The existence of neutrino was supposed in 1933 by the Austrian physicist
Wolfgang Pauli (Nobel prize winner in 1945 ) to explain the variable kinetic
energy of the electrons emitted by radioactive nuclei subjected to b decay.
Pauli postulated the existence of a neutral particle without mass, that is a particle suitable to explain the fact that the emitted electrons have a variable kinetic energy in the range between zero and a maximum value.
Admitting the existence of neutrino justifies for b
decay the validity of the energy and linear momentum conservation principles.
In fact, if it is
postulated the neutrinos are emitted with velocities opposite to the ones of
electrons and with kinetic energies such that, if they are added to the ones of electrons,
give a constant value, immediately are derived the characteristics observed experimentally
for b decay.
Besides, because the mass of neutrino is
considered nearly zero, the supposed particle must be considered to be moving with a velocity nearly
equal to the one of light.
Exist three types of neutrino with the respective
antiparticles, associated respectively to electron, muon and t particle (lepton) .
Neutrinos belong to the lepton family (electrons,positrons,muons, tauons and respective antiparticles) and are subjected only to weak interaction.
The discovery of neutron is owing to the English physicist Chadwick (Nobel prize
winner in 1935), which gave a new interpretation of the experiments effected by Bothe and
Becker (1930 ) and by the French physicists Mr. Joliot e Mrs. Joliot-Curie (1932 ), who
bombarded boron and berillium targets with the a particles
(helium nuclei) emitted by radium.
Bothe and Becker and Joliot-Curie thought that the
colliding aparticles determined emission of high energy g photons by the B o Be nuclei.
The experimental tests produced
from Chadwick showed instead the existence of neutrons in atomic nuclei, that before the
1932 were thought to contain protons and electrons.
Neutron has no electric
charge and its mass is about equal to the one of proton.
It belongs to the family of
hadrons and is subjected therefore to strong interactions, besides the weak ones.
Neutron besides, although it is neutral, has a magnetic moment by which it can interact
with an electromagnertic field,
Neutron and proton are both called nucleons, as they form atomic nuclei.
Enrico Fermi and his co-workers Emilio Segrč, Franco Rasetti, Edoardo Amaldi, Ettore
Maiorana and Oscar D'Agostino, in 1934 at the physics institute of Rome University, effected a series of famous experiences finalized to studying the radioactivity induced by bombarding nuclei by neutrons.
In the same year Mr. e Mrs. Joliot-Curie discovered induced (artificial) radioactivity by bombarding aluminium by the a particles emitted by polonium.
They were able to produce some radioactive nuclei emitting positrons instead of electrons, as it happens for natural radioelements.
Fermi thought to use neutrons instead of a particles, considering that neutrons, being neutral, aren't subjected to the repulsion by positive atomic nuclei, and they are able therefore to induce much easily the reactions that make radioactive atomic nuclei.
These experiments, effected using about sixty chemical
elements, showed the possibility to induce much easily radioactivity, by bombarding
nuclei by slow neutrons, that is by slowed down neutrons which were produced by interposing between neutron source and the substance to be activated, a thickness of hydrogenate materials as water or
paraffin .
The experiences of Fermi's school were of fundamental importance for the
discovery of the nuclear fission of uranium and the realization of the first nuclear
reactor at the Chicago University (1942 ).
The conclusive phase of the researches of Enrico Fermi and his co-workers on the
radioactivity induced by slow neutrons, concerned uranium, the last natural element of the periodic
table.
However, instead of some results similar to the ones produced by experimenting on all the other elements, the experiments produced some radioelements with different decay times , separable chemically with an extreme difficulty, for the smallness of the produced quantities.
Fermi thought that were produced some artificial chemical elements (the so-called transuranic elements ), with atomic numbers greater than the one (92) of uranium .
Some researches analogous to the ones of Fermi's school were effected in Germany by Hahn and Strassman, who in 1939, by bombarding uranium by
slow neutrons, evidenced unequivocally barium and other elements with atomic numbers intermediate between the ones of uranium and barium.
After they repeated several times analyses of reaction products, they understood to have discovered a new phenomenon: the fission of uranium in two fragment, together with 2 or 3 neutrons emitted.
Subsequently, Enrico Fermi, who in 1938 was given the Nobel prize for physics for his researches on the induced radioactivity by
slow neutrons, enunciated the possibility to use the fission of uranium to produce a chain reaction, that is a nuclear reaction that, started by an only neutron, is supporting itself by 2 or 3 emitted neutrons in every fission, that are able to produce the fission of other uranium nuclei, with an exponential increasing number of produced neutrons and the emission of the considerable energy of 200 MeV, that is 200
millions greater than the one a particle with an electric charge of one electron, acquires after having been accelerated between two electrodes between which there is a voltage of 1 Volt .
The kinetic energy of the fission fragments ( two nuclei with atomic numbers intermediate, whose sum is 92, with 2 or 3 fast neutrons) comes out as heat energy transferred to the moderator material , whose function is to slow down neutrons until reduce their energy to some electron-volt fraction, that is comparable with
the energy of thermal agitation.
