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you can do this! > physics 2 > Flashcards

physics 2 Flashcards

1
Q

clinical application X-ray tube examples?

A

diagnostic radiography
fluroroscopy
interventional radiology

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2
Q

Why is a metal envelope better than glass?

A
  • 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
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3
Q

how does the amount of a x-ray from a tube change?

A
  • it depends on the current applied to filaments
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4
Q

how are the electrons emitted?

A

from filament surface
from thermionic emission of electrons into the high voltage field

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5
Q

the hotter the ? the greater number ?

A

The hotter the filament, the greater number of electrons emitted.

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6
Q

how to get thermionic emission?

A

the filaments are at a temperature where metallic atoms are emitted from the filament surface

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7
Q

how to get thermionic emission in a glass tube?

A

metallic atoms are deposited on the glass envelope between the cathode and anode components.

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8
Q

explain tungsten deposition in glass tube x-rays

A
  • if deposit heavy, it will absorb and conduct electrons across surface of glass
  • jeopardises high voltage stability
  • eventually breakdown
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9
Q

explain tungsten deposition in metal x-rays

A
  • metallic atoms attach to metal surface
  • does NOT jeopardise high voltage stability
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10
Q

explain the stationary anode

A
  • made of massive cylindrical block of copper with tungsten target inserted
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11
Q

good things about stationary anode?

A
  • good thermal conductivity
  • size allows good thermal capacity
  • conducts heat away from target minimising heat damage
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12
Q

benefits of tungsten in anode?

A
  • efficient x-ray producer
  • mechanical strength
  • thermal conductivity
  • melts at 3387c
  • low vapour pressure
  • ductile
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13
Q

explain and level rotating anode components!

A

yeah u got this girl :) don’t give up

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14
Q

what is the rotating focal point?

A

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.

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15
Q

what is the rotor?

A

made of copper: high electrical conductivity, allows induction of strong currents

drives the anode disc

has a blackened surface: due to heat dissipation

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16
Q

what are bearings? what do they do?

A

they are steel balls

lubricated
made from:
- lead
- silver
- liquid gallium

they maintain vacuums
- no oil or grease

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17
Q

why use a hooded anode?

A
  • 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

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18
Q

what is X-ray tube filament?

A
  • 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)

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19
Q

explain the production of x-rays

A

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.

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20
Q

X-ray tube cathode:

A
  • 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
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21
Q

tube housing - stationary unit parts

A

OIL

BELLOWS

THERMAL SWITCH

HOUSING

WINDOW

22
Q

oil

A

OIL
convection: heat from glass envelope convected by oil
electrical insulation: between high voltage connections

23
Q

bellows

A

BELLOWS:
- alleviate internal pressure on tube insert when oil expands and contracts with temp

24
Q

thermal switch

A

THERMAL SWITCH:
- activated to prevent more exposure to damage tube

25
Q

housing

A

HOUSING
steel and lead: provide support and protection for glass insert, prevent oil leakage and protection from stray radiation emission

26
Q

window

A

WINDOW
- allows primary radiation to emerge with minimal attenuation

27
Q

Tube housing - rotating unit provides:

A
  • radiation protection
  • electrical protection
  • thermal protection
  • physical protection
28
Q

Why do we not want the anode to heat?

A

we do not want the anode stem to heat and damage the rotor or bearings

29
Q

where is heat radiated from?

A

Heat is radiated from the anode across the vacuum to the glass envelope where it is dissipated by conduction to the oil.

30
Q

tube cooling in target & disc?

A

conduction
radiation

31
Q

tube cooling in glass envelope?

A

conduction

32
Q

tube cooling in oil?

A

convection

33
Q

tube cooling in tube housing

A

conduction

34
Q

tube cooling in atmosphere

A

convection

35
Q

tube cooling in assisted cooling?

A
  • fans
  • oil circulation
  • heat exchangers
36
Q

what happens with an ageing tube?

A
  • 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
37
Q

define thermionic emission?

A
  • Thermionic emission is the emission (escape) of electrons from the surface of a hot body as a result of their thermal energy
38
Q

define thermionic emission in X-ray tube

A

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).

39
Q

what is dynamic equilibrium look like?

A

fish leaping out of water and returning

40
Q

explain dynamic equilibrium

A
  • 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.

41
Q

explain thermionic emission diagram:

A

all the little people are able to leave, no one is coming back

42
Q

define thermionic emission

A

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.

43
Q

explain thermionic emission of tungsten

A

just goes up, no electrons emitted until 2000 degrees reached

after this, v small inc in temp makes LARGE INC in EMISSION

44
Q

what is tube current?

A

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

45
Q

thermionic emission - the theory

A
  • that atoms and electrons vibrate
  • degree dependent on energy
  • energy dependent on temperature
46
Q

define space charge

A
  • at 2000c electrons are emitted which then form a diffuse cloud
47
Q

what factors control rate of thermionic emission?

A
  • material
  • temp
  • filament surface area
48
Q

graph 1:

A
  • 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
49
Q

graph 2:

A

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.

50
Q

graph 3

A
  • 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

51
Q

graph 4

A

just look .. make sense of it

you can do this! (5 decks)

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