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Abstract
Pyroelectric crystals are anisotropic/noncentrosymmetric crystals that are polarized at equilibrium conditions. The net dipole moment per unit volume of the crystal is not zero. The magnitude of polarization without the application of an applied field or temperature gradient is known as the spontaneous polarization Ps. Spontaneous polarization creates charges on the surface of the crystal. However, these surface charges are compensated by extrinsic effects, such as surface flashover and the attraction of stray charges to the surface of the crystal in ambient air, so the surface is electrically neutral. Pyroelectric crystals are cut such that the two faces are normal to the dipole moments (polarization) of the unit cells, effectively creating -Z and +Z faces. During thermal equilibrium, the -Z face has negative spontaneous polarization, and the +Z face has positive spontaneous polarization. The change in polarization due to a temperature gradient causes a charge to build on the crystal surface in vacuum that gives rise to an electrostatic potential. This charge creates an electric field capable of accelerating charged particles to energies approximately hundreds of keV. Fields of around 100 kV/cm can be achieved with temperature changes in tens of degrees if a lithium tantalate crystal with spontaneous polarization of 50 μC and a dielectric constant of 45 is heated or cooled in vacuum, thereby suppressing the ambient screening of the bound polarization charges. Furthermore, barrier tunneling can take place as the electric field strengthens, causing electron emission from the -Z surface during cooling. Upon striking the surrounding environment, these ions and electrons create Bremsstrahlung x rays. Research by various investigators has shown that the above effect is strong enough to create compact sources of x rays, electrons, ions, and neutrons via deuterium-deuterium (D-D) fusion. Recently, Kukhtarev et al. demonstrated the production of electron and x-ray beams via pyroelectric and photogalvanic effects. The electric field of the crystal also creates a beam of accelerated ions in a direction opposite to the electron beam, due to emitted electrons creating ion pairs in the fill gas. An opportunity to create ultraportable x ray and electron sources is offered by the pyroelectric crystals' ability to be heated using only a few watts of power. The production of radiation must be controlled for intensity, energy, and production period in order to develop a compact radiation source.
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CHAPTER 6
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