A-LevelPhysicsMedical physics9702/42 · Feb/Mar 2020 · Q119 Marks

11 Electrons are accelerated through a potential difference of 100kV. They are then incident on a metal target, they decelerate, and X-ray photons are emitted.

(a)Calculate the maximum possible frequency of the emitted X-ray photons. frequency = ................................................................ Hz [2][2]
(b)Explain why an aluminium filter may be placed in the X-ray beam when producing an X-ray image of a patient. [3][3]
(c)(i)A beam of these X-rays is incident on a person. Calculate the percentage of the intensity of the X-ray beam that has been absorbed after passing through 0.80cm of blood. percentage of intensity absorbed = ................................................................ % [2][2]
(c)(ii)In an X-ray image, white regions show greater absorption of X-rays than dark regions. State and explain the difference between the X-ray image of bone compared to that of muscle. [2][2]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

[1] eV = hf
[1] f = 1.60 × 10⁻¹⁹ × 100 000 / 6.63 × 10⁻³⁴ = 2.41 × 10¹⁹ Hz
[1] (aluminium filter) absorbs (most) low energy X-rays
[1] X-ray beam contains many wavelengths
[1] so low energy X-rays are not absorbed in the body
[1] low energy X-rays can cause harm but do not contribute to the image
[1] I / I₀ = e⁻^(μx)
[1] 17% absorbed
[1] bone is seen as lighter / muscle is seen as darker
[1] bone has a higher μ value so absorbs more / muscle has a lower μ value so transmits more

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