohlrausch as manager of the institute of physics of Würzburg, where November 8 th 1895 discovers the rays X.

From 1876 up to the death it received numerous recognitions and honors and you/he/she was named member of the principal scientific Academies.

The discovery of the X rays
it happened by chance November eight th 1895 to work of

Wilhelm Conrad Röntgen
near the institute of Physics of the university of Würzburg, Germany

W.K. Roentgen was born in 1845 from a wealthy family of dealers in the small city of Lennep, in north-western Germany; after having spent the most greater part of the infancy in the Low Countries, to the twenty year-old age he transferred later to Zurich and three years he/she graduated him in engineering near the Technische Hochschule. Although you/he/she had not followed some course of experimental physics during the studies, definite to develop searches in this sector after the diploma.

After having received the doctorate in 1869, Roentgen got a series of charges as teacher in various German universities and in collaboration with Kundt it performed careful studies on the behavior of the subject; for example it was the first one to show, with a thermometer done in house, that is easier to heat the damp air that the dry air.

Roentgen had quarantatré years when it became teacher of physics and manager of the institute of Physics of the university of Wurzburg, a prosperous Bavarian town; he/she lived with his/her wife Bertha in an ample apartment to the second floor of the institute that understood a communicating study with a private laboratory. In June 1894 it started to study the cathode rays, to that time matter of very popular search and the night of November 8 th 1895 during one of his/her experiments it reached the discovery of a type of rays of unknown nature that called "X rays."

Three weeks later spread Roentgen the news of his/her discovery: the fact to be able to see through the objects without breaking them and inside the human body aroused great sensation. In consequence of this he acquired a great fame and in 1901 you/he/she was assigned him the first Nobel prize for the physics

Roentgen died in 1923.

First article of Roentgen
December 1895

published by the magazine
Sitzungsberichte der Physikalisch-Medizinischen Gesellschaft zu Würzburg

Eine Neue Art von Strahlen
"a kind of new rays"

Roentgen immediately realizes the importance of his/her discovery both for the fundamental physics and for his/her manifold applications. Despite this it decides to make her/it public and not to cover her/it from brevet.
In italic character we bring some passages, freely translated, of the first article on the X rays of Roentgen.

1. This new type of radiation can be produced using different types of pipes that are available in many laboratories.

.... If the discharge of of a spool of enough great induction is made to pass through a pipe under empty of Hittford, or through a pipe of Lenard, of Crookes or other similar apparatuses, sufficiently voided, and the pipe has covered with care through a thin black cardboard, and if the whole apparatus is set in a completely dark room, to every discharge a bright illumination of a screen of paper is observed covered with cyanide of platinum and barium, set in proximity of the spool of induction, the fluorescence so produced it is entirely independent from the fact that the screen is turned toward the pipe to discharge with the dressed again surface or that not dressed again. This fluorescence is also visible when the fluorescent screen is prepared to a distance of 2 meters by the apparatus...

In 1901 you/he/she was conferred to professor Röntgen, for the studies done on the rays x, the Nobel Prize for the Physics. The 50.000 crowns of prize were versed at the scientist to the university of Wurzburg, neither Röntgen ever wanted to patent his/her discovery, convinced as it was that "every discovery or invention it belongs to the whole humanity... ".
All for the best? They didn't miss the bitterness, that came above all from a physicist of origin austro-ungarica that he/she lived in Germany, professor Lenard, that accused Röntgen not to be anything else other than the thief of his/her studies, having been him the first one to study the cathode rays. In Nazi period (Röntgen had already died) Lenard also looked for, strong of his/her one man show friendship with Hitler, to make himself/herself/themselves recognize "officially" the paternity of the discovery. But in 1951 the German Federal Republic, on the occasion of the cinquantenario of the conferment of the Nobel Prize, it definitely truncated the discussions, with the issue of a commemorative postage stamp with the effigy of Röntgen.

The dispute among the two scientists doesn't do whether to confirm how much we told the beginnings of this narration of ours: the great discoveries are only rarely fruit of the job of a man, isolated in its study. Neither some great discovery is definitive, but it is generally one it covers on the walk of the knowledge. Unfortunately however the disputes on the paternity of the discoveries of the science often assume sour tones, at times also, as we will see, of bassa.bottega.

The Nine hundred began so with the emotions aroused by the mysterious X rays, one of the scientific discoveries that it mostly excited the popular imagination. But if the studies of the German physicist opened the road to a jaw of the medicine, the radiology, that has in turn innumerable sectors of intervention, our century will stay marked above all by the victory of the medicine against the infectious illnesses, with the discovery of the sulfamidicis and the antibiotic. You minds well, when we speak of "victory", we don't use this term in absolute sense.


