Topic 14: XRAY Detectors

Question 1

Four examples of detectors?

Answer 1

1. Film / Film screen
2. CR
3. Image intensifiers
4. Solid-state detectors

Question 2

Why does an insulator not conduct electricity?

Answer 2

because the conduction and the valence band are far apart

Question 3

whats normally the conduction band of a semiconductor?

Answer 3

1ev or 1.5 ev The value of the band gap determines the behaviour of the semiconductor

Question 4

what is the most commonly used semiconductor?

Answer 4

silicon based

Question 5

what is the band gap?

Answer 5

how much energy you need to give an electron to make it jump from the valence to the conduction band to conduct

Question 6

a semiconductor is not a good conductor what do we need to do to it?

Answer 6

add impurities creates levels in bandgap.

Question 7

if we want to make a semiconductor a detector what do we do?

Answer 7

we dope it, 2 ways

Question 8

what are the two ways of doping a semiconductor

Answer 8

1. 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.

Question 9

What is a PN junction?

Answer 9

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

Question 10

what happens when you put the P AND N together?

Answer 10

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.

Question 11

Again what is an n-type excessive in? and a p-type excessive in?

Answer 11

n-type - excess electrons

Question 12

why does it stop half way to recombine?

Answer 12

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

Question 13

what happens when the xray enters the charged area in a semiconductor detector?

Answer 13

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.!!!

Question 14

how do you improve the basic PN-Junction detector?

Answer 14

you inversely polarise your pn junction

Question 15

What is inversely polarising your pn junction.

Answer 15

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.

Question 16

how do you read out the charge generated through ionisation?

Answer 16

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.

Question 17

what is the single photon counting readout scheme?

Answer 17

detector - > preamplifier and shaper - > threshold discriminator

Question 18

film structure?

Answer 18

emulsion then base then a protective layer.

The emulsion contrains photosensitive grains.

Question 19

How does the film processing work?

Answer 19

1. exposure - photons liberate electrons in halide
2. Latent image - electrons produce silver atoms
3. Development - chemical process reduces grains with number of silver atoms above threshold
4. Fixation - removes unreduced grains.

Question 20

what is the problem with films?

Answer 20

inefficient - thin layer of silvergrains to stop the xray (2-3%)

Question 21

What do we want from a digital detector?

Answer 21

Appropriate area coverage
Uniformity
Stability
Linearity
High dynamic range

Question 22

what is uniformity?

Answer 22

each detector element (pixel) must provide the same response when exposed to the same amount of radiation

Question 23

what is stability?

Answer 23

providing the same response to the same input over different times

Question 24

linearity

Answer 24

direct proportionality between the radiation input and the detector response.

Question 25

High dynamic range

Answer 25

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.

Question 26

four steps of the process of an image in a computer radiography: photostimulable phosphor plate

Answer 26

1. Exposure: photons liberate electrons in phosphor
2. Latent image: electrons trapped
3. Development: laser beam scans plate, light emitted detected by PM tubes
4. Erasure: uniform exposure releases any remaining electrons

Question 27

what are the advantages of CR over film?

Answer 27

1. digitised image
2. Wide dynamic range
3. Large latitude
4. Reusable plates

Question 28

Intensification in an intensifier due to ?

Answer 28

1. Energy given to electrons (flux gain)

2. Minification gain.

Question 29

Two types of conversion

Answer 29

direct conversion: amorphous selenium

Indirect conversion: scintillator and amorphous silicon

Question 30

features of CCD?

Answer 30

Crystalline silicon, small and requires optical coupling

Question 31

what is a ccd?

Answer 31

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)

Question 32

How do gas detectors work?

Answer 32

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