Detectors and dosimetry Flashcards
Types of detectors?
Ionisation chambers
Thimble chambers (QA measurement, Output measurement)
Iontomat chambers (equipment output)
Geiger Muller tubes
Chemical devices
Film
Thermoluminescent dosimeters (TLD)
Electrical Diodes
MOSFETS1 – silicon and diamond detectors
Which of the following are correct statements relating to a Thermoluminescent
Dosimeters?
.
1 Protects the wearer against ionising radiation
2 Can be worn on specific parts of the body to record radiation dose e.g. fingers
3 Records the dose of radiation received by the wearer
4 Emits light when heated.
Can be worn on specific parts of the body to record radiation dose e.g. fingers.
Records the dose of radiation received by the wearer.
Emits light when heated.
Purpose of a Radiation Detector
To measure the direct or indirect effects of a beam of radiation which can be interpreted as a consistent value (and which can be related to the effects of the radiation) as physical dose or dose rate.
Applications of dosimetry
Personnel monitoring
Equipment – Commissioning, radiation output and quality assurance
Radioactivity and contamination
Patient monitoring
What does ionising radiation do, that can be measured?
Energy deposition
Ionisation
What are the radiation types
Radiation type – alpha, beta, gamma, proton, neutron, X rays
How does a thimble chamber work?
Radiation ionizes air in the chamber which will be attracted to a pair of electrodes and this will cause an electrical charge to flow in the electrodes which can be measured and converted to a value
A current flows - proportional to the amount of irradiation (exposure)
How does a Geiger muller detector work?
Similar to a thimble chamber, expect has an open end which then means it measures the air around the end of the detector.
It has a grid on the end so that it only measures in front of the end.
Directional detector.
Sometimes they click.
Detector requirements?
Accurate
Wide range
Not affected much by environmental factors
Good resolution (measure low and high doses)
Response is proportional
Independent of dose rate. (mGy/min)
Directional dependent (positioning to the rad beam)
What does direct and indirect mean?
Direct- measures the amount of rad, the energy
Indirect- measures the effects of rad.
Dosimetry Terminology
Dose Area Product (DAP) Dose length Product DLP) Dose Reference levels (DRL) CT Dose Index (CTDI) Air KERMA (K)
How do TLDs work?
Crystal is irradiated
Energy is captured by electrons from radiation
Electron excite other electrons and their energy is raised to the Conduction Band.
Electrons lose energy and fall into ‘traps’.
To release the trapped electrons heat is required (Thermo) ~ 3000C
After dose reading the TLD must be annealed to remove any residual energy by heating to a higher temp.
Pros and cons of Photogrphic film (they work similarly to TLDs)
Pros Easy to use Processed quickly Cons Single use Damaged easily Not tissue equivalent Energy dependent and variably response over dose range
Factors that affect does reading on TLD
Delay in reading the dose (not handing in on time)
Energy fade
Exposure to heat e.g. radiator or sunlight (reduce the reading)
Releases the energy and so reading is reduced
Environmental damage – e.g. Putting in the washing machine.
How do MOSFETs (Metal Oxide Semi-conductor Field Effect Transistor) work?
Uses P-N materals (postivie and negative)
N material is doping agent and creates a space of no electrons.
Area is irradiated meaning the electrons can move across doping agent and create a electrical current.
This current is measured to tell us the dose.
