15.16. Measurement of radiation, dosimetry Flashcards
What is the type of damage done by the physical phase of direct radiation?
Ionization
What is the type of damage done by the chemical phase of direct radiation?
free radical reaction
What are the 2 types of damages done by the biological phase of indirect radiation?
alteration in the tissues, somatic damage
Time to cause damage for the physical phase
10^(-17) - 10^(-12) seconds
Time to cause damage for the chemical phase
10^(-10) - 10^(-1) seconds
Time to cause damage for the biological phase
- Tissue alteration : hours
- Somatic damage : days- years
What is a stochastic effect?
random effect, severity is independant of the dose. Probability of damage increases with the dose.
What is the deterministic radiation effect?
Occurs when dose is above the “threshold dose”. Severity increases with increasing dose.
What is the absorbed dose?
Energy absorbed from the radiation by the mass. Can be used for all types of radiation
What is the unit of the absorbed dose?
J/kg : Gray
How is the absorbed dose measured?
Indirectly measured : ionization chamber, semiconductor detector, thermoluminescent detector
What is the exposure? (+unit)
The positive charge produced (ionized) in the air of mass m. Coulomb/kg
What radiations can exposure be used for?
for gamma and X-rays, in the AIR
How is exposure measured?
In an ionization chamber
What is the equivalent dose?
stochastic health effect of low level ionizing radiation on human body : Ht = sum of (Wr * Dtr)
What is the unit of the equivalent dose?
Sievert : J/kg
What is a radiation weighting factor?
how many times greater the effectivity of the given radiation is compared to x-ray or gamma radiation
What is the effective dose?
risk of stochastic effect of a whole body : takes into account the different tissues
What is a tissue weighting factor?
relative contribution of an organ or tissue to the total damage in case of stochastic effects
What is the dose rate?
the amount of radioactive dose received by a person over a period of time : sv/h
What is a scintillation counter?
an instrument used to detect and measure ionizing radiation (usually X-ray or gamma photon) by using the excitation effect of incident radiation on a scintillating crystal
What is the role of the scintillator?
- incident photon produces high E electrons by photoeffect, compton or pair production
- electrons excite atoms of crystal (valence band to conduction band)
- crystal emits photon in the blue light range
What is the “dopant” used to activate the crystal?
thalium : shortens the energy gap between valence band and conduction band
What is the current of the anode proportional to?
to the energy of the incident photon
How do detectors based on gas ionization work?
- high E particles interacts with matter, ionizes it and creates ion / electron pairs.
- electrons lead to a potential drop across RC, which is detected
- Size of pulse is dpdt of the E of the incident radiation
What is an ionization chamber?
- Low voltage difference but high enough to have no recombination
- NO secondary ionization
- Pulses are weak but make it easy to differenciate bw strong and weak E particles
Which region of the I/U curve does the ionization chamber belong to?
ionization region
What are the proportional counters?
- Energy is high enough that there are secondary ionizations for each electron: avalanche
- Good for detecting even low E signals
In which region of the I/U curve do Geiger muller tubes work?
In the plateau region
What does a thermoluminescent dosimeter do?
detect low quantity of radiation by measuring the intensity of visible light emitted by a crystal inside the detector, when the crystal is heated (200°)
What is the crystal used for thermoluminescent dosimeters?
calcium sulfate crystal
What is the material used for the trap in the thermoluminescent dosimeter?
dysprosium
What is a semiconductor detector?
uses a semiconductor (Si/Ge) to measure the effect of incident charged particles or photons (radiation)
What is the depletion region of the semi-conductor detector?
Region where electrons and holes recombines = no charge
What happens when radiation arrives on semiconductor detector?
- creates e- / hole pair (excites an electron)
- Number of pairs is proportional to the E of radiation
- electrons transferred to conduction band from valence band
- electrodes detect electrons and electron holes
Why is a semiconductor detector advantageous?
3eV needed to create e/h pair but 35eV needed for e/ion pair, so this detector required less energy for a pulse : can detect different energies easier
ALAR principle (3)
- application of radiation must be useful
- risk of applying should be lower than risk of not applying
- radiation source must not cause deterministic effects on humans
Dose limit for working personel (whole body)
20mSv/year
Background radiation dose
2,4 mSv/year
Radiation dose conventional X-ray
0,2-1 mSv
Radiation dose CT scan
2-8 mSv
Radiation dose radiotherapy
45-60 Gy
How much energy is needed to produce 1 pair of ions in air?
34eV
What is the relationship bw dose and exposure?
dose = J/kg, exposure = C/kg - since volt = J/C, 1C = 34J
