Page 524 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
512
small lamp
large lamp
object screen
object screen
penumbra (partial shadow)
umbra
(total shadow)
penumbra
anode
uncollimated beam lead slit
partly collimated beam
lead slit
collimated beam
Figure 32.12 Collimating an X-ray beam. The first set of slits produces a partly collimated beam but, due to the finite size of the anode target, there is still some spreading of the beam. The second set of slits reduces this spread further, making the final beam almost parallel-sided.
aluminium) which is transparent to X-rays. The plate is placed just above the screen, and the lead absorbs the scattered X-rays.
Improving contrast
Good contrast is said to be achieved if there is a clear difference in the blackening of the photographic film as the X-ray passes through different types of tissue. The contrast is largely determined by the hardness of the X-rays. Bone is a good absorber of the radiation. If the doctor is diagnosing a break in a bone, he or she will use
penumbra
umbra penumbra
Figure 32.9 The small lamp casts a smaller penumbra and this improves the sharpness of the shadow.
electron beam anode target electron beamanode target
narrow X-ray beam
wide
X-ray
a beamb
Figure 32.10 A wide anode target results in a wide X-ray beam, giving fuzzy edges to the shadow image.
adjustable, overlapping lead plates
aperture
Figure 32.11 The smaller the aperture, the narrower the X-ray beam.
Inevitably some X-rays are scattered as they pass through the body. If these reach the detector they cause fogging and this reduces the sharpness of the image. Scattered X-rays approach the detector screen at an angle, and so an anti-scatter screen (Figure 32.13) can be used to absorb them. This consists of a series of plates made of a material (such as lead) which is opaque to X-rays, separated by plates made of a material (such as
collimated beam of X-rays
body
anti-scatter screen
scattered X-rays lead
detector aluminium
Figure 32.13 An anti-scatter screen absorbs X-rays which arrive at an angle to the main beam.