Quantum Physics Part 2 - X ray generation

Production of X-rays

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• Caption: Fig 1. X-ray tube
• Step 1: The cathode is heated to high temperature causing it to emit electrons.
• Step 2: The cathode and anode have an accelerating potential, causing the electrons to be accelerated through a potential difference of several thousand volts. acquiring very high kinetic energy.
• Step 3: The electrons strike a metal target embedded in the anode. The loss in kinetic energy in the electron is converted into high energy electromagnetic radiation known as X-rays.

The continuous X-ray spectrum

• high speed electron from cathode collides with a target atom nucleus in the anode
• As the high speed electron approaches the nucleus in the target atom, it deflects due to the attractive force between the nucleus and the electron.
• loss in KE converted into X-ray photon of energy hf.
• the closer the electron is to the nucleus when approaching it the larger the deflection thus higher the energy of the emitted photon
• will continue to collide with remaining KE -> more X-ray emissions
• min wavelength = maximum energy photon (happens during a SINGLE collision) -> only a single X-ray photon is emitted
• numerous electrons strike the target thus deflections occur at a range of proximities -> distribution of photon energies thus wide range of wavelengths -> continuous.

X-radiation Spectrum

• Q1. Explain the existence of a minimum wavelength.
• The minimum wavelength corresponds to maximum photon energy resulting from a collision in which an incident highly energetic electron stops abruptly in a single collision and all the KE of the electron is converted into a single X-ray photon,
• Q2. Explain the existence of characteristic Ka and Kb lines of high intensity.
• When high energy electrons strike the target atom nucleus from the anode, the electron ionises inner shell electrons of the target and electrons from higher energy shell de-excite to the inner shells, causing x-rays to be produced in a characteristic spectrum.
• Q3. Explain why the wavelength of Kb is shorter than ka line.
• when an electron de-excites from a higher energy shell to a lower shell a photon with energy equal to the energy difference between the shells is emitted Ka is due to transition from M-shell to K-shell while Kb is produced due to a transition of L-shell and K-shell hence energy if kb is higher than ka.