Neutron

Quark structure neutron
Quark model of the neutron.

The neutron is an uncharged, subatomic particle \((\sim 10^{-15}\;\textrm{m})\) consisting of an up quark and two down quarks, with a net electric charge equal to zero, located in the nucleus of an atom.

As it consists of quarks, it belongs to the family of hadrons and in particular to the group of baryons. Having half-integer spin is a fermion. As regards the strong interactions, the neutron and the proton constitute the two different states of charge of the same particle: the nucleon.

It has a resting mass of \(939.57\;\textrm{MeV/c}^2\), slightly higher than that of the proton, and with the exception of the more common hydrogen isotope (whose atomic nucleus consists of a single proton) the neutron composes the nuclei together with the proton, with which is continually transformed by the emission and absorption of pions.

Outside the nucleus the neutrons are unstable and have a half-life of about 15 minutes. They decay into a proton emitting an electron and an antineutron (beta decay or β-decay).

Neutron beams are used in a variety of areas of nuclear research. In addition to the investigation of the nature of nuclear forces they are used in particular for studies of molecular structure. In fact, being the De Broglie wavelength of neutrons of energy 0.1 eV equal to 0.9 Å and therefore comparable with interatomic distances in solids and liquids, diffraction studies, based on physical principles analogous to those involved in X-ray diffraction, allow to collect information on the relative location of atoms in a crystal or in a molecule.

History and discovery

The discovery of the neutron originated from some experiences of Walther Bothe and Herbert Becker, who in Berlin-Charlottenburg in 1930 observed a penetrating secondary radiation emitted by various light elements (Li, Be, B, F, etc.) bombarded with alpha particles emitted from a sample of polonium, and interpreted as consisting of hard gamma rays, i.e., very penetrating.

Shortly after that, John Chadwick, in Cambridge (Great Britain), experimentally demonstrated that the penetrating radiation in question was capable of transferring energies of the same order of magnitude even to nitrogen nuclei, which are endowed with a mass about 14 times that of the proton. Chadwick immediately realized that all the phenomena observed up to then could be fully explained if one assumed that the penetrating radiation was constituted, at least in part, by a new type of electrically neutral corpuscles with a mass very close to that of the proton.

Chadwick published the results of his experiences and their interpretation in a letter to the scientific journal Nature February 17, 1932, letter that is universally considered as the birth certificate of the neutron.

The fact that the neutrons were emitted by nuclei of light elements under the action of alpha particles suggested a new model of the nucleus. Before the discovery of the neutron, it was thought that the nuclei of all atoms consisted of aggregates of the two elementary corpuscles then known, protons and electrons, in such quantities as to give rise to a system endowed with the right value of electric charge and mass.

However, with the advent of quantum mechanics, a few years earlier, this model had become unacceptable. As a consequence of the uncertainty principle, an electron confined within the dimensions of a nucleus was necessarily equipped with a quantity of motion, and therefore also with kinetic energy, so high as to be incompatible with what was already known then about the energies involved in the nuclei. Thus was born a new model of nucleus consisting only of protons and neutrons (F. Perrin, W. Heisenberg, D. Ivanenko, 1932), widely confirmed by all subsequent experiences.

Two discoveries made by E. Fermi and collaborators, closely linked, gave an extraordinary impulse to the study of nuclear reactions and neutron properties. In January 1934, I. Curie and F. Joliot had discovered that some light elements, in particular, boron and aluminum, bombarded with alpha particles, gave rise to radioactive substances (artificial radioactivity due to alpha particle bombardment). Immediately after the disclosure of these results, Fermi in Rome thought of causing artificial radioactivity using neutrons as projectiles, although these, being corpuscles produced in nuclear reactions initiated by alpha particles, were then only available with very small intensities, of the order of 104-105 times lower than the intensities of the alpha particle sources used by the Joliot-Curie spouses.

