The X-ray Tube (Cathode assembly)

What supplies the electrons?

the cathode

What attracts the electrons?

the anode

How are xrays emitted at the focal spot? hint: in all directions

isotropically

What is unusable that occurs in an x-ray tube?

the x-rays absorbed by tube shielding

List the 3 functions of the cathode assembly

1. to produce a thermionic cloud

2. conduct the high voltage between cathode and anode

3. focus the electron stream

What are the parts of the cathode?

• filament (s)

• focusing cup

• associated wiring

What is the filament made of?

a small, thin coil of thoriated tungsten

How thick is the filament?

0.1-0.2 mm thick

What is the atomic number of Thorium (Th)?

z=90

Why is tungsten used for the filament?

because it has a high melting point (3370C) and it is hard to vaporize

What is the melting point of thoriated tungsten?

3420 C

List the two other types used for the filament with the atomic number and melting point

• Rhenium (Re) z=75, melting point = 3170C

• Molybdenum (Mo) z=42, melting point = 2620C

What does the filament provide to cause thermionic emission (electron cloud or space charge)?

enough resistance

What has a great effect on recorded detail (focal spot size)?

the length and width of the filament

(T/F) Most modern tubes have two filaments “dual focus tube”.

true

What does a “dual focus tube” refer to?

large and small focal spots

What is the length of the small focal spot?

0.3 to 1 mm

What is the length of the large focal spot?

1 to 3 mm

What is the shallow depression in which the filament sits?

the focusing cup

What is the charge of the focusing cup?

negative

What is it called when the focusing cup is not working properly, and the efficiency of the tube decreases?

focal bloom

What will too many electrons around the filament cause?

space charge effect