PNPI  
National Research Center "Kurchatov Institute"
Petersburg Nuclear Physics Institute

NEUTRON RESEARCH DEPARTMENT

Petersburg Nuclear Physics Institute
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7th International Workshop "Ultra Cold & Cold Neutrons. Physics & Sources." 8-14 of June 2009
 History
History review of neutron
research in PNPI
   (File PDF)

WWR-M history
   (File PDF)

The first experiments
on reactor WWR-M

From the workshop
"40-th years of WWR-M"

 

WHAT ARE NEUTRONS NEEDED FOR


In 1932 on February, 17th James Chadvick has directed to a press the article "Possible existence of a neutron" (Chadwick, J. Nature, 1932, v.129, p.312) which has opened a new nuclear era in history of development of humanity. Two years later, in 1935 he has been awarded the Nobel Prize. Professor H. Pleijel, Chairman of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences in his presentation speech said, "This year the Nobel Prize for Physics is awarded as a reward for a discovery, confirmed in an experimental way, of a new fundamental building-stone of atoms and molecules, viz. the discovery of the so-called neutron. By a combination of intuition, logical thought, and experimental research Professor J. Chadwick, the laureate of this year, has succeeded in proving the existence of the neutron and establishing its properties."

On the one hand, realization of controlled nuclear reaction with use of neutrons has allowed humanity to achieve on a new level of power engineering and to get practically inexhaustible energy sources. On the other hand, appearance of a neutral "elementary" particle, and also its properties, have presented a number of surprises and at that time have made a revolution in notion of a structure of matter and itself elementary particles. For example, existence of the neutron magnetic moment is simple evidence of complex structure of neutron, i.e. neutron is not quite "elementary" particle.

Possible existence of the electric dipole moment (EDM) is concerned with violation of CP-invariance, i.e. invariance of our world about replacement of particles by antiparticles and, by indirect way, with the baryon asymmetry in the Universe. Since the neutron takes part in all forms of known interactions, research of its electromagnetic properties, as well of its weak and strong interactions gives a unique opportunity to understand how particles and their interactions "are arranged". On the other hand, this research allows getting to the bottom of formation and a structure of the Universe. Therefore, a search and measurement EDM of a neutron, measurement of its electric charge and electric polarizability, a lifetime, a search of a neutron-antineutron oscillations, measurement of angular correlations at beta-decay of a neutron, a research of a neutron-neutron scattering, etc. have huge importance for modern physics.

Neutrons and protons are those "elementary" particles of which atomic nuclei of usual substance are formed. Besides, they play the important role in processes of nuclear synthesis in stars, which defines an origin of elements in the Universe and, in particular, on the Earth.

Thus, the free neutron is an unstable, electric neutral particle with following properties: (the Review of Physics of elementary particles. Euro. Physics. J., 2000, v. C15, number 1-4).

Spin

½

Charge (e - a charge of the electron)

qn = (-0.4 1.1) 10-21

Weight
in atomic mass units

mn =939.56533 0.00004 MeV,
= 1.00866491578 0.00000000055 u

Difference of weights of a neutron and proton
in atomic mass units

mn - mp = 1.,2933318 0.0000005 MeV,
= 0.0013884489 0.0000000006 u

Lifetime

n = 885.4 0.9stat 0.4syst s

The magnetic moment

n =-1.9130427 0.0000005 n

Electric the moment

dn <0.6310-25excm (% CL=90)

Electric polarizability

= () 10-3 Fm3

These properties of a neutron allow to use it, on the one hand, as the object, which is investigated, and, on the other hand, as the tool by means of which researches can be carried out.

In the first case unique properties of a neutron are investigated that is topical and enables most reliably and precisely to define fundamental parameters of electroweak interaction and as a consequence to confirm, or to disprove the Standard model.

Search and measurement of the neutron electric dipole moment is one in the most precision and important experiments in the physicist. Much of offered theories of CP-violation are already rejected by the EDM neutron value limit of 6.3x10-26 xcm (% CL=90) received to the present time. Decrease in 15-20 times of an experimental limit on EDM of neutron, that not seems impossible, is exclusively important in respect to check of super symmetric theories.

In the second case, interaction of non-polarized and polarized neutrons of different energies with nuclei allows to use them in investigations for nuclear physics and elementary particles. Studying of effects of violation of P-parity and T-odd correlations in various processes (from neutron optics to nucleus fission by neutrons) is far not the full list of the topic trend of researches now. That fact, that thermal neutrons have lengths of waves comparable with intra-atomic distances in substance, does them by irreplaceable tool for research of the condensed matter. Interaction of neutrons with atoms is rather weak what allows neutrons to get deeply enough into substance. This property of neutrons is essential advantage in comparison with X-rays and -rays, and also with the charged particle beams. As the neutrons have a mass, they have much smaller energy than X-rays or -rays with the same momentum (wavelength). This energy is comparable with energies of different kind of elementary excitations in substance what enables researching into not only the average static atomic structure of substance, but also occurring dynamic processes. Presence of the magnetic moment allows using neutrons for studying magnetic structure and magnetic excitations that is very important for understanding properties and nature of materials with magnetism.

Scattering of neutrons by atoms is caused, basically, by nuclear forces; hence, coherent scattering cross-sections are not connected at all with atomic number (as distinct from X-rays and -rays. Therefore, "lighting" of materials by neutrons allows defining the positions of atoms of light elements (hydrogen, oxygen, etc.) whose identification is almost impossible with use of X-rays and -rays. For this reason, neutrons are successfully applied at studying biological objects, in materiality, in medicine, etc. Besides, distinction in coherent scattering neutron cross-sections of different isotopes allows not only to distinguish elements with close atomic numbers, but also to investigate their isotopic composition. Presence of isotopes with negative amplitude of coherent scattering gives a unique possibility to contrast under investigation medium, what also very often use in biology and medicine.

V.V.Fedorov, Yu.L.Khazov

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