detectors Flashcards
what is absorbed dose?
energy absorbed per unit mass (Gy)
what is exposure?
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.
what do detectors do?
count interactions
KERMA - Kinetic energy released in matter
describe ionisation chambers
simple
widely available
detect wide range of energies
count dependant on voltage set
no dead time - can miss ionisation events
fragile
what happens in ionisation chambers?
need to know volume of air
penumbra
electric field weakens at edges and no longer uniform - ionisation happens but not counted.
what is solution to weakening of electric fields in ionisation chambers?
guard electrodes
boost magentic field
what does count change with
voltage
graph?
- recombination - voltage not high enough to attract ion pair apart so biggest force they have is each other so they recombine
- ionisation - stable part of graph. right voltage = right count
- proportional
4.geiger muller - inflated count - higher voltage - caused extra events
describe thimble chambers
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
what is needed for thimble chambers?
electronic equilibrium
balance between electrons created in air leaving chamber and electrons created in wall entering chamber
explain pressure as correction factor for thimble
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
explain temperature as correction factor for thimble
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
explain chamber factor as correction factor for thimble
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
what do you use ionisation chamber for?
- reference dosimetry
- producing %DD
- Lateral dose distributions
describe geiger muller tube
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
what is in geiger muller tube and how does it work
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
what are semiconductor diodes
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
what are p-type semiconductors
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
what are n-type semiconductors
silicon bonded to group 5 (phosphurus)
5 electrons in outer shell
4 electrons of silicon bond and leaves excess electron
slight negative charge
what is a p-n junction
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
what is the structure of semiconductor diodes?
small silicon crystal conductor
mounted on perspex plate
connected to coaxial cable (in protective sheath)
build up cap dependant on radiation quality being measured
what are advantages of semiconductor diodes?
- 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
what are the limitations of semiconductor diodes
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
what are thermoluminescent detectors?
on patient
crystalline material within radiolucent package - created as powder pressed into little shapes
use:
- personal dose monitor
- environmental dose monitor
- patient dose monitor
what is the structure of TLD
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
what does a TLD looks like
Plastic holder
nickel coated aluminium card with TLD discs
three filters over each disk: aluminium and copper, perspex, open window
how do TLDs operate
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
How are TLDs read
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
How are TLDs reused
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
what are the advantages of TLDs
- atomic number similar to soft tissue
- more accurate film
- wide range of doses
- reusable
- small size
- wide variety of uses
what are disadvantages of TLDs
- time consuming = one reading at a time when heating
- precision affected by wrong handling (scratches..)
- sensitive to light/heat, recorded dose affected if exposed
