neutron emitting sources

Reactor as a neutron source

The neutron emitting sources with suitable examples and sketches are discuss here advanced. neutron emitting sources is needed to read as engineer student.

Types of neutron sources:

  1. Isotopic (ampoule) sources.
  2. The accelerator as a source of neutrons.
  3. Neutron generators.
  4. Reactors:
  5. industrial research: – continuous; – impulse.
  6. Plasma traps.

Reactor as a neutron source

One of the most important neutron sources is the fission process of the heaviest nuclei, which is accompanied by the emission of fast neutrons. For these purposes, nuclear reactors are used. Reactors are divided into energy (for energy), research (for research on nuclear and neutron physics and material testing), reproduction (for nuclear fuel). Often the same reactor is used to perform different functions. Reactors are also heterogeneous (nuclear fuel is distributed discrete in the core in the form of a lattice) and homogeneous (nuclear fuel and moderator is used in the form of a homogeneous mixture, solution, suspension).Reactors are also divided according to which neutrons the fission process is going on (thermal, intermediate, fast). The neutron source in the reactor is the fission reaction, so the spectrum of neutrons appearing in the reactor is the 235U or 239Pu fission neutron spectrum (the average energy is about 2 MeV). Faced with the nuclei of structural materials and the moderator, neutrons lose energy and in most cases reach thermal speeds before being trapped again. This means that inside the reactor at distances from uranium blocks comparable to the average range of fast neutrons in the moderator, there are both fast and slow neutrons, i.e. neutrons of all speeds, starting from the smallest – thermal and ending with a maximum order of 15 MeV. The velocity distribution in the thermal region agrees quite well with the Maxwell distribution (Fig. 2.9) N (v) dv = kv2exp (-v / v0) 2dv, where N (v) is the number of neutrons with velocities of about v in the interval dv, k – constant, 0 2 / v kT m = is the mean square velocity of thermal motion. In the region of velocities greater than thermal, N (v) is approximately inversely proportional to the square of the velocity, i.e. neutron energy: N (v) dv = (k1 / v2) dv = (k2 / E) dv. The neutron spectrum in a thermal neutron reactor is very wide. It distinguishes 3 components – thermal, epithermal (resonant) and fast neutrons.

Maxwell distribution neutron emitting sources
Maxwell distribution
Typical neutron spectrum of a thermal neutron reactor neutron emitting sources
Typical neutron spectrum of a thermal neutron reactor

Thermal neutrons are neutrons with energies <0.5 eV. They are in thermal equilibrium with the atoms of the reactor material. At room temperature, they have a Maxwell-Boltzmann energy distribution with an average energy of 0.025 eV and a most probable velocity of 2200 m / s. As a rule, 90-95% of the neutron flux is made up of thermal neutrons. A 1 MW reactor provides a neutron flux of ~ 1013 (cm-2s-1). The fraction of fast neutrons (> 0.5 MeV) in the reactor is ~ 5%. They cause the reactions (n, p), (n, n ‘) and (n, 2n) and practically do not cause the reaction (n, γ). Fast neutron reactors of the BR type were developed as experimental facilities. An example of the spectrum of neutrons emerging from the channel of a fast neutron reactor is shown in Fig. 2.11. Uranium monocarbide was used as fuel, and sodium was used as a coolant. The neutron reflector is made of a thin layer of natural uranium and a thicker layer of nickel.

Neutron spectra measured in the channels of a fast neutron reactor
Neutron spectra measured in the channels of a fast neutron reactor

It can be seen from the examples that the neutron spectra depend on the structure of the reactors. Even for one type of reactor, the neutron spectra inside the experimental devices are different. Therefore, talking about a typical spectrum of any reactor design is possible only conditionally. Special support irradiation facilities based on research reactors are being created. The neutron field inside such an apparatus should be well known, and the spectrum calculated and measured in several ways.

The neutron emitting sources with suitable examples and sketches are discuss here advanced. neutron emitting sources is needed to read as engineer student.

Reference neutron emitting sources

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