As neutron moderators are used substances containing
hydrogen ( paraffin, water, heavy water ), because, being the mass of a neutron about
equal to that of an hydrogen nucleus (a proton), there are enough many neutron-proton collisions
to dissipate the greatest part of the kinetic energy of a fast neutron ( from 1 to 5 MeV
).
The study of the secondary particles ( particle showers ) produced by cosmic
rays, that are X-rays and charged particles, mainly high energy protons coming from
extraterrestrial space, allowed since 1932 (by Anderson) to discover some new elementary
particles, whose existence was been foreseen theoretically.
In such a mode were discovered
positron, p meson (pion), muon, that derive by decay from pion and
moreover, since the early fifties, many other particles, both electrically charged or
neutral, that were more heavy than the proton ( particles L,S,X)
and weren't been foreseen theoretically.
The physicists recognized that, starting from three particles (that is electron, proton and neutron), considered initially the only particles of matter, were arrived to consider a numerous family of particles, whose existence made very difficult to elaborate a theoretical model that were able to describe satisfactorily their characteristics .
Were observed behaviours not re-entering within the theoretical schemes valid till that
moment, and it was necessary to coin new categories, as that of the " strangeness
", to classify some of discovered particles.
Gradually it was recognized that
researches on cosmic rays, consisting in observing the traces impressed by
particles on a photographic plate or in a cloud chamber (a Wilson's chamber ), were
no more enough to do progress the newborn physics of elementary particles.
Therefore
in the research international centers it was decided to start the construction
of some great particle accelerators, that permitted to produce high energy particles to bombard
a metallic target, for example a foil of copper, iron or tungsten with the aim of examining
reaction products and studying easily the characteristics of scattering events in relation to
the energy and type of used "bullets".
The systematic study of
collision events of particles against the target produces huge quantities
of information on the structure of matter, and allows to verify experimentally the
validity of the theoretical models that are proposed with the aim of constructing a
general theoretical model.
The systematic application of the Galilean experimental method
is fundamental not only to get confirmations about the theoretical forecasts that have been
elaborated relatively to a certain collision typology , but above all to use
coherently accelerators as if were some special ultra-microscopes suitable to observe
subnuclear matter, with a resolution that is the greater, the higher is the energy.
It is well known, in fact, that the resolution of a light microscope increases with decreasing
wavelength of the visible radiation used to effect observations.
In an analogous
mode, with increasing energy of particles bombarding the target, increases
the resolution increases, because decreases the length of the "probability waves",
given by De Broglie's relationship l = h/( MV ),
that is fundamental in microcosm mechanics.
In this context, by re-elaborating the project of the cyclotron, which is the prototype of a circular
accelerator, invented in 1931 by Lawrence (Nobel prize winner in 1939) at the California
University, were constructed in USA and in Europe ( CERN at Ginevra and national high
energy laboratories in France, Germany and Italy ) the first electrosynchrotrons and
proto-synchrotrons for researches in the field of high-energy physics, that differs from
the nuclear physics, as the matter is studied at relatively low energies.
Since early fifties, in about a decade, the family of elementary particles was so
large to include about twenty particles, of which only some (proton, neutron,
electron and neutrino ) could be considered truly elementary, while, by
considering the results of the experiments of high energy physics, that were effected by
accelerators always more powerful, was strengthened the suspect that all the other ones
were quite different than elementary and that it was necessary to seek the truly
elementary particles, by whose combinations it was possible to explain the properties of
all other particles.
In this context, in 1964 the physicists Murray Gell-Mann (Nobel prize
winner in 1969) and George Zweig advanced the hypothesis that the truly elementary
particles were leptons, that is electrons, muons and neutrinos with the respective
antiparticles, and that all the other particles, that is hadrons , that are sensitive to the
strong subnuclear interaction, were instead formed by 2 or 3 truly elementary
particles, with a fractional electric charge ( 2/3 or 1/3 of the electron charge ), for
which was conied the name of "quark",a word with a obscure meaning, used for the first
time in the novel "Ulysses" by James Joyce.
The quark model was founded
originally on three fundamental particles with spin 1/2, with the respective
antiparticles: the up-quark , with a positive electric charge equal to 2/3 of the
electron one, the down-quark , with a negative electric charge equal to 1/3 of the
electron one and the strange-quark, with a negative electric charge equal to 1/3 of the
electron one, introduced to explain the slowness of the decay of K mesons and other
strange hadrons.
Mesons(r, K , etc...) are formed by a
quark and an antiquark, while baryons (proton, neutron, particles L,
S, X, etc...), are formed by three quarks.
Antiparticles of mesons and
baryons are gotten by substituing each quark with the respective antiquark.
The
three-quark model permitted to foresee the existence of some new particles, for example the
baryon W-, that was subsequently identified in a particle of mass nearly equal to the one of
the particle foreseen theoretically.
