Atoms

Neutron Scattering / Diffraction and Spallation

Spallation is defined as fragments ejected from a material by the impact of a projectile or through internal process within. Proton spallation is the production of neutron fragments via proton impact. It is then used for neutron scattering experiment similar to X-ray diffraction

		(in energy ~ 10 meV and size domain ~ 5 ) with the exception that neutrons can probe deeper because it is neutral so that there is no electrostatic interaction with the constituents of the sample. Figure 13a shows a simple example of the process. The neutrons beyond the 100 meV range are used for inducing fission chain reaction (Figure 13b), which belongs to another topic about Nuclear Bombs.
Figure 13a Neutron Scattering Experiment [view large image]	Figure 13b Fission Chain Reaction [view large image]	1 meV = 10-3 eV



• Neutron Characteristics -
  1. Slow (cold) neutrons have energies comparable to structural and magnetic excitations (1 - 50 meV).
  2. They have wavelengths comparable to inter-atomic spacings (5 - 0.1 ).
  3. Neutron carries no charge (for probing the nucleus directly), but possesses magnetic moment (for studying magnetic materials).
  4. It has high penetrating power through bulk samples (owing to the above-mentioned property).
  5. Neutrons scattered differently from element to element, it is much more sensitive to light element and its isotopes.
  6. It can detect both the structural and dynamical (time development) behavior of the material.
  7. Its short range nuclear interaction with matter makes theoretical formulation easier.
  8. Neutron exhibits both wave and particle properties. Essentially, the low energy variety shows up as wave in diffraction, while the particle

     	aspect is more obvious in high energy collision with heavy nucleus. The wave-particle duality is expressed by the de Broglie relation = h/p, where is the wavelength, p the moment, and h = 6.625x10-27 erg-sec is the Planck constant.



• Neutron Sources -
  • Spallation Neutron Source (SNS) - It is a facility specifically designed for the generation of neutron beam. The process runs in steps as shown in Figure 13c and in text below :
    1. It begins with negatively charged hydrogen ions (1 proton + 2 electrons) that are produced by an ion source.
    2. The ions are injected into a linear particle accelerator (linac), which accelerates them to high speed (~ 90% the speed of light).
    3. The ions pass through a foil, which strips off each ion's two electrons, converting it to a proton.
    4. The protons pass into a ring-shaped structure, a proton accumulator ring, where they cycle around at very high speeds and accumulate in bunches. Each one of these is released from the ring as a pulse, at a rate of 60 times per second (60 hertz).
    5. The high-energy proton pulses strike a target of liquid mercury, where the protons are conversed into fast neutrons. The yield is about 40 neutrons per spallating proton. A modern facility could produce about 10¹⁶ neutrons/sec-cm² (the flux or brightness).
    6. The fast neutrons are then slowed down in a moderator to about 10 meV and guided through beam lines to perform a wide variety of scattering experiments. (Figure 13a).
  • Photo neutron sources - A high energy photon (gamma ray) can eject a neutron from a nucleus if its energy is greater than the binding energy of the least bound neutron in the nucleus (threshold nuclear binding energy). Most nuclei have binding energies in excess of 6 MeV, which is above the energy of most gamma rays from fission. However, there are a few nuclei with sufficiently low binding energies to be of practical interest, they include ⁹Be (1.667 MeV), ²H (2.225 MeV), ¹³C (4.9MeV) and ⁶Li (5.67MeV). For example with a minimum 2.225 MeV gamma ray, ²H₁ + γ → ¹H₁ + n. The portion of photo-neutrons in a heavy-water moderated CANDU reactor is about 3x10^-4 of the total measured and even less in a common light-water reactor.

  		Nuclear spallation - Particle accelerator is used to produce a beam of neutrons by bombarding mercury, tantalum, lead or other heavy metal target. It can produce 20 to 30 neutrons after each impact. Nuclear Reactor - Nuclear fission in a nuclear reactor has the advantage that the beam can be pulsed with relative ease. For example, the fission of each U235 nucleus will produce 3 neutrons. Some of these can be used as neutron source for scattering experiments.
  Figure 13c Spallation Neutron Source (SNS)	Figure 13d List of Sources [view large image]	Figure 13d lists major neutron sources past and present, from reactor and spallation including the 2017 addition of CSNS in South China.

  
  
• Application - As indicated in Figure 13e, ultra cold neutrons are used to look for the neutron electriic dipole Moment. The experiment has something to do with fundamental physics about the internal charge distribution inside neutron. The absence of any moment (down to about

  		10-36 e-cm) has led to the proposal of a hypothetical particle by the name of axion; while the thermal neutrons are important in nuclear fission. Anyway, cold neutron scattering with energy in the 10 meV range and length scale about 1 (Figure 13f) has become a complementary method to x-ray diffraction in physics, chemistry, biology, life science, and material science.
  Figure 13e Applications [view large image]	Figure 13f Scattering by Nano-structures [view large image]	The followings lists some of the scattering applications :

		Time-Of-Flight Spectroscopy - In this method, the initial position and velocity of a pulse of neutrons is fixed, and the final energy is determined by time-of-flight between the sample and the detectors in Direct Geometry (DG) spectrometers (Figures 13g, 13a). Polarized Neutron Diffraction (PND) - Neutron polarization refers to the alignment of neutron spins in neutron beam. It is used to the determination
Figure 13g TOF [view large image]	Figure 13h PND [view large image]	of magnetic structures, to the separation of magnetic and nuclear contributions and to magnetization distributions mapping (Figure 13h).



• Diffractometer - This instrument is for analyzing the structure of a material from the scattering pattern produced by a beam of radiation or particles (such as X-rays or neutrons).



	Reflectometer - It also utilizes diffraction for measuring the structure of thin films. The technique provides valuable information over a wide variety of scientific and technological applications including chemical aggregation, polymer and surfactant adsorption, structure of thin film magnetic systems, biological membranes (Figure 13i).
Figure 13i Reflectometer [view large image]




Radiation Hardening Test - This is the process to make electronic components and systems resistant to damage or malfunctions caused by ionizing radiation (particle radiation and high-energy electromagnetic radiation), such as those encountered in outer space and high-altitude flight, around nuclear reactors and particle accelerators, or during nuclear accidents or nuclear warfare. Neutrons and protons are used to test displacement damage, and single event effects (SEE).

Figure 13j Specific Applications, CNSS [view large image]

See some specific applications in Figure 13j as envisioned by CNSS.

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