Chapter 27 Medical Physics Flashcards
Recap on anode vs cathode
Anodes attract anions thus must be positive, cathodes attract cations thus must be NEGATIVE
What was themrionic emission again and how did it work
It is the process by which a current is used to heat up the filament, giving electrons here Ke
When they had enough kE, they could escape the filament all together
We make the assumption that they escape the filament with 0KE
Once thermionic emission happened, how are electrons accelerated? What’s the energy they gain by the time these finished accelerating and what speed travel at once done?
In an electron gun
After they escape, there is ANOTHER CIRCUIT.
Here a voltsge supply is connected to two parallel metal plates to create a uniform electric field
- the Parallel plates involve the CATHODE which is the filament and an ANODE which is something else
- the cathode needs to be the filament, as this is negative, so it will repel the electrons (electric field needs to run in this way for electrons to be acclersted correct way)
2) the electrons will accelerate due to constant F being applied towards the Anode. If by they reach the whole anode, that means they have experienced the whole voltage and thus have gained energy = QVas V= WD / Q.
- this converted entirely to Ke so Ke = VQ and thus know the velocity
3) if there is a hole in the anode, then they all escape with a certain velocity !
Why must there be a vacuum
Must be a vacuum so electrons don’t collide with gas particles and transfer energy to the,
What’s the aim with the electrons in an X ray tube to produce s rays (hows it done)?
We want to make X day photons
- this is dine by the accelerated electrons to COLLIDE with tungsten , transfer there KE to excite electrons in tungsten and thus decelerate colliding electrons , and when the excited electrons unexcited, they produce X RAYS!
So now describe each component of an x day tube used to produce an x ray
- two circuits, with an EVACUATED TUBE
1) first circuit connects to a filament in the tube - a current is run across the filament which caused electrons to be emitted by thermionic emission
2) then there is a second circuit connecting the filament to a piece of TUNGSTEN METAL
- this makes the filament a cathode and the tungsten an anode
- a HIGH voltage is run across this circuit
- this acceleatyed the emitted electrons towards the tungsten anode
3) with the energy of the electrons, electrons in tungsten excite and then de excite later. This produces X RAYS
Finally how is the x ray tube allow for x rays to be emitted in certain direction?
The tungsten is aligned such that they are emitted through a window
How to find the MINIMUM wavelength that a colliding electron can produce?
Assumign that electron had NO ke when thermionically emitted
- that means energy gained = vQ = 1/2mv2
- assuming all this energy was transferred to electron and de excited back
Then eV = energy of x ray photon = h f (where f is max frequency )
Thus csn find min wavelength ntd
How to increase the frewuencies or decrease min wavelengths of the x rays?
Increasing the ACCELERATING VOLTAGE of 2nd circuit means more energy = higher f = lower wavelengths of x rays!
How to increase the INTENSITY ( number of x rays)
Important
As each electron produces 1 x ray, to incresse number of x rays, have to increase number of electrons
Thus to increase number of electrons, need to increase emitted
- do this by INCREASING THE CURRENT of the FIRST CIRCUIT, increased the amount of electrons emitted thus amount of x rays emitted too!
In reality , is all the kE from colliding electron transferred to electron in tungsten shell?
Hell nah!
99% is transferred to HEAT upon colliding, only 1 % goes snd acc transferred to excite the electron snd so x ray acc gonna have way less energy and thus way higher min wavelength
How to ensure heat felt by tungsten isn’t a lot as 99% of electron energy converted to heat here?
1) rotate the tungsten so heat spread out
2) use o Icl to cool the anode
Why does the x rays produced actually a CONTINOUS spectrum of different wavelengths ?
What happens when increase accelerating v / initsl current ?
The electrons colliding don’t necessarily collide with the OUTER SHELLS of tungsten
= some could go deeper in , and thus excite electron less ( as it was at higher energy level) and thus lower res length of x ray produced
- as this happens at all levels, a CONTINOUS spectrum of x rays are produced , like a normal distribution
2) increase current = intensity up so all shift up extend
- increase pd = increase max frewuency so reduced min wavelentgh so graph increases to the LEFT and it stretches !
DONT NEED TO KNOW
What are the k lines Prdouced on the CONTINOUS spec and why?
These are random lines on the curve
- this is due to the Fingerprint of the TARGET METAL
- when electron excites, electron of higher energy shell moves down to fill Valentine gap, releasing an x ray as a result
- this causes the spike
- it’s unique to each target metal because each target metal had its own divisions between energy levels so different x rays produced when this happens etc
Causing different spikes due to different wavelengths being produced at MUCH HIGHER INTENSITIES
How does x ray imagine bsdicslly work
It works on the fact that human body is SELECTIVLY ATTENUATING
- as a result, x rays will be able to pass through soft tissue (blood muscle fat ) but not hard tissue (bone)
- x rays travel to another screen behind which is digital. Where it picks up an x ray it can colour whatever, typically it covers it in BLACK, so thst is the absence of the x ray (bone) it becomes WHITE
