Transcript xray

X-ray tube and detection of X-rays
Lecture 5
Reminder: The rough schematics of an X-ray tube
photon flux
filament
e-
target
anode
cathod
electron kinetic energy= e x (Voltage difference)
Filament is the source of the electrons. Filament is heated to
high temperatures. Atoms are heated, leading to heating of
electrons. Some electrons can escape to vicinity of the wire.
Thermionic emission. (Edison effect).
Tungsten (W) crystallized wires. Why? Not the best electron
emitters. Need T>2200C. Pluses: Can make a thin wire which is
strong - melting T=3300C, small evaporation, can be repeatedly
reheated 25 - 2600 C, dimensional stability → long life expectancy.
17
Typical current: 100 mA - 6.25 x 10 electrons/second.
Electron repulse each other - tendency for the beam to spread.
Use of focusing cup. It confines electrons and focuses them on
a focal spot on anode.
Current 3-5 a, V=10 volt - heats the
filaments.
Anode. Two key considerations. Enhancing higher energy
component of the photon spectrum; Large flux. ---> Tungsten is
a good target. The main problem - heat.
Solution: (a) target angle
Molybdenum (copper)
base due to higher heat
capacity
(b) Rotating anode (1936)
Heel effect: The X-rays are
generated inside the anode,
with those at the anode side
stronger absorbed than at the
cathode side.
➔ Put the thicker side of the
object at the cathode side.
Usual shape of the focal point is
double banana.
X-tube cannot operate well if alternative voltage is applied to
anode and cathode. Electrons hitting cathodes when
cathode’s potential is positive would destroy it. Solution chop off the wrong part of the cycle.
Disadvantage 1/2 time no
X-rays. Can use three phase
power to reach nearly
constant V(t).
The spectrum of photons produced in the scattering of
electrons off the anode is too broad for many applications.
(a) Diagnostics - low energy photons are useless since will
not penetrate through the patient, but would increase the
dosage.
(b) Therapy - low energy photons will not penetrate the
tumor.
Solution - Filtering - system of thin absorbers which are
more transparent to higher energy photons.
See fig. in next page.
Take a 0.2 cm slab of Al . At 10 keV the flux will be
suppressed by a factor exp(-0.2/009) = 8. At 100 keV by exp(0.2/2) =0.9. A slab of lead should be much thinner.
Graph showing how flux of energy per energy interval for the
radiation generated by 200 keV electron beam bombarding a
thick W target (A) changes with filtration. B - layer of Al is
added; C in addition a layer of Cu is added; D - Sn is added.
In radiography one needs to choose the beam intensity
such that “optimal” fraction of the beam went through. (will
discuss math later). Also, the patient should receive as small
dose as possible.
Hence the intensity of the beam should be chosen
proportional to the thickness of the scanned part of the body.
use of the trough filter
for examination of the chest.
Use of the wedge filter