Dosimetry Instrumentation Flashcards
5 reasons why we need to measure radiation in HC?
- Patient dose (risk) - effective dose and organ dose
- Occupational Dose ALARP - <irr17>
</irr17><li>Area Monitoring - dose of other persons, assessment of sihelding</li><li>Calibration - how much </li><li>Equipment performance/validation (QA)</li>
</irr17>
Give examples of:
Protection quantities
Physical quantities
Operational quantities
- How do you get from physical to protection?*
- How do you get from operational to protection?*
Protection quantities
- Equivelent organ dose (HT)
- Effective Dose (E)
Physical quantities
- Exposure
- Absorbed Dose
- Kerma
Operational quantities
- Ambient dose equivelent H*(d)
- Personal Dose Equivelent Hp(d)
Physical > protection : calculations & phantoms
Operational > protection : measurable approximation
What is an ICRU sphere?
Concept of expanded & alligned field?
ICRU sphere is theoretical, 30cm diameter, density = 1000Kg/m3, mass composition equal to tissue.
Radiation field is expanded to encompass entire sphere
aligned so the quantity is independent of angular distribution of radiation field (collimated)
I.e. you define this sphere 30cm diameter, patientequivelent, you say there is a radiation field to cover the whole sphere and the rays are coming parallel.
What is Ambient dose equivalent - H*(10)
What is it used for?
Why do we use it?
H*(10) is the dose equivelent that would be produced in expanded/alligned ICRU sphere at depth of 10mm, on radius opposing field-direction. I.e. in this patient-type scenario, this is what the equivelent dose would be 10cm into the phantom/patient.
Use: area monitoring
We use it because it’s a good estimate of effective dose (E(Sv))
What is Personaldose equivalent - Hp(10)
What is used for effective dose, skin, eyes?
How do you measure it?
Hp(d) is the equivelent dose that would be generated in a depth d (mm) from the surface of a person.
Hp(10) = effective dose to individual
Hp(0.07) = skin
Hp(3) = lens of eye
You can measure Hp(d) with a dosimeter that is worn on the surface of the body but contains a layer of tissue equivelent material on top.
What are some good characteristics of a dose measurement instrument?
- Measures the relevant dose quantity
- Appropriate range (energy & dose rate)
- Linear response with dose
- Flat response with energy
- Appropriate accuracy & Precision
Ion chambers
How do they work?
5 positives
2 negatives
Already covered this content in RT extensively
Positives
- Measures Kerma
- Lienar response with alrge dynamic range, changes predictably with x-ray energy
- Variable chamber size
- Good for standards
- Good for dose rate
Negatives
- Needs an electrometer for readout
- Temp and pressure corrections
*
GM Tube:
How does it work?
- Sealed chamber with gas at low pressure
- Two electrodes with high voltage (400-800V)
- Radiation produces ion pairs, accelerated by E-field increasing their energy
- Increased energy from ion pairs creates more ion pairs, chain reaction until saturation, pulse is detected by electroncics
- Measures Counts/s but can be calibrated to uSv/hr with energy range
GM Tube:
4 positives
2 negatives
positives
- Better sensitivity than ion chamber - good for low dose measurements
- Detects all radiation types
- cheap
negatives
- Poor at high doserates due to dead time
- Poor energy response output
Scintilliation Detector
How does it work?
- Phosphor is detective medium e.g. NaI, when irradiated they give of flash of light
- Light is amplified by PMT and measured with circuitry
- Individual ionisation events detected, high sensitivity
- PHA
- Used for low-dose measurements
- Material must match radiation tupe e.g. NaI for y-camera but not for alpha or betas
Solid State detectors
How do they work?
- Semiconductor material exposed to radiation produces free electrons and holes, number of pairs is proportional to energy of radiation
- Under influence of electric field, electrons & holes travel to either side of detector, measurable voltgage
Thermo-luminescent Dosimetry
How does it work?
- Irradiation of TLD causes electrons to be stored in meta-stable states within crystal structure (latent signal)
- When material is subsequently heated to few hundred degrees, electrons released from traps -> light scintilations measured with PMT
- Used for personal monitoring
Film Dosimetry
How does it work?
- GaFc contains dye which changes colour when exposed to IR, exposure and beam shape measured
- Unlike conventional x-ray film you don’t need to develop, results obtained instantly
- Dose from lookup table / calibration curve
- Poor response below 1cGy, useful in RT
What dose measuring equipment would you use for:
- Equipment performance
- Staff dose assessment
- Activity for NM injection
- Contamination monitoring
- Survey outside LINAC
- Equipment performance
- Air Kerma (mGy)
- Staff dose assessment
- TLD, Hp(10,3) etc.
- Activity for NM injection
- Well-type ion-chaber (counts/s)
- Contamination monitoring
- GM tube Air Kerma (mGy)
- Survey outside LINAC
- GM tube Air Kerma (mGy)
Check to see if these are right, not entirely sure.
SUMMARY
•Before choosing a instrument decide what need to measure (6 things)
What else?
- Radiation type (alpha, beta gamma)
- Energy range (keV-MeV)
- Doserate or total dose
- Likley doserate (<0.1uSv/hr – Gy/s?)
- Dose Quantity (AK, H*(10), Hp(d), count/sec )
- What accuracy /precision
Match an instrument to a particular task and understand it’s limitations
