X-ray tube and detection of X-rays Lecture 5

Reminder: The rough schematics of an X-ray tube filament cathod target anode photon flux e-e- electron kinetic energy= e (Voltage difference) x

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 C, dimensional stability → long life expectancy.

Typical current: 100 mA x 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. Usual shape of the focal point is double banana. ➔ Put the thicker side of the object at the cathode side.

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