The Tube

Question 1

To produce x-rays, must have a ____ of electrons

• means to ____ the electrons
• means to ____ the electrons (target)
• _____

Answer 1

Source, accelerate, decelerate, and a vacuum

Question 2

Cathode assembly

Answer 2

• negative side of the tube
• provides the source of electrons required for x-ray production
• made of two main parts: filament and focusing cup
• function: to produce or emit electrons

Question 3

Filament

Answer 3

• most tubes have 2 (dual focus tubes)
• emits electrons when heated
• typically made of thoriated tungsten (increases thermionic emission, extends filament life, high melting point)
• set inside focusing cup
• determines focal spot size

Question 4

Functions of filament

Answer 4

• provides resistance to the flow of electrons so heat is produced (causes thermionic emission to occur)
• this happens as the exposure switch is pushed halfway down and the rotor is activated

Question 5

Focusing cup

Answer 5

• what the filament sits in
• negatively charged to force the electrons together into a cloud as they come off the filament (electromagnetic repulsion)
• size, shape, and charge of the focusing cup and how the filament is situated will affect how well the electrons are “focused” onto the target

Question 6

Space charge effect

Answer 6

• focusing cup shapes electrons from filament into a cloud (called space charge)
• self limiting (when it reaches the size set by current used it is difficult for additional electrons to be emitted)

Question 7

Focal spots

Answer 7

• large filament = large focal spot
• small filament = small focal spot
• selection made with the mA station selector on the console

Question 8

Anode assembly

Answer 8

• positive end of the tube
• functions: ELECTRICAL CONDUCTOR (some electrons interact with the target to produce x-rays, the rest continue as current flow through the x-ray circuit), MECHANICAL SUPPORT FOR TARGET (target required for electrons to interact with and produce x-rays), THERMAL DISSIPATER (tremendous amount of heat generated during x-ray production)

Question 9

Anode design

Answer 9

• two designs: stationary and rotating
• STATIONARY: target does not move, ex dental units, disadvantage is electrons always hitting same small target area, heat builds up rapidly, limited to small exposures
• ROTATING: (use in our units), always electron beam to interact with a greater target area

Question 10

Induction motor

Answer 10

• anode rotates using an electromagnetic induction motor

- two major parts are: stator (OUTSIDE the glass/metal envelope) and rotor (INSIDE the envelope)

Question 11

Induction motor-how it works

Answer 11

• staters are energized in opposing pairs, and induce a current in the rotor with an associated magnetic field
• induced field opposed that of the stator pair, and the rotor turns to correct orientation
• as the two fields align, the next pair of stators is energized, and again, another magnetic field is induced, causing the rotor to turn again
• this continues with energizing each pair of stators in sequence so the rotor will continuously turn in to orient with the continually changing external fields
• at the end of the exposure, the stators will reverse direction to slow down the rotor

Question 12

Rotating anode

Answer 12

• head is spread out over a larger surface area
• electrons strike a small part of the anode surface area, and the area changes due to the rotation (becomes a focal “track”, heat build up over the focal track circumference vs one spot)
• large increase of the heat load capacity and exposure techniques that can be used

Question 13

In what situation would a dual filament unit have filaments “end to end” instead of “side by side”?

Answer 13

When in a biangle X-ray tube

Question 14

On the anode, where are the x-rays produced?

Answer 14

The target