Final answer:
The intensity of the x-ray beam is greatest along the cathode end due to the anode heel effect, where the geometry causes an uneven absorption of x-rays. Characteristic x-rays are highly energetic emissions from filling inner-shell vacancies in heavy element anodes like tungsten, requiring high accelerating voltage. These energies increase significantly with the atomic number of the element used for the anode material.
Step-by-step explanation:
The student asked why the intensity of the x-ray beam is greatest along the cathode end of the beam, referring to the phenomenon known as the anode heel effect. In an x-ray tube, x-rays are generated when energetic electrons strike the anode material, typically made of heavy elements such as tungsten due to its high melting point. These electrons interact with the anode's atoms, occasionally leading to the emission of EM radiation, including both bremsstrahlung and characteristic x-rays.
Characteristic x-rays are particularly energetic because they result from the filling of inner-shell vacancies, where electrons are tightly bound. As the atomic number (Z) increases, the energy of these emissions also increases, approximately as Z². This explains why heavier elements emit more energetic characteristic x-rays. For example, tungsten requires a significant accelerating voltage, at least 72.5 kV, to create these inner-shell vacancies.
The anode heel effect implies that the x-ray intensity is not uniform across the beam; it's stronger on the cathode side because the angle of the anode causes a greater absorption of x-rays on the anode side, leading to a reduction in intensity towards the anode end. This effect is taken into account when positioning patients in medical imaging to optimize image quality.