Absorption of X-rays (A-level only) Flashcards – AQA A-level Physics (7408)

Why does the intensity of an X-ray beam decrease as it passes through a material?

Photons are removed from the beam.

For a single-energy X-ray beam passing through a homogeneous material, what is the transmitted intensity $I$ after thickness $x$?

$I = I_0\mathrm{e}^{-\mu x}$, where $I_0$ is the incident intensity and $\mu$ is the linear attenuation coefficient.

Why is X-ray attenuation exponential rather than linear?

A constant fraction of the remaining intensity is removed per unit thickness.

What two factors determine the linear attenuation coefficient for X-rays?

The material and the photon energy.

What are the units of the linear attenuation coefficient?

Inverse length.

How is the mass attenuation coefficient $\mu_m$ related to the linear attenuation coefficient $\mu$ and density $\rho$?

$\mu_m = \frac{\mu}{\rho}$

What quantity of material does the mass attenuation coefficient relate attenuation to?

The mass per unit area traversed.

What is the half-value thickness for X-ray attenuation?

The thickness that reduces transmitted intensity to half its incident value.

For exponential attenuation, what is the half-value thickness $x_{1/2}$ in terms of the linear attenuation coefficient $\mu$?

$x_{1/2} = \frac{\ln 2}{\mu}$

What does each successive half-value thickness do to the remaining X-ray intensity?

It halves the remaining intensity.

Why do different tissues attenuate X-rays by different amounts?

They have different attenuation coefficients.

What three factors affect X-ray attenuation by tissue?

Tissue composition, tissue density, and X-ray photon energy.

For a given X-ray beam, how does stronger tissue attenuation affect transmitted intensity?

Stronger attenuation results in less transmitted intensity.

For a given X-ray beam, how does tissue thickness affect transmitted intensity?

Thicker tissue transmits less intensity.

What produces contrast in an X-ray image?

Differences in transmitted X-ray intensity.

For comparable thicknesses, how does X-ray transmission through bone compare with transmission through soft tissue?

Bone attenuates X-rays more strongly, so fewer X-rays reach the detector through bone.