Fermi started from the idea that the lack of electric charge of the neutron, thanks to which it is not rejected by a nucleus when it passes in its proximity, could more than compensate for their low number. The discovery of artificial radioactivity by neutron bombardment, made by Fermi in March 1934, was particularly interesting also for the variety and importance of the phenomena discovered in the systematic study, performed in various research centers and in particular in Rome by Fermi and collaborators (E. Amaldi, O. D’Agostino, F. Rasetti, E. Segré). Already during 1934, it was shown that neutrons could produce processes of different types, indicated with the symbols (n, α), (n, p) and (n, γ), where the first and second letters in brackets represent the incident particle and the emitted particle respectively. It was also demonstrated, in Rome, that the last of the processes as mentioned above, often referred to as the radiative neutron capture, can be produced in all chemical elements, from the lightest to the heaviest, such as thorium and uranium.

In later years, many other types of neutron-induced nuclear reactions were discovered in several laboratories. Particularly noteworthy was the discovery made in 1939 in Berlin, by O. Hahn and F. Strassmann, who showed that, under the action of neutrons, heavy elements such as thorium and uranium undergo the phenomenon of fission.

Neutron properties

Neutrality

The property par excellence of neutron, which differentiates it from other particles, is its zero electric charge, as it is composed of two down quarks and one up quark:

\[q_n = 2q_d + q_u = 2 \times -\dfrac{1}{3}e + \dfrac{2}{3}e = 0\]

The zero electric charge is the basis of their high penetration capacity and difficulty in manipulation. In addition, important natural sources are absent, another reason for the delay in the discovery of the neutron compared to the other two particles that make up the atom.

Penetration

Charged particles (such as protons, electrons and alpha particles) lose energy in passing through matter, mainly due to electromagnetic forces that ionize the atoms with which these particles interact. Neutron is weakly affected by these forces because it has zero electric charge. However neutron is subject to strong nuclear force action, characterized by a short range, effective only near a nucleus. So neutron behaves similar to a solid sphere (probably the farthest from neutron in many other aspects) that impacts other solid bodies (nuclei in the medium). A free neutron goes on its way undisturbed until it hits “frontally” with a nucleus.

Because of the small cross section of nuclei compared to the space between them, these collisions happen very rarely and neutrons travel large distances before colliding. Even greater distances are covered before being absorbed by a nucleus, the greater is the speed of the neutron: the dose absorbed by a material irradiated with neutrons is greater and more concentrated on the surface the lower is their speed:

  • fast fission neutrons (10 keV-10 MeV) deposit energy primarily through elastic collision with multiple light nuclei in succession, with average free transport paths ranging from centimeters to tens of centimeters in water and biological tissues. The absorbed dose is due to ionizations caused by the recoil of secondary light nuclei.
  • resonance neutrons (1 keV-1 MeV) deposit energy primarily by absorption when their energy corresponds to a resonant frequency of a nearby nucleus.
  • slow neutrons (energy < 0.5 eV) have low transport length, from some fraction of millimeter to centimeter, and large absorption cross section: also in this case absorption is the main type of interaction.

Hazardousness

Their higher rest mass than that of other types of radiation makes them more dangerous to biological tissues once they have interacted and can result in biological tissues and ordinary materials in secondary release of gamma rays by neutron capture (e.g. with hydrogen results in a deuteron and a 2, 2 MeV), transmutation and sometimes spallation (e.g. activation of nitrogen-14 into carbon-14 with release of a proton or spallation of lead exploited in accelerator driven systems), causing transmutation and for transuranic fertilization. Their hazard factor once absorbed with respect to gamma radiation holds:

\[w_n=5+17\mathbf{e}^{-\frac{(\mathbf {ln}{\frac{2E}{MeV}})^2}{6}}\]

i.e., depending on their energy they go from being 5 times more dangerous than photons if they are thermal (E < 10 keV) or 25 times more dangerous if they are fast (100 keV < E < 2 MeV), respectively. Note, however, that this factor does not take into account the previous penetrance property, so the relative hazard of a fast beam compared to a thermal one is actually overestimated by this factor.

Evanescence

Indirect neutron detection is based on the transmission of motion to light atoms in the medium that occurs in elastic collisions: a very heavy nucleus acquires by elastic collision a small fraction of the momentum; instead a proton (which has a mass approximately equal to that of the neutron) is projected forward with a significant fraction of the original speed of the neutron, which in turn slows down. Since the nuclei set in motion by these collisions are charged, they produce ionization and can be easily detected experimentally.