Wilhelm C. Röntgen during the primary and secondary schools had a normal scholastic output, with a rather tall profit, except a solo "insufficient", in physics. Mocking destiny for the future discoverer of the rays X. Nato to Lenepp, in Germany, March 27 th 1845, departed big part of his/her youth in Holland, where his/her family was transferred. To 17, however, while it was frequenting to Utrecht a course that prepared technical for the industry, you/he/she was expelled by the institute because amazed to laugh in front of the caricature of an unpopular teacher. This expulsion prevented him from achieving the necessary title to enter to the university but lose Röntgen him of mind and succeeded in getting first the diploma of mechanical engineer and then the degree in physics. In 1870, the future father of the X rays, won a desk to Würzburg, and there you/he/she transferred him to teach physics. Röntgen was a pragmatico more than a theorist of the physical sciences: its laboratory resembled a lot to a messy shop, obstructed of batteries, of spools and tools of every kind. November 8 th 1895, Röntgen, was completing to the dark of the experiments with a pipe to cathode rays, in his/her laboratory, when it noticed a green light coming from a piece of cardboard that was found in another part of the room. The cardboard had covered of a luminescent chemical substance, that was resplendent if struck by the light. But there was no light in the laboratory. Röntgen removed the tide from the cathode pipe and that green light it disappeared. Current Ridiede and it put the hand between the pipe and the cardboard: with his great amazement, saw projected on the cardboard the shade of the bones of the hand. "I didn't have idea of thing they were that rays" he/she wrote subsequently "therefore I simply called them X rays, being x the mathematical symbol of an unknown greatness." These rays "unknown" they passed through the paper, the wood, the meat, but not through the bones and the metals, and besides they impressed the photographic plates. Today we know that the X rays are electromagnetic radiations of inferior wavelength to that of the ultraviolet rays and from the ample range of frequencies. Their discovery revolutionized the world of the medicine, because for the first time the physicians were able "to look" inside the body. In fact, already in 1896, the X rays were used for examining the bony fractures. To understand the pioneer climate in which you/they operated men as Röntgen, is enough to know that one of the first radiographies of the history immortalized on a plate the hand of his/her wife, with the ring that brought to the finger, because on her Röntgen tested his/her equipment, according to a custom typical of certain researchers of end Eight hundred. Studious for which scientific activity was a solitary adventure in which the rule was worth of "to count on his/her own strengths", and therefore on his/her own darlings. Numerous they were the researchers and the radiologists, above all to the beginnings, that suffered burns because of the radiations or they were stricken from ulcerations or tumors. An unsuspected field of application of the X rays that has had a great development, beginning from the sixties, with to multiply himself/herself/themselves some aerial deviations, it concerns the examination of the baggages, in the airport structures, to generally discover you the possible presence of guns, bombs and weapons. Since then these instruments circumferentors have been adopted all over the world. Even if the discovery of big long more main point of Röntgen stays that of the X rays and the intuition to be able to use them in the diagnostic one in medicine, he conducted searches in other fields. It studied the ownerships piezoelettriche of the crystals, the stringiness and the index of refraction of the different liquids and the movement of an insulating body in an electric field. Searches that contributed to the elaboration of the theory of the relativity. It is certain however that, also before the discovery of the X rays, the name of Röntgen was already famous. After the Nobel received in 1901, his/her life of man and scientist it had a curious course. Lost his wife in 1919, after a period of serious sufferings, and while they crossed titles and honors for his/her discovery, Röntgen it didn't make mystery around his/her perplexity on the searches developed himself/herself/themselves around the rays X. Argomento to which him same he didn't interest anymore. Röntgen didn't want some brevet for the X rays neither for the uses that would be done. To who offered him recognitions and remunerations not disinterested, Röntgen he/she asked a contribution of 10 marks for every pipe you/he/she had been him necessary to produce his/her rays. And when someone tried to involve him/it in an industrial project of exploitation of the X rays, he/she answered that discovered and inventions belong to the humanity and that you/they should not be mortgaged by brevets, licenses and contracts. "You/they should not be checked by great groups", it added. This indifference for the money forced him/it in poverty, in the last years of its life, before the death happened in 1923, during the German economic crisis of the years Winds, covered him of important honors, but without having a penny in pocket.

