Daviddi Arianna & Veronica Cappello 4abio1 29 / 05/2008
ENRICO FERMI
Life
Enrico Fermi was born in Rome September 29, 1901. As a child he showed great interest and propensity for mathematics and physics, which came to the contribution of the teacher gazie Amidei. Quest'ultimo lo aiutò seguendo sistematicamente i suoi studi e proponendogli dei test specifici. La sua preparazione proseguì al liceo, ove si diplomò nel 1918, e successivamente presso la scuola normale di Pisa.
Durante gli studi universitari Enrico Fermi si dedicò ad una intensa attività di ricerca sui caratteri distintivi del suono e della diffrazione dei raggi X da parte dei cristalli, conclusosi nello stesso periodol'assegnazione del premio Nobel per la fisica ad Einsteinmise lo studio della struttura atomica al centro dei suoi interessi e lo portò all'elaborazione della statistica antisimmetrica (statistica Fermi-Dirac) basata sul principio di esclusione di Pauli.
La brillante elaborazione lo mise ai vertici degi interessi Corbin, a renowned politician, who gave him the opportunity to create a research group at the Chambers Street Panisperna Portti in Rome and the Italian physics at the highest levels.
Along with a group of young graduates Fermi was able to make the studies that led to the formulation of the mathematical theory of beta decay and the discovery of slow neutrons. An neutron bombardment of this type allowed to obtain activity induced much more 'intense. More information on the subject are summarized in a work done with the collaboration Amaldi "On the absorption and diffusion of slow neutrons, and theoretical work of the same Fermi:" On the motion of neutrons in hydrogenated substances. " The enormous value of the discovery yielded to stop the award of the Nobel Prize for Physics in 1938.
From then until 1942 the world's precarious condition constrinse Fermi to stay in the U.S., were not in force when racial laws adopted in Germany and Italy instead. There he worked the activation of the first nuclear reactor, which was used to produce a development monitored by energy from a fission process, and construction of the atomic bomb.
abandoned his studies in this area dedicated himself, until his death in 1954, the study of subatomic structure and analysis of reactions between Pions and Nucleons. To honor
ilo his enormous contribution to the physics world when subatomic particles were discovered as yet unknown, was given to them in the name of fermions.
The beta decay of the neutrino hypothesis
Substances with radioactive alpha emitting nuclei of helium, for example, radium (Ra) - element discovered by Marie Curie-decays into radon (Rd), a noble gas according to the scheme:
With the alpha radioactivity originating from the atom are ejected two protons and two neutrons, the mass number is reduced then to four units. This phenomenon reflects a structural instability of the heart "father", a phenomenon explained in the early '30s in the context of quantum mechanics. The origin of beta radioactivity, however, had problems at work until Fermi. The beta rays are electrons, but where they come from? Before the discovery of the neutron is believed that the core was composed of protons and electrons. In this context the beta decay would thus be similar to the alpha decay, which is a rearrangement of the components already present in the initial nucleus, a real disintegration. The discovery of the neutron led quickly to abandon that view. The beta radioactivity was an issue even more serious: the electrons are emitted from a single energy but with an energy spectrum that varies continuously. The situation is quite different from that found in the decay of radioactive alpha or gamma, in which the energy of the emitted particle is determined the energy difference between initial and final nucleus, and is therefore always the same for a given type of decay. In the disintegration of radium, for example, alpha particles are emitted with an energy of 4.88 MeV. This simple argument does not work in the case of beta decay, Bohr had come to propose that in this case the energy is not exactly conserved. The solution to this problem was found by Pauli in the beta decay is not only emitted an electron, but also a second particle that escaped their tools. The two are divided particles emitted energy available, it may be in different proportions, so that the energy given the electron is not uniquely determined. The second particle must be neutral, otherwise it would be easily detected through its ionizing power, and could not be a photon, since the experimental data seem to exclude. It must have been an entirely new plot. The hypothesis of this second plot seemed fanciful at the same Pauli. He wrote it only to close colleagues where this particle called the neutron. In the congress held in Rome on nuclear physics with Fermi nel1931 spoke jokingly offered him the appropriate name "neutrino" because its mass must be less than that of the neutron. In 1932, however, still debated for Fermi beta radioactivity in terms of emissions of particles already present in the nucleus; the solution took place in 1933 would have been radically different. In October 1933, the modern point of view on the structure of the nucleus composed of protons and neutrons, which had its official sanction in Brussels at the Solvay Conference. In the discussion that followed the report on nuclear forces Heisenberg, Pauli finally came out into the open with some remarks on the assumptions of the neutrino. The theory of Fermi
The entrance to the neutron by Chadwick in the structure of the electron and nucleus drove out, leaving little place in the presence of a neutrino Pauli. If an electron and a neutrino are not present in A, they must be created in the transition. This was a view hard to accept because it was used think of the electron as a particle material, with its solidity. The ability to create and destroy particles had a precedent in the case of photons. The light is solely made up of photons that are created when light is emitted and absorbed when it is destroyed. An atom can emit a photon from a higher energy level to a lower level. In the reverse process an atom can absorb a photon passing from a lower energy level to a higher level. These processes concerning the photons are described by quantum theory of the electromagnetic field developed by Dirac. In 1927 Jordan and Klein showed that this quantum field theory could be applied to any particle. The electrons that could be seen as particles but also as a wave phenomenon. In quantum physics the concept of particle and the field are fully interchangeable. In any field is a kind of identical particles with each other, but the opposite is true. The language of the field allows you to describe phenomena in which particles are created or destroyed but the work of Fermi on radioactivity beta is the first in which this possibility has been used outside of the theory of photons.
