Topic 14: XRAY Detectors Flashcards
Four examples of detectors?
- Film / Film screen
- CR
- Image intensifiers
- Solid-state detectors
Why does an insulator not conduct electricity?
because the conduction and the valence band are far apart
whats normally the conduction band of a semiconductor?
1ev or 1.5 ev The value of the band gap determines the behaviour of the semiconductor
what is the most commonly used semiconductor?
silicon based
what is the band gap?
how much energy you need to give an electron to make it jump from the valence to the conduction band to conduct
a semiconductor is not a good conductor what do we need to do to it?
add impurities creates levels in bandgap.
if we want to make a semiconductor a detector what do we do?
we dope it, 2 ways
what are the two ways of doping a semiconductor
- N type
Silicon has 4 covalent bonds, we inset a pentavalent impurity (basically an atom with an extra electron) so a free electron is inserted. the electron is almost free
2) P-doping
Trivalent impurity - missing electron
Hole - energy just above valence level. This hole can move freely as if it were a positive electron. !!! Like Boron.
What is a PN junction?
you put a p-type and n-type silicon. you mask it bombard it with phosphor and then you mask the other side and bombard it with boron
what happens when you put the P AND N together?
some electrons will diffuse the p-type region and likewise holes in the n-type region. Therefore they will combine leaving a region free of charge carriers.
Again what is an n-type excessive in? and a p-type excessive in?
n-type - excess electrons
why does it stop half way to recombine?
the material you leave behind is not electrically neutral. the region free of charge carriers is not electrically neutral, it were the charge carriers themselves that kept it neutral! This is the principle of the detector hence you have an electric field
what happens when the xray enters the charged area in a semiconductor detector?
x ray - interacts. the passage of an ionising particle creates ionisation i.e. electrons and holes and these immeadiately start to drift inopposite directions because of the electric field! ( just remember that for x-rays a photoelectron is created first, and this then creates the e/hole pairs. ) You should notice a little pulse as the electrons and holes move and a field has been created.!!!
how do you improve the basic PN-Junction detector?
you inversely polarise your pn junction
What is inversely polarising your pn junction.
becasue you enlarge the region free of charge carriers i.e. the region in which ionising radiation can be detected and you also have a stronger electric field in your material so you are more effective in transporting those charge carriers.
how do you read out the charge generated through ionisation?
DC coupling and be integrating devices - all generated charge can flow through the wire or AC coupling - mostly used in photon counting devices, the interaction of the x-ray generates a PULSE which is transmitted through the built in capacitance.
what is the single photon counting readout scheme?
detector - > preamplifier and shaper - > threshold discriminator
film structure?
emulsion then base then a protective layer.
The emulsion contrains photosensitive grains.
How does the film processing work?
- exposure - photons liberate electrons in halide
- Latent image - electrons produce silver atoms
- Development - chemical process reduces grains with number of silver atoms above threshold
- Fixation - removes unreduced grains.
what is the problem with films?
inefficient - thin layer of silvergrains to stop the xray (2-3%)
What do we want from a digital detector?
Appropriate area coverage Uniformity Stability Linearity High dynamic range
what is uniformity?
each detector element (pixel) must provide the same response when exposed to the same amount of radiation
what is stability?
providing the same response to the same input over different times
linearity
direct proportionality between the radiation input and the detector response.
High dynamic range
dynamic range = ratio between the maximum signal achievable in the individual detector pixel and the amount of signal that is stored in the individual pixel only because of the noise.
four steps of the process of an image in a computer radiography: photostimulable phosphor plate
- Exposure: photons liberate electrons in phosphor
- Latent image: electrons trapped
- Development: laser beam scans plate, light emitted detected by PM tubes
- Erasure: uniform exposure releases any remaining electrons
what are the advantages of CR over film?
- digitised image
- Wide dynamic range
- Large latitude
- Reusable plates
Intensification in an intensifier due to ?
- Energy given to electrons (flux gain)
2. Minification gain.
Two types of conversion
direct conversion: amorphous selenium
Indirect conversion: scintillator and amorphous silicon
features of CCD?
Crystalline silicon, small and requires optical coupling
what is a ccd?
a sensor material that converts x rays into electrical signals directly (as opposed to a scintillator and photodiode which converts xrays to light then to electrical signals)
How do gas detectors work?
gas between electrodes
photon ionises gas
voltage causes charges to move
charge collected at electrodes