Immanoeuvrability

Charged particles can be accelerated, decelerated and deflected by the electrostatic field, which however has practically no effect on neutrons, making them maneuverable only with magnetic field given their rather high magnetic dipole moment relative to the intrinsic momentum. The negative sign of this magnetic moment simulates the rotation of negative charges counterclockwise around the spin direction. The only means of controlling free neutrons is to place nuclei in their path so that the neutrons are slowed down, deflected, or absorbed in the collision. These are the main regulatory effects in reactors and nuclear weapons.

Instability

A pulsed, collimated beam of free neutrons decays by passing between two magnetic lens spectrometers arranged so that one collects protons the other electrons with average lifetimes of 880 ± 1 s.

As for the neutron bound to the other nucleons in the nucleus, the grand unification theory predicts an average lifetime of the order of 1031 years (more than a trillion of billion times the current age of the universe of about 1010 years), similar to that of the proton.

Neutron sources

The simplest neutron source is a compound consisting of a suitable radioactive substance and a light element such as beryllium or boron. The radium-beryllium source provides, for example, neutrons as a result of processes (a, n) and, for the long average life of radium (1622 years), is a standard emitter with sufficiently constant flux of neutrons of energy between 1 MeV and 12÷13 MeV.

Often are also used sources of photonutrons (i.e. neutrons liberated by the action of X-rays and γ-rays), where processes (γ, n) are exploited; they have the advantage of producing neutrons practically monoenergetic. Most neutron sources are based on the Be9(γ, n)B8 and H2(γ, n)H1 reactions.

The γ source consists predominantly of nuclides activated in nuclear reactors. Neutrons can also be produced in particle accelerators. Bombardment of heavy water (D2O) with deuterons accelerated with a Van de Graaff generator provides a simple and efficient source. Other accelerators also produce virtually single-energy neutron beams with energies on the order of a few MeV. In many applications neutrons with energies from 0.01 eV up to a few eV are needed; these are obtained by channeling the fastest neutrons into materials where they can lose energy as a result of elastic (predominantly light nuclei) or inelastic (predominantly medium or heavy nuclei) collisions.

If a neutron source is congealed in a material with low neutron absorption cross section and high collision cross section, and if the geometry of the system is appropriate, neutrons lose energy until their velocity becomes comparable with that of thermal agitation of the nuclei of the materials. Neutrons are then said to be thermal and have Maxwellian distribution of velocities. A material that performs the function of slowing down neutrons is called moderator. The most used moderators are water, heavy water, kerosene, graphite and beryllium.

The most intense sources of neutrons are those obtained with nuclear reactors. In a system, the site of controlled chain reactions, consisting of fissile material and moderator, neutrons are present at different energies and can be used to activate artificial radionuclides. If thermal neutrons are needed they can be obtained in the so called thermal columns consisting of graphite blocks adjacent to the reactor in which neutrons are slowed down and, if necessary, collimated in a beam to be used outside the column.

Neutron detection

The detection of neutrons depends on the secondary effects of their interactions with nuclei. In fact, since neutrons have no electric charge, crossing matter they produce a negligible ionization and cannot be detected directly by instrumentation based on this effect. The reactions on which neutron detection is based are:

  • neutron absorption by a nucleus with fast charged particle emission for which ionization chambers, or proportional counters, filled with natural BF3 or B10F3 are used;
  • neutron absorption with fission of the formed compound nucleus for which ionization chambers containing gaseous UF6 are used. Using the different uranium isotopes, neutrons are revealed from thermal up to energies greater than 1 MeV;
  • neutron absorption with formation of radioactive nuclides whose activity is measured. Choosing properly the nuclide to be activated it is possible to detect neutrons in a wide range of energies;
  • deflection (scattering) of neutrons by a light nucleus, for example hydrogen, with deflected neutron producing ionization for which hydrogen gas ionization chambers are used or containing a solid hydrogen-rich target.

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