The X rays

The X rays are so that electromagnetic whose wavelength is around 3 times that of the visible radiation. They is produced by the strong deceleration of the electrons in the collisions with the atomic nucleuses and from the transitions of the electrons in the deepest orbits inside the atoms. They was open from W.C.Roentgen (1845 - 1923) in 1895 bombing a metallic target with a bundle of electrons (cathode rays) sent forth by the cathode of a pipe of discharge containing become less frequent gas. Because of their dwarfish wavelength they weakly interacts with the subject. After in 1912, Max von Laue (1879 - 1960) it observed that a bundle of X rays shows effects of interference passing through a crystal, it resulted clear that they differs only from the light as it regards the wavelength. The regular disposition of the atoms in the crystalline network simulates a network of diffraction. The same result was gotten by W.L.Bragg (1890 - 1970) analyzing the reflection of the rays X. Egli drew their wavelength from the knowledge of the direction of the constructive interference and from the distance among the plain reticolaris (law of Bragg). Vice versa the figure of diffraction can be used for drawing the structure of the crystal (X rays cristallografia of W.H.Bragg (1862 - 1942)). already postulated by Michael Faraday to the purpose you Coo of it unloads The electron Him "atoms of electric position" they were already postulations from Michael Faraday to the purpose to explain the phenomenon of the electrolysis. Idea was taken back by william Crookes (1832 - 19199 and from Arthur Schuster (1851 - 1934) which were convinced that the cathode rays produced in their experiments were negatively bundles of loaded particles. In 1897 the existence of the discreet unity of position was established from J.J.Thomson. Using cross electric and magnetic fields in the pipes of discharge he showed that the cathode rays were formed of loaded particles in speed very smaller motion of that of the light and it measured the relationship between their mass and their position. J.J.Thomson admitted that the value of the position of the particle was identical to that that G.Johnstone Stoney (1826 - 1911) it found in 1891 to be brought by the ions of the univalent elements in the electrolysis. In such way he/she succeeded in also appraising the mass of the particle to which, using the name already introduced by Stoney, the name of electron was given. Well soon to work of Hendrik Lorentz, Philipp Lenard (1862 - 1947), Henry Becquerel (1852 - 1908) and Ernest Rutherford (1851 - 1937), it was established that the presence of the electron was necessary to explain many physical phenomenons as the thermionic issue, the photoelectric effect and the radioactivity. In the same years Robert Millikan (1868 - 1953) he/she succeeded in measuring with big precision the position of the electron and his/her mass. In the 1924 Louis de Broglie advanced the hypothesis that the electron also possessed undulated ownership. On this hypothesis Erwin Schroedinger founded the new undulated mechanics. In the 1927 Clinton Davisson (1881 - 1958), Lester Germer (1896 - 1971) and George P.Thomson (1892 - 1975) they experimentally verified the undulated ownerships of the electron. In the meantime Samuel Goudsmit (1902 - 1978) and George Uhlenbeck (1900 -) they introduced the spin. Paul's Dirac job "The relativistic theory of the electron" of 1928 it justly framed all the hypotheses and the experimental data in the famous "equation of Dirac." The X rays The X rays are so that electromagnetic whose wavelength is around 10-3 times that of the visible radiation. They is produced by the strong deceleration of the electrons in the collisions with the atomic nucleuses and from the transitions of the electrons in the deepest orbits inside the atoms. They was open from W.C.Roentgen (1845 - 1923) in 1895 bombing a metallic target with a bundle of electrons (cathode rays) sent forth by the cathode of a pipe of discharge containing become less frequent gas. Because of their dwarfish wavelength they weakly interacts with the subject. After in 1912, Max von Laue (1879 - 1960) it observed that a bundle of X rays shows effects of interference passing through a crystal, it resulted clear that they differs only from the light as it regards the wavelength. The regular disposition of the atoms in the crystalline network simulates a network of diffraction. The same result was gotten by W.L.Bragg (1890 - 1970) analyzing the reflection of the rays X. Egli drew their wavelength from the knowledge of the direction of the constructive interference and from the distance among the plain reticolaris (law of Bragg). Vice versa the figure of diffraction can be used for drawing the structure of the crystal (X rays cristallografia of W.H.Bragg (1862 - 1942)). already postulated by Michael Faraday to the purpose.



With the term Radioactivity he intends the spontaneous issue of particles e/o radiations from the nucleus of an atom.

It results evident that the discovery of the radioactivity goes together with the search on the atomic structure and on the nuclear characteristics.

In the 1896 Henri Becquerel it noticed that a photographic plate became black if sets in the proximities of a mineral container composed of the uranium. These mixtures had to send forth therefore radiations able to release energy inside the plates impressing her. In the 1899 Pierre Curie and his/her wife Maries succeeded in extracting from the mysterious mineral the radioactive substance responsible of the strange phenomenon, that had called radio. One year later Ernest Rutherford identified the radiations sent forth by the radioactive substances in issues alpha and beta, while Paul Villard individualized the rays range. Rutherford observed besides that the atoms that send forth radiations are turned into different atoms, in other words endowed with different chemical ownership from those characteristics of the atoms of departure. Many experiments were taken place in the following years to the purpose to individualize the composition of the three types of radiation. Their results have brought to conclude that the radiation alpha is constituted by nucleuses of helium (two protons and two neutrons), the radiation beta from electrons (or from their antiparticelles, the positrons) while the radiation range is an electromagnetic radiation (and therefore composed by photons) particularly energetic.

In the 1911 Rutherford it used the rays alpha to study the atoms and it conceived the model of atom that brings his/her name: a "nucleus" container the most greater part of the mass of the atom, load of positive electricity and avente a smaller ray of that atomic; around the nucleus a certain number of electrons on circular orbits. In base to his/her atomic model, radioactivity was attributed to the transformations that happen in the nucleuses of the atoms. Two years later Niels Bohr introduced his/her theory on the structure of the atom. It completed the model of Rutherford and, above all he/she explained the electronic disposition in base to the processes of issue and absorption of photons from the atoms of hydrogen.

This result stimulated the studies of atomic physics and conducted, around 1920, to the formulation of the mechanical quantistica from Louis de Broglie, Werner Heisenberg, Erwin Schrödinger, Paul Dirac, Wolfgang Pauli and others. The new theory, experimented with success in the description of the atom, it was well soon applied to the study of the atomic nucleuses. They understood so the laws, established in the first decades of the XX century, related to the nuclear decadences accompanied by the issue of particles alpha and beta.

The structure of the nucleus became clearer when, in 1932, James Chadwick discovered the neutron, a particle avente around the same mass of the proton but with position electric nothing.

In 1895 the German physicist Wilhelm Conrad Roentgen, studying the effect of the electric discharges in the attenuate gases, it discovered radiations able a piece of paper to make phosphorescent covered of a salt of barium set in the proximities. Roentgen called X rays these radiations of unknown origin.

You arrived so to the hypothesis of Heisenberg that the atomic nucleuses consisted of protons and of neutrons.

The knowledge of the nuclear ownerships progressed notably thanks to experiments during which the nucleuses were bombed with light particles (protons, electrons, particles alpha...). Great importance also assumed the study of the artificial trasmutazione of a chemical kind in another.

