detectors

Question 1

band structure/band gap

for diff materials

Answer 1

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

Question 2

N-type semiconductor

Answer 2

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

Question 3

P type semiconductor

Answer 3

positive

trivalent impurity
missing electron
eg. boron has 3 electrons

hole
energy just above valence level

Question 4

PN-junction

Answer 4

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

Question 5

ionising particle in PN junction

Answer 5

the passage of an ionising particle creates ionisation, electrons and holes, which immediately drifts in opposite directions due to electric field

Question 6

how to improve PN junction

Answer 6

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

Question 7

DC coupling

Answer 7

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

Question 8

AC coupling

Answer 8

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

Question 9

single-photon counting readout scheme

Answer 9

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

Question 10

film structure

Answer 10

emulsion contains grains

emulsion bottom, then base, then protective layer

Question 11

film process

Answer 11

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)

Question 12

HD curve

Answer 12

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

Question 13

film + fluorescent screen

Answer 13

mu is higher
thicken than emulsion
higher stopping power

reduced spatial resolution

Question 14

structure of screen

Answer 14

phosphors

Question 15

film adv and disadv

Answer 15

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

Question 16

digital detector qualities priorities

Answer 16

appropriate area coverage
uniformity
stability
linearity
high dynamic range

Question 17

BaFBr:Eu2+ process

Answer 17

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

Question 18

computed radiography
photostimulable phosphor plate
PSP

Answer 18

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

Question 19

stimulation and emission spectra

Answer 19

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

Question 20

CR adv over film

Answer 20

digitised image, post processing
wide dynamic range
large latitude
reusable plates

Question 21

image intensifier

Answer 21

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

Question 22

scintillator

Answer 22

cesium iodide

grown in columns
-tight packing
-light spread reduced

Question 23

flat panel detectors

Answer 23

direct conversion
-amorphous selenium

indirect conversion:
-scintillator
amorphous silicon

Question 24

amorphous selenium

Answer 24

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.

Question 25

amorphous silicon

Answer 25

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

Question 26

CCD

Answer 26

charge coupled devices

crystalline silicon
small
requires optical coupling

high quality, low noise

Question 27

how CCD works

Answer 27

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.

Question 28

CCD potential wells

Answer 28

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

Question 29

CCD charge transfer

Answer 29

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

Question 30

CMOS sensors

Answer 30

complementary metal oxide semiconductor

less expensive
lower power usage
lower quality
more noise

Question 31

how CMOS sensors work

Answer 31

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

Question 32

gas detectors

Answer 32

gas between electrodes
photon ionises gas
voltage causes charges to move
charge collected at electrodes

no, of ions collected vs voltage graph

Question 33

xenon ionisation chamber

Answer 33

used in CT
operates in ionisation region

Use of xenon gas ensures higher sensitivity and thinner design of the detector

Question 34

MWPC

Answer 34

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)

Question 35

silicon pixel/strip detectors

Answer 35

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: