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'A' Level Medical Option Questions - X-Rays

Q9.

(a) Explain what is meant by the half-value thickness of lead for X-rays.

The half-value thickness is the thickness of lead needed to reduce intensity of the X-rays of a specific energy by half

If you look at how the half-value thickness is quoted in part b, it gives you a clue that the value is quoted for particular X-Ray energies.

(b) Calculate the linear attenuation coefficient of lead for 90 keV X-ray photons. half value thickness of lead for 90 keV X-ray photons = 12mm.

= ln2/(12 x 10-3)

= 58 m-1

(c) Calculate the thickness of lead needed to reduce the intensity of a beam of 90 keV X-ray photons to 5.0% of the intensity incident on the lead.

I =Ioe–x

0.05 = e–57.8x

x = 0.052 m (or 52 mm)

(Total 6 marks)