In this phase of the search it also began the study of the nuclear strengths those that hold together protons and neutrons. You immediately understood that nuclear strengths are very stronger than those electromagnetic and gravitational and that they act only on very short distances, comparable with the ray of the nucleus. In the 1939 Hideki Yukawa, following a suggestion of Heisenberg, it hypothesized that the strengths that practice him among the constituent ones of the nucleus were due to heavy particles (300 times more than the electrons). These particles, call mesoni, they were indeed open in the cosmic radiation in 1946.



In a hypothetical trip inside the atom, after having gone beyond the hulls that entertain the electrons it would be found there to cross a zone of completely empty space. After a long journey relatively it would finally come upon us in the nucleus. The nucleus occupies, in fact, the center of the atom and its dimensions are about ten thousand of inferior times to the distance that separate him/it from the nearest electrons. Its role can be compared to that some Sun in our planetary system: as the planets orbit around our star for gravitational attraction, so the electrons, loads of negative electricity, "they orbit" around the nucleus because attracted by its positive position.

Inside the atomic nucleus two types of particles are found: the protons and the neutrons. They are particles that are resembled a lot (so much to be been suitable both with the name of "nucleoni") but while the neutron is electrically neutral, the proton has a position that is exactly worth how much that, of opposite sign, transported by the electron.

The number of present protons in a nucleus is said atomic number and is suitable with Z; therefore the nuclear electric position is equal to turned Z the position of a proton. We remember that normally the atoms are neutral and that this is owed number to the peer, Z I sharpen, of protons and electrons that compose them. All the atoms that have equal Z, even if they differ for the number of neutrons, damage origin to the same chemical element, has in practice the same ownerships and they occupies the same place in the periodic table of the elements. For this motive atoms with the same atomic number Zs are said isotopes (same place).

In the nucleus the mass of the atom is assembled almost all. In fact, neutrons and protons have greater masses (around 1800 times) of that of the electrons. To appraise the mass of a nucleus is fundamental to know the number of neutrons that you/they appear you; such number is generally pointed out with N.

If it neglects him the dwarfish existing difference among the masses of the proton and the neutron, you/he/she can be concluded that the mass of a nucleus is worth Z + turned N the mass of the proton. The quantity Z + N is pointed out with the letter Á. and calls number of mass. How term of comparison for the atomic masses (and nuclear) a particular isotope of the very abundant carbon is chosen in nature: the carbon-12. In his/her nucleus they are present 6 protons and 6 neutrons; his/her number of mass Á. is worth therefore 12. How unity of measure of the atomic masses is chosen the twelfth part of the mass of the carbon-12.

Not always however the mass of an atom is equal to an integer of times this unity of measure; it is often a decimal number. The reason for this resides in the existence, for a same chemical element, of isotopes of different weight.

They contributes to the mass of the element in way more or less accented according to their abundance in nature.

The evaluation of the mass of the atoms, and therefore of the nucleuses, it has a great importance in the nuclear physics. The famous formula And = mc2, written for the first time by Albert Einstein in 1905, it establishes that an equivalence exists among mass and energy, as if they were two forms under which introduces him the same physical entity. The interpretation of the formula is simple: it allows to calculate to how much energy (And) it corresponds a certain mass (m); all it takes is multiplying the mass for the speed of the light (c) elevated to the square. In some nuclear trials (nuclear fission, fusion) fractions, also very small, of the mass of the nucleus they are turned into energy. If then it is known with precision the mass of an atomic nucleus and his constituent, using the formula of Einstein the energy you/he/she can be appraised that it sends forth during nuclear reactions as those that happen in the stars, in the nuclear reactors or in the devastantis atomic bombs.


The atomic nucleus can be imagined as a loaded sferetta of positive electricity whose ray is worth around a decimillesimo of billionth of centimeter (10-13 cms). It is not easy however to imagine as can coexist in such a small space Z protons and N neutrons. Nevertheless many experiments, conducted by the beginning of the XX century, you/they have made light on the inside structure of the nucleuses. Particularly you/he/she is understood as is distributed in the nucleus the in partnership electric position to the protons.

The information of which it prepares him on the nuclear dimensions, the bombardment of nucleuses they are owed to through electrons to tall energy. There are good reasons to believe that the volume that contains the electric position coincides indeed with that busy from the nucleus. The results of these experiments have shown that electric density is constant from the center of the thin nucleus to a certain value of the ray; then it starts to decrease thin to annihilate himself/herself/themselves. The point in which it annuls him the external limit of the nucleus can be considered.

The way according to which the density of electric position decreases to increase some distance from the center, is almost the same for the nucleuses of all the chemical elements. What appears evident is the growth of the space occupied by the electric position for that nucleuses that are composed from an increasing number of protons and neutrons.

It results, in fact, that the volume of a nucleus (defined as the space occupied by the distribution of electric position) it is proportional to the number of mass Á., therefore the density of subject is the same one for all the atomic nucleuses. Besides it has a value that surprises for his/her greatness. In fact, if a common object were dense as a nucleus, every die of his avente the sides of a centimeter it would weigh 200 million kg.

