Question 9.2.Q1: Neutrons by virtue of being neutral particles can approach a......

(a) Six principal processes by which neutrons interact with nuclei of the absorber are:

(1) Elastic scattering.
(2) Inelastic scattering.
(3) Non-elastic scattering.
(4) Neutron capture.
(5) Spallation.
(6) Fission.

A brief description of each of these neutron interactions is as follows:

(1) Elastic scattering of neutron on absorber nucleus is the most important process for slowing down neutrons. The neutron collides with a nucleus of mass M that recoils with an angle 𝜙 with respect to the neutron initial direction of motion and the neutron is scattered by a scattering angle θ . Total energy and momentum are conserved in the elastic scattering interaction which means that kinetic energy lost by the neutron is equal to recoil energy of the target nucleus. The lighter is the target nucleus, the larger is the energy transfer from the neutron to the nucleus in an elastic scattering process; however, the target nucleus remains in the ground state.

(2) Inelastic scattering of neutron with absorber nucleus is similar to elastic scattering except that some of neutron’s kinetic energy is transferred to the nucleus not only to manifest itself as nuclear recoil kinetic energy but also to raise the nucleus from the ground state to an excited state. The nucleus de-excites by emitting highenergy γ rays and the neutron is scattered and moves on with kinetic energy that is lower than its incident energy. For inelastic scattering to occur, kinetic energy of the incident neutron must exceed the excitation energy of the nucleus. In contrast to elastic scattering, inelastic scattering is a threshold process and, when it occurs, neutron loses more energy in inelastic than in an elastic collision with absorber nucleus in order to account for the energy of the emitted γ ray. Therefore, only fast neutrons undergo inelastic scattering.

(3) Non-elastic scattering is in certain respect similar to inelastic scattering except that the secondary particle that is emitted is not a neutron. On the other hand, non-elastic scattering can also be considered neutron capture, except that the term neutron capture usually applies to capture of thermal neutron while non-elastic scatter typically deals with fast neutrons. An example of non-elastic scattering is

{ }_6^{12} \mathrm{C}(\mathrm{n}, \alpha){ }_4^9 \mathrm{Be}          (9.14)

(4) Neutron capture is a term used to describe a nuclear reaction in which a thermal neutron collides with a target nucleus leading to neutron absorption in the target nucleus, formation of a new nuclide of different atomic mass number and/or atomic number from those of the target nucleus, and emission of a proton [(n, p) reaction: neutron capture with particle emission] or gamma ray [(n,γ) reaction: neutron capture with emission of γ radiation] in the process. Neutron capture, even after an immediate emission of a particle or γ ray, often results in an unstable radionuclide which decays with its own half-life that can range from a fraction of a second to many years depending on the nature of the neutron capture product. The majority of artificial radionuclides produced during the past decades have been discovered by means of thermal neutron capture in stable samples placed into nuclear fission reactors. When neutron capture is used for production of radionuclides or for analysis of trace elements in material samples, it is usually referred to as neutron activation instead of neutron capture.

(5) Spallation is defined as fragmentation of a target into many smaller components as a result of impact or stress. Consequently, nuclear spallation is defined as disintegration of a target nucleus into many small residual components such as α particles and nucleons (protons and neutrons) upon bombardment with a suitable projectile such as light or heavy ion beams or neutrons. Nuclear spallation can also occur naturally in earth’s atmosphere as a result of exposure of nuclides to energetic cosmic rays such as protons.
An example of spallation is as follows

{ }_8^{16} \mathrm{O}+\mathrm{n} \rightarrow 3 \alpha+2 p+3 n           (9.15)

Most of the energy released in the spallation process is carried away by the heavier fragments that deposit their energy in the absorber locally. On the other hand, neutrons and de-excitation γ rays produced in spallation carry their energy to a remote location. Spallation can be used for production of radionuclides and for generation of intense neutron beams in spallation neutron generators.

(6) Fission is a particular type of neutron interaction produced by bombardment of certain very high atomic number nuclei (Z ≥ 92) by thermal or fast neutrons. The target nucleus fragments into two daughter nuclei of lighter mass and the fission process is accompanied with production of several fast neutrons. Nuclei that are capable of undergoing fission are called fissionable nuclei in general; nuclei that undergo fission with thermal neutrons are called fissile nuclei. Fission fragments combined with the nuclei that are subsequently formed through radioactive decay of fission fragments are called fission products.

(b) Neutron source is defined as a device that emits mono-energetic neutrons or a spectrum of neutrons. A wide variety of neutron sources are available ranging from small, encapsulated sources through particle accelerators and neutron generators to nuclear fission reactors.

(1) Nuclear fission reactor is the most abundant source of neutrons producing neutrons with an energy spectrum in the range from a few keV to over 10 MeV and average neutron energy of 2 MeV. Neutrons produced in research nuclear reactors are used for neutron scattering experiments, non-destructive testing, production of radionuclides for use in science, industry and medicine, and on a limited scale have been and still are also used in boron neutron capture therapy.

(2) Particle accelerators generate neutron beams by means of nuclear reactions with accelerated projectiles (protons or deuterons) striking a suitable target (typically of low atomic number) resulting in a product nucleus and a mono-energetic neutron beam. Best-known nuclear reactions for neutron production with neutron generators are exothermic reactions { }_1^3 \mathrm{H}(\mathrm{d}, \mathrm{n}){ }_2^4 \mathrm{He} with a Q value of 17.6 MeV and { }_1^2 \mathrm{H}(\mathrm{d}, \mathrm{n}){ }_2^3 \mathrm{He} with a Q value of 3.3 MeV. Cyclotron-produced fast neutrons rely on acceleration of protons to about 50 MeV and directing them onto a beryllium target in which fast neutrons are produced for use in radiotherapy. Most intense pulsed neutron beams for industrial research are produced by spallation neutron sources that are accelerator based.

(3) Radioactive neutron sources are produced by means of mixing an α emitter (such as radium-226 or americium-241) with a light metal (such as beryllium or boron) in powder form and encapsulating the mixture to make a neutron source generating neutrons through (α, n) reactions. The source intensity is governed by the half-life of the α emitter and the energy spectrum of emitted neutrons is continuous with maximum energy equal to the sum of the reaction Q value and kinetic energy of the α particle striking the nucleus.

(4) Photoneutron sources make use of photonuclear (γ, n) reactions and use mixtures of a mono-energetic γ emitter with beryllium metal. Photonuclear sources with mono-energetic γ emitters produce mono-energetic neutrons in contrast to (α, n) neutron sources that produce a spectrum of neutrons because of the random energy degradation of the α particles before they interact with the nucleus. The intensity of the photonuclear sources is governed by the half-life of the γ emitter component of the neutron source.

(5) Spontaneous fission neutron sources contain encapsulated high atomic number elements that undergo spontaneous fission and emit neutrons in the process. The best-known example of an intense spontaneous neutron fission source is californium-252 that was found useful in a wide range of specialized areas of science, industry, and medicine, such as the study of fission, neutron activation analysis, neutron radiography, well logging, nuclear reactor start up, and brachytherapy.