the base of the Fermi theory is the hypothesis that the beta decay of a nucleus is due to a new type of interaction between particles that causes the transmutation of a neutron into a proton with the simultaneous creation of an electron and a neutrino,
.
Since proton and electron have opposite electrical charge while neutrons and neutrinos are electrically neutral, this process has kept the value of the total electric charge. Second stop there was an analogy between this process and the emission of gamma rays at the base,
in which one of the protons in the nucleus changes from a higher to a lower energy state, emitting a photon.
Fermi proposed the existence of a new type of current, now called low-current, which occurs at the transformation of a neutron proton pair leading to the creation of e - v.
In his work of 1933 presented the mathematical structure of his new theory and its application in the study of radioactive beta decay. These can be divided into two classes:
Permits - could take place even if the nucleons (protons, neutrons) were still within the nucleus;
banned - are only made possible by the fact that the nucleons move, they proceed more slowly and their average lifespan is about 100 times longer than the allowed decays. It was only with Fermi
that this phenomenon is explained quantitatively. A second important result of the work is in the determination of the Fermi energy distribution of electrons emitted. Showed that this distribution allows to determine the mass of the neutrino. Fermi's theory contains only one parameter Incognito, G, now called the Fermi constant which can be determined by measuring the average lifespan of an allowed beta decay and determines the intensity of new interactions.
We have said that the transition between neutron and proton in the aforesaid process generates a weak current that leads to the creation of the pair electron - neutrino. The mechanism of this phenomenon is similar to the phenomenon of 'magnetic flux when the current in a circuit generates a magnetic field. In Fermi's theory has a sort of short circuit between the nucleon weak current, activated by the transition from neutron to proton, and a corresponding current of leptons (electron - neutrino), whose activation leads to the creation of the pair electron - neutrino. The weak interaction is then the second stop for a direct interaction between currents, without the action of an intermediate field, such as in the case of magnetic induction.
The stability of the nucleus A nucleus is stable when its mass is less than the sum of the masses of each pair of cores can be obtained from its division. A magnitude that clearly defines the stability of a given species is the atomic binding energy per nucleon or specific, is defined by the following relation
Experience shows that the value of this energy varies little for all species and nuclear is between 7.4 MeV and 8.8 MeV. It's huge energy when you consider that those bond of electrons to their nuclei are only a few electron volts. The binding energy per nucleon is very small for light elements and increases rapidly until it reaches the maximum value for those of average atomic number (at between 40 and 60), then it tends to decrease gradually until it becomes again at least for heavier nuclei. It thus appears that both nuclides lighter and heavier ones are less stable. For the former, consisting of a few protons and neutrons, the effects of the surface of the core are more significant. In fact, the nucleons located near the outer surface of the nucleus are not completely surrounded by other nucleons and are subject to a smaller force of attraction. It follows that in the light elements surface nucleons are relatively larger and therefore heavier than the binding forces are lower. For heavier nuclei instead of the large number of protons leads to increased electrostatic repulsion of their charges to the detriment of the stability of the nuclei themselves.
processes radioactivity
The nuclei of atoms can emit not only heavy particles and electrons as well as gamma radiation: if the issue occurs spontaneously, as occurs in nature for the heavier elements (Z> 80), the process is name of natural radioactivity, in the opposite case we speak of artificial radioactivity. Radioactive decay is the phenomenon by which a nuclide, resulting from the issuance of heavy particles is transformed into a more or less long, in a different nuclide. The transformation can occur through several stages tends, more or less rapidly to a stable form, so, for example, the radioactive series headed by uranium 238 that passes through a succession of decays leading finally to the last item stable consists of the lead. Nuclear fission
The phenomena of radioactivity can be adequately explained by the occurrence of specific nuclear reactions. A nuclear reaction is called when the fission involves the splitting of an unstable nuclide in two or more nuclides with masses comparable with each other.
The study of fission reactions originates from research conducted by Fermi and the boys on the street Panisperna bombardment with neutron beams, produced using the gun to neutrons. The neutrons, having no electric charge, can penetrate inside the nucleus but have little kinetic energy. The odds, therefore, to make nuclear reactions with these particles are greater, especially for the heavier nuclei that have a lower binding energy. We need, however, neutron sources and those rich enough, already in the thirties, were available to physicists working in this field of scientific research. During the experiments
carried out by Fermi and his collaborators was noticed by bombarding uranium with slow neutrons, was obtained by the formation of several radioactive nuclides. It was thought however that these elements have an atomic number close to that of uranium, and because there were recognized as among the known nuclear species, it is proposed that this was transuranic elements.