Protons and neutrons, the constituent ones of the nucleus, are very similar particles. If, in fact, she is also excluded important difference related to the electric position, they has so alike characteristics to induce to think that I/you/he/she am had to whether to do only with a type of particle. That's why it has sense speak of nucleoni without subsequently distinguishing between protons and neutrons. This choice is subsequently justified by the mechanical quantistica: in this circle, in fact, they introduce him some greatness, the numbers quantici, whose different values correspond to different states of the particle to which refer. This way, for example, to every nucleone you/he/she has been in partnership the number quantico of "isotopic spin." It can assume two values: a correspondent to the "is" proton, the other correspondent to the "is" neutron. According to the value of the isotopic spin, the nucleone involves as a proton, endowed with electric position, or as a deprived neutron of position. Proton and neutron must be considered as the two possible "is" of a same particle, the nucleone.

Also the nucleus in its complex is endowed with an isotopic spin. Naturally it keeps in mind of the isotopic spins of all the Á. nucleoni that composes him/it: this means that, for example, the nucleuses of the trizio (the isotope of the hydrogen with two neutrons and a proton) and of the helium-3 (the isotope of the helium containing two protons and a neutron) as two equal nucleuses aventi can be thought however different values of the isotopic spin.

The similarity between protons and neutrons is particularly accented if it confines us to consider the nuclear strengths, those that is that they practice him among the components of the nucleus. It results, in fact, that such strengths don't depend from the electric position of the nucleonis involved in the interaction; or rather that the strength that practices him between a proton and a neutron is exactly the same one of that that practices him between two protons or two neutrons.

Nuclear strengths are essential for the stability of the nucleus. All it takes is thinking about the fact that in a dwarfish portion of space two coexist or more protons, particles endowed with positive position. If strong nuclear strength didn't exist, the electrostatic repulsion would estrange the protons the one from making the existence of the nucleuses impossible. This makes to understand how come the presence is necessary of the neutrons: them, to be subjects of it, practices further on the other particles of the nucleus a strong interaction, contributing to brake the tendency of the protons to estrange the one from the other.

That's why the number of neutrons N grows to increase some number of protons Z. In the light nucleuses, those with few protons, Z and N they coincide. When instead Z grows, the electrostatic repulsion among the protons becomes so intense that, so that stable nucleuses exist, an elevated number is necessary of neutrons able to practice an enough strong interaction. For example, while the isotope of the most abundant carbon in nature has an equal number of protons and neutrons (Z = N = 12), the iron has 26 protons and 30 neutrons. Difference grows as soon as him it flows the periodic table (periodic system of the elements): the most abundant isotope of the lead (Z = 82) it has well 116 neutrons.


If they was able "to weigh", first an atomic nucleus and his/her separate components, would subsequently be us of forehead to an amazing fact: the mass of the nucleus is slightly inferior to the sum of the masses of the protons and the neutrons that you/they constitute him/it; it is had in practice a defect of mass. This is one of the consequences of the relationship of equivalence among mass and energy realized by Einstein.

When two or more nucleonis unite him to form a nucleus, part of their mass you/he/she is converted in energy of bond. This phenomenon introduces him in all the physical systems in which more components are tied up among them from strengths of any nature. Nevertheless, in the most greater part of the cases, the fraction of mass escaped to the components of the system to be converted in energy is so small to be been able to be neglected. For example in the system Earth-sun only a part on diecimila million of the mass you/he/she has been "sacrificed"; in a crystal, the atoms have had to abdicate to a centomiliardesimo of their mass to be able him to tie; in an atom of hydrogen instead you/he/she is turned into energy a decimilionesimo of the general mass of the electron and the proton. If however an atomic nucleus is considered, it realizes us that the effect is not absolutely negligible: a cent of the mass of the nucleonis is converted in energy of bond. The comparison with the cases quoted in precedence, in which the gravitational interaction and the electromagnetic interaction intervene, clarifies because the strength that holds together the nucleonis has been denominated interaction "strong."

The energy of bond is that that is had to furnish to a nucleus to succeed in separating one from the other the Zs protons and the Ns neutrons that compose him/it.

And' then evident that a nucleus characterized by a great energy of bond results particularly stable.

Stability also explains the abundance in nature of certain isotopes: they is privileged in comparison to the other isotopes of the same element because they have an energy of greater bond. The experimental study of the energies of bond has underlined some important situations: it for example results that among the nucleuses with number of mass Á. protects, those with Z and odd N are very stable (in other words they have an energy of inferior bond) of those aventis Z and N you protect. And' is this observation to suggest that nuclear strengths are strengths that are practiced among couples of bodies.

To establish what atoms or isotopes of an element are more stable than others the following rules they are applied:

  1. Greater it is the energy of bond for nucleone, more stable it is the nucleus.
  2. Nucleuses of lower part elements atomic number, with relationship neutrone/protone of 1: 1 are very stable.
  3. The most stable Nucleuses extend to have an equal number of protons and neutrons.

Energy for nucleone

We consider the nuclide 16 of the oxygen: it contains 8 protons, 8 neutrons and 8 electrons, we can imagine then composes him/it from 8 atoms of hydrogen more 8 neutrons.

Every atom of hydrogen has a mass of 1,0078252 u.m.as., every neutron has a mass of 1.0086652 u.m.as., for which its total mass should be of:

(1,0078252 xes 8) + (1,0086652 xes 8) = 16,1319232 u.m.as., instead the real mass of the oxygen 16 are of 15,9949150 u.m.as.. The difference between the calculated mass and the real mass has called "defect of mass."

For the atom of oxygen 16 this defect of mass is quantified in 0,1370082 u.m.as., this mass has been turned into energy that develops him in the moment of formation of the nucleus, and it is the same quantity of energy that must be furnishes to the nucleus so that the separation of its nucleonis happens.

