detectors

Question 1

what is absorbed dose?

Answer 1

energy absorbed per unit mass (Gy)

Question 2

what is exposure?

Answer 2

total charge of one sign produced in dry air when all electrons liberated by photons in a unit mass of air are completely stopped in air.

Question 3

what do detectors do?

Answer 3

count interactions

KERMA - Kinetic energy released in matter

Question 4

describe ionisation chambers

Answer 4

simple
widely available
detect wide range of energies
count dependant on voltage set
no dead time - can miss ionisation events
fragile

Question 5

what happens in ionisation chambers?

Answer 5

need to know volume of air
penumbra
electric field weakens at edges and no longer uniform - ionisation happens but not counted.

Question 6

what is solution to weakening of electric fields in ionisation chambers?

Answer 6

guard electrodes
boost magentic field

Question 7

what does count change with

Answer 7

voltage

Question 8

graph?

Answer 8

1. recombination - voltage not high enough to attract ion pair apart so biggest force they have is each other so they recombine
2. ionisation - stable part of graph. right voltage = right count
3. proportional
   4.geiger muller - inflated count - higher voltage - caused extra events

Question 9

describe thimble chambers

Answer 9

plastic
graphite
aluminium
carbon

small hole to allow changes in atmospheric pressure and temperature

air equivalent wall - no interaction of photon
-> if not, interaction = secondary photons = infalted count

insulator - current inside

thin wall - air volume small
caps to alter thickness

Question 10

what is needed for thimble chambers?

Answer 10

electronic equilibrium
balance between electrons created in air leaving chamber and electrons created in wall entering chamber

Question 11

explain pressure as correction factor for thimble

Answer 11

low pressure - air molecules less present so less chance of interactions so lower reading for same dose

high pressure - air molecules more present so more chance of interactions so higher reading for same dose

low reading if you dont account for low pressure
high reading if u dont account for high pressure

Question 12

explain temperature as correction factor for thimble

Answer 12

raise temp - less molecules present so less chance of interaction so lower reading for same dose

decrease temp - more molecules present so more chance of interaction so higher reading for same dose

Question 13

explain chamber factor as correction factor for thimble

Answer 13

chamber wont be air equiv as nothing is
wall = imperfect
so can do:
stem irradiation - radiation will interact with base and cable - inflate count if field is too big

free air chamber - calibrated against a primary standard

Question 14

what do you use ionisation chamber for?

Answer 14

• reference dosimetry
• producing %DD
• Lateral dose distributions

Question 15

describe geiger muller tube

Answer 15

simple widely available
inefficient
cannot distinguish between radiation types
cannot determine energy
high dead time - count only one event at a time
can be inaccuarate

good at counting particles in the tube

Question 16

what is in geiger muller tube and how does it work

Answer 16

90% argon - photon has something to interact with
10% ethanol

electron cascade happens due to acceleration across tube (secondary events)
-> whilst this happens it cant count anything else
everything is counted cascade and original events

Question 17

what are semiconductor diodes

Answer 17

sensitive
tapes on skin
made of silicon
- group 14 = 4 electrons in valence band
- atoms covalently bonded
impurities with different outer shell electrons
- create holes or excess electrons

Question 18

what are p-type semiconductors

Answer 18

silicon bonded to group 3 ( boron)
3 electrons in outer shell so 4th electron in silicon cannot bond so positive hole formed

slight positive charge

Question 19

what are n-type semiconductors

Answer 19

silicon bonded to group 5 (phosphurus)
5 electrons in outer shell
4 electrons of silicon bond and leaves excess electron

slight negative charge

Question 20

what is a p-n junction

Answer 20

when p and n type form in centre

negative type has few electrons - migrate towards holes in p type

charge pulled one way
holes and electrons combine - depletion zone

Question 21

what is the structure of semiconductor diodes?

Answer 21

small silicon crystal conductor
mounted on perspex plate
connected to coaxial cable (in protective sheath)

build up cap dependant on radiation quality being measured

Question 22

what are advantages of semiconductor diodes?

Answer 22

• dose sensitive
• mechnanically stable in detector (not cable)
• small - allow small measurment volume
• independant of atmospheric pressure = no correction needed as no air going in or out
• immediate readout
• small stopping power variation with energy = suitable for electron beam dose

Question 23

what are the limitations of semiconductor diodes

Answer 23

n type:
- sensitivity decreases due to radiation damage
- non-linear response to dose rate
- p tyoe preferred in clincical use as they are less susceptible to effect

temperature:
- affects sensitivity
- must be used at same temp of tissue or correction factor needed
- sensitivity stable with accumulated radiation dose
- cable difficult to manage and easily damaged = affects recorded dose

Question 24

what are thermoluminescent detectors?

Answer 24

on patient
crystalline material within radiolucent package - created as powder pressed into little shapes

use:
- personal dose monitor
- environmental dose monitor
- patient dose monitor

Question 25

what is the structure of TLD

Answer 25

made using lithium fluoride/calcium fluoride crystal
from powder form - extruded as chips (teflon coated)

impurties added to lithium fluoride (mg)
mg sits with spare electrons
electron moves out of valence bond when irradiated and raises in energy
electron trapped in impurity
electron traps located in valence and conduction bands - hidden zone

Question 26

what does a TLD looks like

Answer 26

Plastic holder
nickel coated aluminium card with TLD discs
three filters over each disk: aluminium and copper, perspex, open window

Question 27

how do TLDs operate

Answer 27

TLD exposed to radiation at room temp
radiation interacts with lithium fluoride
photoelectric interactions occur
electrons given energy to move into conduction band
causes hole in valence band
electrons lose energy and fall into electron traps
electrosn retain energy as they havent returned to valence band

no of e- stuck in traps proportional to absorbed dose recieved by TLD

Question 28

How are TLDs read

Answer 28

throught TLD reader
electrons need more energy to escape traps
240 degree heat applied
increases interatomic vibration so trapped electrons move up conduction band
electrons fall back into valence band and lost energy is emitted as light
light detected by photomultiplier tube

intensity of light proportional to amount of radiation incident on TLD

Question 29

How are TLDs reused

Answer 29

annealed
allows all electrons that are trapped to return to valence band
heated to 100 degreed for an hour
resets lithium fluroide lattice
batches heated at same time

Question 30

what are the advantages of TLDs

Answer 30

• atomic number similar to soft tissue
• more accurate film
• wide range of doses
• reusable
• small size
• wide variety of uses

Question 31

what are disadvantages of TLDs

Answer 31

• time consuming = one reading at a time when heating
• precision affected by wrong handling (scratches..)
• sensitive to light/heat, recorded dose affected if exposed