Neutron interactions

In nuclear fission reactors, neutrons cause the fission. When a neutron hits the fuel nucleus, say, U-235, the neutron is absorbed and an isotope, U-236 is formed. The U-236 is invariably in its excited state and has to de-excite. As said earlier, one possibility of de-excitation is fission, and the entire reactor programme depends on this possibility. Let us see the types of interactions that are possible, and how these are quantified. A nuclear reaction is generally symbolised as Target (projectile, ejectile) Residue.

It indicates that a particle (projectile, say neutron) hits a nucleus (Target, say U-235), and the interaction results in a nucleus (Residue, say U-236), and a particle (ejectile, say g).

Examples:

U-235 (n,g) U-236: Capture of a neutron and ejection of g.

U-235 (n,f) Fission Products: Neutron fission

Fe-56 (n,n) Fe-56: Scattering of a neutron.

Many types of reactions are possible when a neutron hits a nucleus. The probability of each type of reaction happening depends on the nucleus and is very sensitive to the energy of the neutron. In this regard, two neighbouring nuclides (say, of masses A and A+1) may differ drastically. Some of the prominent reactions are explained below:

Elastic scattering: The neutron and the nuclide collide and share a part of their kinetic energies. They rebound with speeds different from the original speeds, such that the ‘total kinetic energy’ before and after the collision remains the same. If the nucleus is stationary before collision, it will gain energy from the neutron and start moving, and the neutron gets slowed down due to loss of kinetic energy. However, the residual nucleus is not excited but is in its ground state.

This is the type of reaction that mostly helps fast neutrons to be slowed down to low energies in a reactor. Can the neutron gain energy? Yes. However, this is obviously possible when the nucleus has higher kinetic energy than the neutron.

Inelastic scattering: The neutron and the nuclide collide and rebound with speeds different from the original speeds, but the rebounding nuclide is left in an excited energy state. Hence the ‘total kinetic energy’ after the collision is less than that before the collision, and this difference accounts for the energy of excitation. If the nucleus is stationary before collision, the neutron must have kinetic energy exceeding the excitation energy, so that such a reaction is possible. Hence inelastic scattering is said to be a ‘threshold reaction’, the threshold being the minimum kinetic energy of the neutron required for the reaction to be possible. The excited nucleus subsequently de-excites by emitting g radiation. Heavy nuclides have lower thresholds than light nuclides. Though the probability of inelastic scattering is generally lower than elastic, the energy loss to the neutron is higher in an inelastic collision. Inelastic scattering in heavy nuclides degrades fission neutron energies heavily.

Capture: The neutron is absorbed by the target nucleus to form the next higher isotope (of mass A+1), in an excited state of energy. The new isotope de-excites by emitting g rays. The neutron is thus lost in this reaction. This is often known as ‘radiative capture’.

(n,x) reaction: In this reaction, ‘n’ represents neutron, and ‘x’ represents any particle like neutron, proton, deutron, a particle, etc. or a combination of such particles. It means that a neutron interaction with a nuclide results in emission of the particle(s) represented by ‘x’. (e.g.) if the emitted particle is a, it is called (n,a) reaction. If a neutron and a proton are emitted, then it is called (n,np) reaction. If 2 neutrons are emitted, it is then (n,2n) reaction. Such reactions are generally threshold reactions.

Fission: This is the most important reaction upon which the present day nuclear energy programme depends. Nuclear fission is a phenomenon in which a heavy nucleus, splits into two smaller nuclei, called the fission fragments, mostly of unequal masses, one often with nearly half the mass as the other, and rarely of equal masses. This reaction gives off a large amount of energy and emits two or more neutrons, and gamma rays. When a neutron hits a heavy nuclide like U-235, the neutron gets absorbed in the heavy nuclide that gets energetically agitated (or excited). If the new energy state of the heavy nuclide is sufficient for it to split, then it can split to cause fission.

The neutrons produced in fission are fast, with an average energy of 2 MeV.

It must be noted that the fission fragments themselves are in excited state, and they de-excite generally by b, g and neutron emissions. The neutron emitted during fission are called prompt neutrons, and those emitted by the fragments after a delay are called delayed neutrons. Similarly, prompt and delayed gammas are also emitted. About 80 % of the energy released in fission is carried away by the fission products (and the rest by the other particles), which in turn transfer the energy to the surroundings, making the energy recoverable. Some energy is carried away by particles known as neutrinos, which are chargeless and light, do not interact with any material, and hence their energy is not recoverable.

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Нейтроны являются одними из частиц, находящихся в ядрах атомов и могут быть испущены при их делении или при ядерных реакциях. Все свободные нейтроны начинают жизнь как быстрые нейтроны, с энергиями больше 0.10 МэВ. Быстрые нейтроны замедляются, и их энергия уменьшается при столкновениях с ядрами атомов поглотителя. Затем нейтроны переопределяются как промежуточные (диапазон энергий от 0.025 эВ до 0.10 МэВ) или тепловые (ниже 0.025 эВ).

Нейтроны взаимодействую с веществом тремя способами:

• Упругое рассеяние
• Неупругое рассеяние
• Поглощение нейтронов

Упругое и неупругое рассеяние являются процессами, в ходе которых быстрые и промежуточные нейтроны замедляются. Когда нейтроны достигают тепловых энергий, они поглощаются в процессе нейтронного захвата.