detectors Flashcards
band structure/band gap
for diff materials
conduction band: where electrons are when excited
valence band: where all electrons are (ground state)
insulator: gap E>5eV
semiconductor: gap E~1eV
metal: no gap
N-type semiconductor
negative
pentavalent impurity
eg phosphor has 5 electrons
silicon has 4
extra electron
easily detached
electron almost free, energy just below conduction level
creates additional energy level just below conduction band
P type semiconductor
positive
trivalent impurity
missing electron
eg. boron has 3 electrons
hole
energy just above valence level
PN-junction
n type has excess electrons
p type has excess holes
join together: electrons diffuse to p type region and vice versa
they will recombine
leaving a region free of charge carriers
but that section is not electrically neutral
phosphor nuclei without electron is positive
boron nuclei with extra electron is negative
which creates an electric field
ionising particle in PN junction
the passage of an ionising particle creates ionisation, electrons and holes, which immediately drifts in opposite directions due to electric field
how to improve PN junction
currently small electric field and small region free of charge carriers
inversely polarise the PN junction
positive end pulls electrons that way
vice versa
enlarges the region free of charge carriers
ie the region in which ionising radiation can be detected
full depletion is reached
this is the photodiode
DC coupling
direct:
all generated charge can flow through the bonding wire, all charge in one go
in integrating devices
metallic bonding wire touches PN junction
DC coupling
AC coupling
indirect:
capacitor added
the interaction of the individual x-ray generates a pulse which is transmitted through the built in capacitance
current cant pass but pulse can
AC coupling
in counting devices
single-photon counting readout scheme
circuit:
input, preamplifier, shaper, buffer, high-pass filter, discriminator, threshold, 16 bit counter and shift register
discriminator can find threshold
set threshold above intensity of individual signal
to separate noise from signal
leaving only poisson fluctuations
film structure
emulsion contains grains
emulsion bottom, then base, then protective layer
film process
exposure: photons liberate electrons in silver halide
latent image: electrons produce silver atoms
development: chemical process reduces grains with number of silver atoms above threshold
fixation: removes unreduced grains (makes image permanent)
HD curve
film inefficient
H-D curve
optical density over log x-ray exposure
flat then straight line gradient then flat
under exposure then latitude (ideal section) then over exposure
film + fluorescent screen
mu is higher
thicken than emulsion
higher stopping power
reduced spatial resolution
structure of screen
phosphors
film adv and disadv
adv:
practical
easy to manufacture and use
reliable
cost effective
spatial resolution
disadv:
intrinsic background due to film granularity
low dynamic range
low efficiency
analog info unless digitisation
no image processing
working with chemicals and disposal
no storage, transfer of info
no real-time
digital detector qualities priorities
appropriate area coverage
uniformity
stability
linearity
high dynamic range
BaFBr:Eu2+ process
screen, no film
materials in band gap create energy levels
electrons rise to conduction band and falls to F centre for long enough to be read (trapped)
shine red laser light to send electron back up to conduction band
electron falls through cascade mechanism down to valence band and emits blue light
blue light amount is proportional to electrons
computed radiography
photostimulable phosphor plate
PSP
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
stimulation and emission spectra
relative intensity and energy graph
if low energy
only part is trapped, other emits prompt light (not captured)
if high energy:
too much falls back into traps, instead of producing luminescence
CR adv over film
digitised image, post processing
wide dynamic range
large latitude
reusable plates
image intensifier
The x-ray image intensifier converts the transmitted x rays into a brightened, visible light image.
the input phosphor converts the x-ray photons to light photons, which are then converted to electrons within the photocathode.
The electrons are accelerated and focused by a series of electrodes striking the output phosphor, which converts the accelerated electrons into light photons that may be captured by various imaging devices.
Through this process, several thousand light photons are produced for each x-ray photon reaching the input phosphor
intensification depends on
energy given to electrons (flux gain)
minification gain
scintillator
cesium iodide
grown in columns
-tight packing
-light spread reduced
flat panel detectors
direct conversion
-amorphous selenium
indirect conversion:
-scintillator
amorphous silicon
amorphous selenium
The incident X-rays make the selenium layer generate electron-hole pairs. Under the action of an externally biased electric field, the electrons and holes move in opposite directions to form a current, and the current forms a stored charge in the thin film transistor. The amount of stored charge of each transistor corresponds to the dose of incident X-rays, and the charge amount of each point can be known through the readout circuit, and then the X-ray dose of each point can be known.
Since amorphous selenium does not produce visible light and has no influence of scattered rays, a relatively high spatial resolution can be obtained.
amorphous silicon
First, the scintillator converts X-ray energy into visible light
photodiode: light is converted to electrical signals
electric circuit
image
second way:
x-ray is converted to electrical signals in sensor material
electric circuit
image
CCD
charge coupled devices
crystalline silicon
small
requires optical coupling
high quality, low noise
how CCD works
The CCD is divided up into a large number of light-sensitive small areas (known as pixels) which can be used to build up an image of the scene of interest. A photon of light which falls within the area defined by one of the pixels will be converted into one (or more) electrons and the number of electrons collected will be directly proportional to the intensity of the scene at each pixel. When the CCD is clocked out, the number of electrons in each pixel are measured and the scene can be reconstructed.
CCD potential wells
the pixels are defined by the position of electrodes above the CCD itself. If a positive voltage is applied to the electrode, then this positive potential will attract all of the negatively charged electrons close to the area under the electrode. In addition, any positively charged holes will be repulsed from the area around the electrode. Consequently a “potential well” will form in which all the electrons produced by incoming photons will be stored.
prevent full well
light must be prevented from falling onto the CCD for example, by using a shutter as in a camera. Thus, an image can be made of an object by opening the shutter, “integrating” for a length of time to fill up most of the electrons in the potential well, and then closing the shutter
CCD charge transfer
Each pixel actually consists of three electrodes IØ1, IØ2, and IØ3. Only one of these electrodes is required to create the potential well, but other electrodes are required to transfer the charge out of the CCD.
all 0V, now IØ2 has 10V
charge being collected under one of the electrodes.
To transfer the charge out of the CCD, a new potential well can be created by raising IØ3 voltage, the charge is now shared between IØ2 and IØ3 .
If IØ2 is now 0V, the charge will be fully transferred under electrode IØ3
CMOS sensors
complementary metal oxide semiconductor
less expensive
lower power usage
lower quality
more noise
how CMOS sensors work
CMOS sensors convert photons into electrons to a voltage & after that into a digital value through an on-chip ADC
array of photodiodes
can add transistors:
source follower transistor to amplify signal
reset transistor to clear pixel of charge
row select transistor to select row for readout
passive and active designs
gas detectors
gas between electrodes
photon ionises gas
voltage causes charges to move
charge collected at electrodes
no, of ions collected vs voltage graph
xenon ionisation chamber
used in CT
operates in ionisation region
Use of xenon gas ensures higher sensitivity and thinner design of the detector
MWPC
multi wire proportional chamber
a type of proportional counter that detects charged particles and photons and can give positional information on their trajectory, by tracking the trails of gaseous ionization.
gives x and y coordinates
uses an array of wires at high voltage (anode), which run through a chamber with conductive walls held at ground potential (cathode)
silicon pixel/strip detectors
solid-state ionisation chambers
Take a p-n-diode
* Segment it
* Apply a voltage
* Wait for a MIP to
deposit charge
* Charges separate
and drift in E-field
* This gives a signal
in the p-strips
This detector will deliver
2D information – we need
one more coordinate:
