physics 2 Flashcards
clinical application X-ray tube examples?
diagnostic radiography
fluroroscopy
interventional radiology
Why is a metal envelope better than glass?
- more rugged construction
- greater focal spot stability
- less fragile
- anode and cathode assemblies attached to metal are more studier and stable assembly
- more localised cooling of components
how does the amount of a x-ray from a tube change?
- it depends on the current applied to filaments
how are the electrons emitted?
from filament surface
from thermionic emission of electrons into the high voltage field
the hotter the ? the greater number ?
The hotter the filament, the greater number of electrons emitted.
how to get thermionic emission?
the filaments are at a temperature where metallic atoms are emitted from the filament surface
how to get thermionic emission in a glass tube?
metallic atoms are deposited on the glass envelope between the cathode and anode components.
explain tungsten deposition in glass tube x-rays
- if deposit heavy, it will absorb and conduct electrons across surface of glass
- jeopardises high voltage stability
- eventually breakdown
explain tungsten deposition in metal x-rays
- metallic atoms attach to metal surface
- does NOT jeopardise high voltage stability
explain the stationary anode
- made of massive cylindrical block of copper with tungsten target inserted
good things about stationary anode?
- good thermal conductivity
- size allows good thermal capacity
- conducts heat away from target minimising heat damage
benefits of tungsten in anode?
- efficient x-ray producer
- mechanical strength
- thermal conductivity
- melts at 3387c
- low vapour pressure
- ductile
explain and level rotating anode components!
yeah u got this girl :) don’t give up
what is the rotating focal point?
Rotation of the disc carries heated spot out of the path of oncoming electrons
Focal track can be more than 100 times larger than stationary target
This enable higher intensity X-ray beams due to increase thermal capacity.
what is the rotor?
made of copper: high electrical conductivity, allows induction of strong currents
drives the anode disc
has a blackened surface: due to heat dissipation
what are bearings? what do they do?
they are steel balls
lubricated
made from:
- lead
- silver
- liquid gallium
they maintain vacuums
- no oil or grease
why use a hooded anode?
- some of the electrons emitted by cathode bounce off the target
- since there is no where else for them to got the electrons head back to target
- then more likely to strike body and stem of anode rather than focal spot
hood reduces this problem
what is X-ray tube filament?
- electron source
- temp controlled by tube current (mA)
made of tungsten: due to high melting point
- 3387c
- low work function
- low vapour pressure
ductile (can be made into fine wire and stays even when hot)
coiled
(inc surface for electron prod)
(controllable electron emitter)
explain the production of x-rays
The source of the electrons is the cathode, the negative electrode.
Electrons are stopped or decelerated by the anode, the positive electrode.
Electrons move between the cathode and the anode as there is a difference in potential between the two.
X-ray tube cathode:
- neg charged
keep stream of electrons emitted from filament from spreading apart - filament
- focusing cup
made of nickel, surrounds filament
open end, permit free flow of electrons across tube from cathode to anode
tube housing - stationary unit parts
OIL
BELLOWS
THERMAL SWITCH
HOUSING
WINDOW
oil
OIL
convection: heat from glass envelope convected by oil
electrical insulation: between high voltage connections
bellows
BELLOWS:
- alleviate internal pressure on tube insert when oil expands and contracts with temp
thermal switch
THERMAL SWITCH:
- activated to prevent more exposure to damage tube
housing
HOUSING
steel and lead: provide support and protection for glass insert, prevent oil leakage and protection from stray radiation emission
window
WINDOW
- allows primary radiation to emerge with minimal attenuation
Tube housing - rotating unit provides:
- radiation protection
- electrical protection
- thermal protection
- physical protection
Why do we not want the anode to heat?
we do not want the anode stem to heat and damage the rotor or bearings
where is heat radiated from?
Heat is radiated from the anode across the vacuum to the glass envelope where it is dissipated by conduction to the oil.
tube cooling in target & disc?
conduction
radiation
tube cooling in glass envelope?
conduction
tube cooling in oil?
convection
tube cooling in tube housing
conduction
tube cooling in atmosphere
convection
tube cooling in assisted cooling?
- fans
- oil circulation
- heat exchangers
what happens with an ageing tube?
- the image, tube is slightly brown and tanned. referred to as sun tanning - leads to tube arcing and failure
- metal deposits from vaporised tungsten begin to accumulate on inside of glass envelope
- gradually reduces x-ray output
define thermionic emission?
- Thermionic emission is the emission (escape) of electrons from the surface of a hot body as a result of their thermal energy
define thermionic emission in X-ray tube
When the tungsten filament (the body) gains enough heat (therm)
The outer shell electrons (ions) of the filament atoms are boiled off (emitted) from the filament (now the hot body).
what is dynamic equilibrium look like?
fish leaping out of water and returning
explain dynamic equilibrium
- electrons can move freely within the wire but they are unable to leave the material surface
why?: due to the other atoms pulling them back.
explain thermionic emission diagram:
all the little people are able to leave, no one is coming back
define thermionic emission
as temp of body inc, inc energy of electrons until posses enough energy to escape binding atomic forces
Their escape from the surface of the material is thermionic emission.
explain thermionic emission of tungsten
just goes up, no electrons emitted until 2000 degrees reached
after this, v small inc in temp makes LARGE INC in EMISSION
what is tube current?
The hotter the metal, the more electrons will escape and travel to the positive plate.
Inc the voltage on the plate will inc the flow and make electrons move faster
thermionic emission - the theory
- that atoms and electrons vibrate
- degree dependent on energy
- energy dependent on temperature
define space charge
- at 2000c electrons are emitted which then form a diffuse cloud
what factors control rate of thermionic emission?
- material
- temp
- filament surface area
graph 1:
- illustrates typical electron emission from tungsten cathode in response to heat being generated with it
- little thermionic emission below 1500 degrees, then inc until filament temp is 2500
- small inc in temp beyond 2500, large inc in electron emission
graph 2:
Space charge limited – some of the electrons are drawn to the anode – increasing voltage increases tube current.
Saturation – All electrons emitted are drawn to the anode
At this point it is possible to increase the tube current only by increasing the filament temperature.
graph 3
- effect on tube current by inc filament temp
increase in kVp at this point will not increase e- yield.
increasing the temperature control factor of the filament will result in an increase in e- emission
graph 4
just look .. make sense of it