  • With the relationship E=m*c2 we convert the defect of mass in energy:

· this energy, (2,0445639 * 10 -11 Js) it is the energy of nuclear bond that divided for the number of the constituent nucleonis the nucleus it gives the unitary energy for nucleone:

We try to consider the atom of uranium 238 and to calculate its defect of mass:

92 atoms of Hydrogen = 1,0078252 xes 92 = 92,719918 u.m.as.

146 neutrons = 1,0086652 xes 146 = 147,265119 u.m.as.

For a total of 239,985038 u.m.as., but the real mass of the uranium 238 are 238,0289 u.m.as. and therefore doing the difference the lacking mass corresponds to 1,956138 u.m.as. from which, always applying the relationship is = m*c2 the energy of nuclear bond is:

· E the unitary energy for nucleone:

If the energy of bond is compared for nucleone of the Nuclide 16 of the oxygen and that of the uranium 238 it is evident that the stability of the first one is greater.

Stability and nuclear instability

When a nucleus is stable it is considered him/it in the fundamental state. As you/he/she is said, stability depends on the particularly tall value of the energy of bond and therefore corresponds to the difficulty to extract from the nucleus someone of his constituent. If instead the configuration of the nucleus is altered through a change of energy induced by the outside, for example bombing him/it with particles, the same nucleus it comes to be himself/herself/themselves in a state "excited." The nuclear ownerships generally come reported to the situation of is fundamental, but the study of the excited states results very useful to the goals of the understanding of the inside structure of the nucleuses.

Very interesting results have been drawn by the examination of the states of the so-called nucleuses speculari, that nucleuses that is with equal number of mass Á., but such that the number of protons Z of the one is equal to the number of neutrons N of the other and vice versa. After having escaped the electrostatic contribution, this examination shows a substantial identity, as if neutrons and protons were interscambiabili: this is a great deal important test of the fact that nuclear strengths are symmetrical in comparison to the substitution of neutrons with protons. In substance, the nucleuses with the same Á. involve equally for that that it pertains to their purely nuclear structure, while they are differing for the electromagnetic structure.

The effected observations have also allowed to draw important conclusions on the existing bond among the number of present nucleoni in a nucleus and its stability. For small values of Z and N the stable nuclear configurations correspond to an equal number of protons and neutrons (Z = N); to the growth of Z the number of necessary neutrons to guarantee the stability increases, overcoming of big long the number of present protons in the same atom. The course just described is made very well by the so-called curve of stability, that is gotten bringing all the existing stable nucleuses in nature on a Cartesian plan whose aces represent the number of protons Z and the number of neutrons N.

A nucleus is anymore away from the curve, or rather anymore the couple Z-N him far from the optimal values, greater it is the instability that countersigns him/it.

The relationship among number of protons and number of neutrons that are found in a nucleus is not casual therefore. If in fact we wanted to build atomic nucleuses admitting at random a certain number of nucleoni, the most greater part of the combinations it would result unstable and it would give origin to the reactions of decadence that are described in the succession.


At the base of the radioactive issues there is the tendency of some nucleuses to go more and more himself/herself/themselves toward stable configurations. This way a nucleus that is found in an excited state, avente in practice superior energy to that of the fundamental state, frees him some energy in excess sending forth particles alpha, beta or photons range. The radioactivity, over how natural, you/he/she can artificially be provoked also. If, in fact, him "it excites" a nucleus bombing him/it with particles as protons or neutrons, it will return, or it will draw near, to the fundamental state sending forth radiations.

Natural radioactivity introduces him in almost all the nucleuses aventi atomic number Z understood among 81 and 92; they is turned into lighter nucleuses, whose chemical characteristics are well distinguishable from those of the initial nucleuses.

The law that describes the radioactive decadence is type exponential. This law shows as it reduces him to spend some time the number of nucleuses of departure because of their decadence. A very important parameter that appears in the formula is her/it "middle life." After it is departed once equal to her "middle life", the two bystanders of the initial nucleuses almost result to have suffered the radioactive decadence.

The "middle life" various according to the considered nucleus: you/he/she can oscillate from the thousandth one of billionth of year to the one hundred million of million of years. Its value is a clear index of the stability of the nucleus which refers: a brief middle life is sign of instability and therefore of predisposition to the radioactive decadence; the stable nucleuses boast of long middle life instead.

It is not said that a radioactive nucleus directly decays in a stable nucleus; you/he/she can happen that it decays in turn in an unstable nucleus decadence radioactive subject. The trial in continuous fall until him doesn't reach a stable nucleus. It speaks then of radioactive series.

Naturally the radioactive elements have been gathered in three series that you/they take name from the three elements that act from founder: the series of the uranium, the series of the torio, the series of the actinium. The founders have very long middle life (respectively 6,5, 20 and 1,3 million years) and they decay in lighter elements; the process of decadence arrests when a stable isotope of the lead is produced. It also exists the series of the nettunio that however it contains not also existing radioactive nucleuses in nature but produced in the laboratory (elements transuranici); the series finishes in a stable isotope of the bismuth.

The belonging nucleuses to a same series differ the one from the other for four nucleonis, since the decadence that makes to have passed since one to the other one corresponds alpha to the issue of particles. In a limited number of cases you/he/she can also verify him the decadence beta which, turning a neutron into a proton, it doesn't behave the change of the number of mass A.