But in 1939 the German physicists O. Hahn and F. Strassmann proved that when the uranium was bombarded with neutrons to promote the formation of two large nuclear fragments which subsequently suffered a series of radioactive transformations, among them the reaction products identified the nuclei of barium and krypton. In this type of nuclear reaction, which consists in the splitting of atomic nuclei of heavier elements into two (rarely more than two) with masses that are in a ratio of the order of 3 / 2, was given the name of nuclear fission. According to the model nuclear drop, designed by N. Bohr, the fission reaction can be interpreted as follows. When a heavy nucleus, for example, captures a neutron, into a state of instability and stands to vibrate. Subsequently, it tends to stretch in one direction and narrows in the middle to break up into two.
Following the first experiments we could prove that the fission with neutrons also occurred in the thorium and protactinium. But while these elements was necessary for high kinetic energy of neutrons used for uranium, however, the reaction took place with both fast neutrons and slow neutrons. Instead, he was able to prove that, while the most abundant isotope of uranium (99,274%), which were needed fast neutrons, for the least abundant (0.71%), ie the 'slow or thermal neutrons were sufficient, the latter designation means that these neutrons have a kinetic energy of the order of magnitude as that caused by thermal agitation of molecules. Another remarkable fact, which was to manifest because of the use of atomic energy, was that in the fission of uranium 235 were issued at least two or three neutrons per nucleus that underwent disintegration.
In the fission process when discovering the convenience of slow neutrons by passing them through or within a block of paraffin or a water tank (the famous fish pond via Panisperna), the bombing more interesting, and initially very mysterious, will be noted that the uranium. The isotope of uranium was able to separate uranium 235 and the reaction against it can be schematized as follows: X and Y are nuclides with mass numbers around 127, the number of neutrons ( n) varies depending on the actually produced nuclides, the energy released is about 200 MeV for the nucleus of uranium nucleus that is broken. The amount of energy produced is enormous, considering that the combustion of a carbon atom free about 4 eV, while the splitting of a uranium nucleus involves the release of an amount of energy 50 million times. This happens because the nuclear reactions have significant changes in mass but that no chemical reactions occurring in the normal combustion. Returning to the reaction described, it should be noted that the production of neutrons can be absorbed by neighboring nuclei of uranium fission giving rise to new processes and new power generation, the first neutron that produced the initial cleavage triggers a chain reaction so that in short time includes the entire amount of uranium available. The first practical use the fission reaction was the appalling destruction of Hiroshima and Nagasaki, which marked the end of World War II. In the reaction becomes explosive atomic bomb because they are rapidly brought into contact two masses "subcritical" so that the system, on balance, exceed the mass critical. The critical mass is the smallest amount of material needed to fissibile self-sustaining chain reaction. In the nuclear chain reaction is controlled and adjusted using rods of cadmium or graphite or boron steel that have the ability to easily capture neutrons. In the reactor involved in these functions the control rods. An adjustment of the reactor is ensured even the water that cools the core, turns into steam as a result: it absorbs, slowing down, a certain amount of neutrons. The steam produced is sent to the turbine, coupled to the alternator is capable of generating electricity.
Fermions
look at the structure of atoms is possible to ask a question: we know that the particles that make up the core affected by the strong interaction, we know that the electrons circling the nucleus like the moon around the earth, but because the electron is not affected by the strong nuclear force much more intense than that in the mail when its motion is very close to the nucleus? This question Fermi in collaboration with the physicist Dirac, tried to answer by introducing the concept of fermion. The class includes
fermions leptons, which have lower mass than the proton, and baryons, with mass equal to or greater than the proton.
stable particles are the photon, the particle of light and "quantum" of energy and leptons, which are the lightest elementary particles with the exception of the muon. The leptons include the electron, the muon (most stable), which equal to the electron mass and electric charge, positive or negative, and the neutrino, whose mass is practically zero and is issued in radioazione beta along with a positron, the m particle, the particle you their antiparticles.
The following are the baryons that comprise the proton (stable), the neutron (the semi), which has an average life of 1.01 • 10 s. ³, the fibulae, most resonances in rapid decline.
The fibulae are particles with mass from 2100 to 2600 times higher than that of electrons, and decay when they give rise to protons, neutrons and mesons. Fermi said, and his statements are considered valid today, that in reality only a few particles are affected by the strong nuclear force, as only the electrically charged particles are affected by the electromagnetic interaction.
We can therefore make a distinction between particles that are affected by strong interactions and those that are not affected: the former are called hadrons seconds are the leptons.
Fermions:
A fermion is any particle whose intrinsic angular momentum (spin) has a value odd multiple of half (1 / 2, 3 / 2 ,...), measured in units of h (h-cut ). As a result of their angular momentum, all the fermions obey the Pauli exclusion principle.
The fundamental matter particles (quarks and leptons) as well as most of the composite particles (like protons and neutrons) are fermions. Therefore, according to the Pauli exclusion principle, these particles can not coexist in the same place. And that, for materials under ordinary conditions, is an important property.
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