RAD 140 EXAM 2 (XRAY TUBE) QUESTIONS AND
ANSWERS
What are the 4 conditions necessary for xray production - Answers - 1. Source of
electrons
2. Target
3. High voltage
4. Vacuum
What are the 2 electrodes - Answers - 1. Anode
2. Cathode
Anode - Answers - positive
Cathode - Answers - negative
-Produces the electron cloud (via thermionic emission)--Conducts high voltage (from
cathode to anode)--Focuses the electron stream (focusing cup)
The __ is the negative electrode and consists __ and a __ - Answers - -cathode
-filament
- focusing cup
What material is used for the filament and why - Answers - - tungsten
- it has a high melting point and is difficult to vaporize
- smaller filament is used when need finer detail
- large filament Is used when marking a larger exposure, causes more heat produced
What does duel focus mean - Answers - diagnostic xray tubes. Have dual filaments
Thermionic emission - Answers - -current flowing through filament will produce heat
-enough heat will cause ionization of the outer shell tungsten electrons
- electrons form a cloud
(heating causes electrons to be emitted from tungsten atoms)
- the ma selected determines the amount of thermionic emission (thus the size of the
electron cloud and the tube current)
What is the purpose of the focusing cup? - Answers - - confines the thermionic cloud
towards the anode
-houses the filament
- composed of nickel
-keeps electrons close together
-prevents thermionic cloud from spreading out
, How does the focusing cup confine thermionic cloud towards the anode - Answers -
travel towards the target
Space charge - Answers - cloud of electrons surrounding the filament
What is the space charge effect - Answers - negative charges of electrons at the
filament oppose emission of additional electrons
- this is why xray tubes are limited to maximum Ma ranges between 1,000-1,200.
Saturation current - Answers - no additional electrons are available to be driven towards
the anode
-the entire space charge is in use
- as kvp increases more of the space charge is driven towards the anode until there are
no more electrons available
-size of space charge is based on ma selection
The positive electrode is called the __ and contains the electron __ - Answers - - anode
-target
The functions of the anode include - Answers - 1. Target surface
(disk has a surface where electrons interact, electrons will collide, actual spot on the
anode where the electron beam interacts)
2. High voltage (conducts voltage from cathode back to xray generator circuitry)
3. Thermal conductors
Give two advantages of rotating anodes - Answers - 1. Provide larger target area
2. Better heat dissipation- the faster anodes rotates more quickly, it will dissipate the
heat
Give 3 reasons why tungsten is chosen material for target - Answers - 1. High atomic
number - allow energy to be converted into xrays
2. High melting point
3. Heat conduction
Target - Answers - a point source of xrays photons
Focal spot - Answers - physical area struck by electrons
Actual focal spot - Answers - physical area struck by electrons
-the area of the focal spot that is projected out of the tube toward the object being
radiographed
Effective focal spot - Answers - - area projected out of the tube and towards the patient,
always smaller than actual focal spot
Small actual focal spot - Answers - pro: thin body parts, small effective focal spot
ANSWERS
What are the 4 conditions necessary for xray production - Answers - 1. Source of
electrons
2. Target
3. High voltage
4. Vacuum
What are the 2 electrodes - Answers - 1. Anode
2. Cathode
Anode - Answers - positive
Cathode - Answers - negative
-Produces the electron cloud (via thermionic emission)--Conducts high voltage (from
cathode to anode)--Focuses the electron stream (focusing cup)
The __ is the negative electrode and consists __ and a __ - Answers - -cathode
-filament
- focusing cup
What material is used for the filament and why - Answers - - tungsten
- it has a high melting point and is difficult to vaporize
- smaller filament is used when need finer detail
- large filament Is used when marking a larger exposure, causes more heat produced
What does duel focus mean - Answers - diagnostic xray tubes. Have dual filaments
Thermionic emission - Answers - -current flowing through filament will produce heat
-enough heat will cause ionization of the outer shell tungsten electrons
- electrons form a cloud
(heating causes electrons to be emitted from tungsten atoms)
- the ma selected determines the amount of thermionic emission (thus the size of the
electron cloud and the tube current)
What is the purpose of the focusing cup? - Answers - - confines the thermionic cloud
towards the anode
-houses the filament
- composed of nickel
-keeps electrons close together
-prevents thermionic cloud from spreading out
, How does the focusing cup confine thermionic cloud towards the anode - Answers -
travel towards the target
Space charge - Answers - cloud of electrons surrounding the filament
What is the space charge effect - Answers - negative charges of electrons at the
filament oppose emission of additional electrons
- this is why xray tubes are limited to maximum Ma ranges between 1,000-1,200.
Saturation current - Answers - no additional electrons are available to be driven towards
the anode
-the entire space charge is in use
- as kvp increases more of the space charge is driven towards the anode until there are
no more electrons available
-size of space charge is based on ma selection
The positive electrode is called the __ and contains the electron __ - Answers - - anode
-target
The functions of the anode include - Answers - 1. Target surface
(disk has a surface where electrons interact, electrons will collide, actual spot on the
anode where the electron beam interacts)
2. High voltage (conducts voltage from cathode back to xray generator circuitry)
3. Thermal conductors
Give two advantages of rotating anodes - Answers - 1. Provide larger target area
2. Better heat dissipation- the faster anodes rotates more quickly, it will dissipate the
heat
Give 3 reasons why tungsten is chosen material for target - Answers - 1. High atomic
number - allow energy to be converted into xrays
2. High melting point
3. Heat conduction
Target - Answers - a point source of xrays photons
Focal spot - Answers - physical area struck by electrons
Actual focal spot - Answers - physical area struck by electrons
-the area of the focal spot that is projected out of the tube toward the object being
radiographed
Effective focal spot - Answers - - area projected out of the tube and towards the patient,
always smaller than actual focal spot
Small actual focal spot - Answers - pro: thin body parts, small effective focal spot