The unstable isotopes that decay sending forth particles alpha, beta or you radiate range, they are said radioisotopes; they know around a thousand but their middle lives and the tied up difficulties to their production they do yes that only a hundred is usable for practical applications.


Many nucleuses are unstable since their energy of bond is not enough to hold together the nucleonis that constitute them. Spontaneous trials of trasmutazione rise up during then which the unstable nucleuses are transformed in more stable nucleuses. These trials are also called radioactive decadences because they are accompanied by the issue of radiations of different nature: you radiate range, particles alpha and particles beta.

These issues that happen during the period of radioactive decadence have also called "Radiations ionizzanti" and posseggono, in different measure, a penetrating power in the subject.

The radiations (energy's transport in the space) meeting the subject you/they can transfer their energy to the atoms or molecules, exciting the electrons of it. If energy is enough to escape the electron to the strengths of attraction of the nucleus an atom or molecule it will be gotten ionized.

The energy of the radiations ionizzanti is expressed in elettronvolt (eV), 1eV define the energy purchased by an electron when it crosses a difference of potential of 1 Volt in the void.

Other characteristic parameter of the radiations is the penetrating power or the ability to cross more elevated thickness before the radiations are arrested.

As the radioactivity it is known it is a normal component of the natural environment for which the man has constantly been exposed to the radiations of natural origin since his to appear on the earth and these have remained the only source of irradiation up to few less than one century ago.

He/she anchors now, despite the breadth employment of artificial radioactive substances and fittingses radiogeni of various kind, natural radioactivity keeps on furnishing the most greater contribution to the dose received by the population and is probable a great deal that this continuous to verify in the future also him.

In the natural radioactivity two components are distinguished, one of terrestrial origin and the other extra-earthling. The first one is due to the contained primordial radionuclidis in various quantity in the inorganic materials of the terrestrial crust (mineral, rocks) since his/her formation.

The second is constituted by the cosmic rays, also known as " Radiation leading ".

The principal primordial radionuclidis are, the Potassium (K-40), the Rubidio (Rb-87), and the elements of the two radioactive series of the uranium (U-238), and of the Torio (Th-232).

The series of the isotope is generally ignored 235 some uranium (U-235), gives the modest relative abundance of the founder, even if this is always not justified in terms dosimetrici.

The concentration of the natural radionuclidis in the ground is not equidistribuita, but it varies from place place in reason for the different geologic conformation of the various areas he/she took in examination.

For instance, in the igneous rocks, the concentration of U-238 is generally greater that in the sedimentary rocks as the limestones or the chalks, even if in some sedimentary rocks of sea origin an elevated concentration of this radionuclide is found.

Besides, in the rocks "sour", both the Torio both the uranium are more abundant than in the rocks "basic."

Typical values of concentrations of activity in the ground are inclusive between the 100 and 700 Bqs * Kg-1 for the K-40, and among 10 and 50 Bq*Kg-1s for the radioactive series of U-238 and Th-232.

The issue of rays range

The issue of rays range happens in almost all the unstable nucleuses and usually corresponds to the elimination of energy in excess: a nucleone can for example be in a state of tall energy having nevertheless free a state of lower energy; the nucleone passes in the inferior state and contemporarily the difference of energy is released in the form of photon range, that is of electromagnetic wave (the strong analogy is evident with the process of spontaneous issue that occurs with the electrons of the atoms-fluorescence and phosphorescence).

The rays range

I am so that electromagnetic, as the light, and not of nature corpuscolare, their frequency depends on the substance and has an inclusive wavelength between 10-11 and 10-14 meters.

Rising: radioactive nuclidi

Energy: their energy is proportional to the frequency: from ca.10 keV to 10 MeVs

Speed: "c" (300.000 Km/secs. speed of the light).

Penetrating power: strongly (100 times greater of the rays beta), some lead centimeter decreases the intensity of a factor of it 2.

To be able ionizzante: indirect ionization of the air through electrons.

Degree of dangerousness: always dangerous even if sent forth by external source to the human body.

The decadence beta

The decadence beta is one of the most important phenomenons in the nuclear physics. It is the more commune, and all the radioactive elements have isotopes that decay in this way, it Corresponds to the transformation of a neutron in a proton or, and in such case it speaks opposite of decadence beta, to the transformation of a proton in a neutron.

When a neutron turns him into a proton, the decadence is accompanied by the issue of an electron and an antineutrino (the antiparticella of the neutrino); the presence of the electron guarantees that the electric position of the system is unchanged before and after the trial, it speaks to this case of decadence "beta-".

When the transformation of a proton is had in a neutron instead, a neutrino and a positron are issued (an electron of positive position), decadence is had "then beta +."

The decadence beta inside an atomic nucleus happens when the same nucleus introduces an excess of neutrons or protons excess that must be eliminates.

In precedence you/he/she is made to notice as the number of protons and neutrons is essential to determine the stability of a certain nuclear configuration.

The privileged configurations are those that, in the diagram Z-Ns, are prepared along the curve of stability. If a nucleus is found above such curve, it means that to its inside there is an excess of neutrons. You baits then a decadence beta: one of the neutrons of the nucleus become a proton and contemporarily the issue of an electron and a neutrino happens.

This way, the nucleus has one more proton and a neutron in less in comparison to the nucleus of departure. The to be himself/herself/themselves below the curve of stability points out instead that the nucleus contains an excess of protons.

The decadence opposite beta handles theirs "elimination": they is turned into neutrons while I am being issued positrons and neutrini.

