X-ray experiment

• Supervisor: Dr. Weifeng Su
• Location: Room 226-232， Henglong Physics Building

• Observe the image of an opaque object on the luminous screen, and to study how the image quality can be improved.
• Investigate the lattice spacing of an NaCl monocrystal.

Lab 9 X-ray experiment

1. Material science
2. Condensed matter physics

　　X-rays are a form of electromagnetic wave with the wavelengths ranging from 10-8 to 10-10 m. X-rays are created when fast-moving electrons are rapidly decelerated in matter. According to the laws of classical electrodynamics, this deceleration gives rise to electromagnetic radiation, which is mainly radiated perpendicular to the direction of acceleration for energies below 50 keV, i.e. for an X-ray tube, perpendicular to the direction of the electrons striking the anode. For historical reasons, this x-ray component is referred to as “bremsstrahlung” after the German word for the deceleration process by which it occurs. The bremsstrahlung has a continuous spectrum which extends to a certain maximum frequency λmax or a minimum wavelength λmin. If the energy of the electrons exceeds a critical value, the characteristic x-radiation is generated, which appears in the spectrum as individual lines in addition to the continuous bremsstrahlung spectrum. These lines are generated when high-energy electrons penetrate deep into the atomic shells of the anode material, and eject electrons from the innermost orbits by collision. The holes created in this process are filled by electrons from the outer orbits with emission of X-rays. The resulting X-radiation is characteristic of the anode material, and is roughly comparable to the optical line spectrum of a material in a gaseous, or vapor state.

• X-ray apparatus（Leybold Didactic）

• Observe the image of an opaque object on the luminous screen
• Measure the lattice spacing of the NaCl monocrystal using the characteristic X-ray radiation of molybdenum, and confirm Bragg’s law of reflection.

• How are X-rays generated? What is the difference between the bremsstrahlung and the characteristic X-radiation?
• Why is the image of the obejct no more visible on the screen when the high voltage is reduced to about 60% of the maximum? Why is the image still clear even when the current is reduced to about 0.1 mA?
• What is the “coupled scan mode”? Why do we use the “coupled mode” for the measurement of crystal lattice spacing?

x-ray_experiments.pdf