Thermal neutron

As Thermal neutrons - are - are not fully consistent free neutron designated whose kinetic energy is less than, say, 100 meV (milli- electron volts is). In the classification of neutrons , they lie between cold and epithermal neutrons.

The designation as thermal neutrons is derived from their formation. Thermal neutrons are created by slowing down neutrons of higher kinetic energy by being elastically scattered several times on atomic nuclei . As a result, they assume the same velocity distribution as the atoms or molecules of the scattering medium; this speed distribution depends only on the temperature of the scattering medium. In many cases, e.g. B. in nuclear reactors or shields , this deceleration is brought about specifically by means of a moderator medium. But also in other material environments with free neutrons, their energy spectrum almost always shows a larger or smaller proportion of thermal neutrons.

The kinetic energy is also linked to the temperature of a system via the equipartition theorem . It applies to neutrons with the Boltzmann constant and the absolute temperature . Although temperature is only defined for many-particle systems, the technical jargon says that the individual neutrons have a temperature at a certain average energy . This temperature is usually given in units of energy by setting the Boltzmann constant equal to one (see natural units ). Furthermore, it is common in particle physics to omit the prefactor when defining the temperature of an individual particle. ${\displaystyle \langle E\rangle ={\tfrac {3}{2}}k_{\mathrm {B} }T}$ ${\displaystyle k_{\mathrm {B} }}$ ${\displaystyle T}$ ${\displaystyle \langle E\rangle }$ ${\displaystyle {\tfrac {3}{2}}}$

The mentioned “upper limit” of 100 m eV thus corresponds (without the factor 3/2) to the temperature 1160  K (887  ° C ).

The nominal energy is usually taken at room temperature ; more precisely is the mean kinetic energy ${\displaystyle E=k_{\mathrm {B} }T\approx 0{,}025\,\mathrm {eV} }$

${\displaystyle \langle E_{\rm {kin}}\rangle ={\frac {3}{2}}k_{\mathrm {B} }T\approx 0{,}039\,\mathrm {eV} }$

use

Thermal neutrons play an important role in most nuclear reactors. However, there (at least in power reactors ) their energy is noticeably above the above-mentioned 0.025 eV because of the working temperature.

They are also used in neutron scattering as an important tool in structural research on materials.

Also for shielding against neutron radiation , i. H. to reduce the radiation intensity , the neutrons are initially thermalized by a moderator, and then of a material with great absorption - cross section for thermal neutrons, such as boron or cadmium to be absorbed.

literature

• K. Wirtz, KH Beckurts: Elementary Neutron Physics . Springer 1958, page 35