Therefore, when it happens inside a nucleus, the decadence beta approaches the nucleus to the curve of stability.

The trial leaves the number of mass unchanged Á., but it changes Z (it increases of an unity in the decadence "beta-", it decreases of an unity in the decadence "beta +").

The decadence beta is described by the weak interactions and the protons they are turned only into neutrons when they are inside nucleuses: the decadence beta of free protons has never been observed.

This behavior of the protons is tied up to their mass. When a particle always decays ago it turning himself/herself/itself into a lighter particle. The neutrons for example, decay in protons. The protons however don't have lighter particles in which to decay. Inside the nucleuses instead the presence of the energy of bond alters, even if of little, the values of the masses of the nucleonis. You/he/she can happen so that a proton turns him into a neutron.

You radiate beta

Flow of particles of electrons (beta -, negative) and of positrons (beta +, positive electrons) sent forth by the nucleus in disintegration. Some of these particles aventi high speeds interact with the subject, with consequent issue of X rays (natural).

Rising: radioactive nuclidi

Energy: from some keVs to many MeVs, but rarely superior to the 4 MeVs.

Speed: from 150.000 km/ses to "c" (speed of the light)

Penetrating power: weak (100 times smaller of the rays range and 100 times greater of the rays alpha), they don't overcome a barrier of the thickness of 5 mms. of aluminum or 2,5 wood cms, besides they don't penetrate for over a centimeter in the skin. With an energy of 3 MeVs a particle beta crosses in the air around a meter.

To be able ionizzante: very low, 4 couples of ions for millimeter with energy of 3 MeVs.

Degree of dangerousness: the limited penetrating power ago him that their dangerousness is limited if sent forth by a source day-pupil to the body; they are harmful if the source is inside.

The issue of particles alpha

The issue of particles alpha from the nucleus can happen thanks to a characteristic trial of the physical quantistica said effect tunnel: the two protons and the two neutrons succeed in practice in also escaping possessing a quantity of insufficient energy to break the nuclear bonds. And' as if a stone tossed up in the air, succeeded in escaping the attraction gravitational earthling and to fly in the space, despite the little energy engraved him by the pitcher. This phenomenon is perfectly explained by the mechanical quantistica and it is especially important for the nucleuses with atomic number greater Z of 82 (lead). Following this decadence a diminution of Z of 2 unities or Á. of 4 unities is had.

You radiate alpha

Particles constituted by nucleuses of Helium (2 neutrons and 2 protons) that they have a double positive position.

Rising: radioactive nuclidi

Energy: rarely inferior to the 4 MeVs.

Speed: from 15.000 to 20.000 km/ses

Penetrating power: very weak (100 times smaller of the rays beta), they don't go beyond a sheet of paper, an aluminum foil of the thickness of 50 microns or the basal layer of the epidermis; in the air if they possess an energy of 3 MeVs they have crossed since 2 to the 8 centimeters. With an energy than at least 7,5 MeVs you/they can penetrate in the skin.

To be able ionizzante: very elevated, (1000 times greater of the rays beta), with an energy of 3 MeVs they produce 4000 couples of ions for millimeter.

Degree of dangerousness: only if sent forth by an inside source to the human body, you/they can create serious damages in consequence of the elevated power ionizzante.

Other source of radiations ionizzantis are the cosmic rays; with this name phenomenons of various nature are identified (atomic nucleuses, electrons, positrons, rays range, swarms photon-electrons) and it results enough easy to realize that their source is of extraterrestrial origin.

Their energy is very elevated, of the order of a lot of thousand of MeV (from 108 to 1020 MeVs), with elevated speed, next to the speed of the light; they have a strong penetrating power and ionizzante but gives their scarce presence they have a negligible dangerousness.


With the term "Activity" of a radioactive substance he intends the number of nucleuses of this substance that you/they disintegrate him in the unity of time:

an old unity of measure of the activity is the Curie (There), now replaced in the International System (S.I.) from the Bequerel (Bq):

1 there = 3.700.000 dises. / sec.

1 Bq = 1 dis. / sec.

To quantify the biological damage of the radiations on the organisms I/you/they have been introduced some unities of measure that define her/it "absorbed Dose", that is the energy deposited by the radiation in the material radiated for unity of mass:

the most ancient is the "RAD"

1 RAD = 100 erg/gs

Currently in the S.I. the is used "GRAY" (Gy):

1 Gy = 1J/Kg

1 Gy = 100 RADs

But the effect of the radiations, parity energy's hips it is dependent from the type of radiation.

Therefore the factor of quality of the radiation is introduced "Q", the greatness that is considered it becomes therefore the equivalent of dose "H" tied up to the absorbed dose "D" from the relationship:

H = QxD

For electrons, X rays and rays range Q = 1

For neutrons and protons Q from 5 to 20

For the particles alpha Q = 20

It is finally had to also keep in mind some different sensibility (Fp) of the various fabrics and organs to the radiation; it is for this that the concept of effective dose is introduced (And)

Values of Fp for the various organs:


marrow, breast, bellows, thyroid, liver, esophagus, colon;

skin, bony surface;

brain, bowel, kidney, spleen, uterus, pancreas, muscles





The unities of measure of the equivalent of dose and the effective dose are:

1 Rem = 100 erg/gs

what in the International System you/he/she has been replaced by the Sievert (Sv):

1 Sv = 1 J/Kg

from which 1 Sv = 100 Rems