General Flashcards
pros of ion chambers
-good long and short term stability
-linear response
-small directional dependence
-dose rate independence
-energy independence
cons of ion chambers
-some small ones have high Z central electrode materials- overly sensitive to low E photons due to PE effect. Causes reponse variation as function of energy, depth, field size, and distance off-axis
-volume averaging is concern in high dose gradient regions
-stem effect
size of typical ion chamber, mini-chamber, micro-chamber
-typical = 0.6 cc
mini = 0.1 cc
micro = 0.01 cc
what should be used to measure penumbra in TPS modelling?
o Ion chambers should generally not be used to measure penumbra in TPS modelling process, due to volume averaging. Diodes, film or diamond detector should be used instead (if using diodes, must be careful with energy-dependent response though since energy spectrum varies with off-axis position.).
stem effect
: irradiation of the stem can induce leakage current, which will perturb the collected charge and will be a relatively more pronounced effect for smaller chambers, where the resulting leakage represents a larger proportion of the signal.
how often do you have to check cross-calibration of the field standard (monthly ion chamber)
annually
how often do you check ion chamber linearity, stem effect
annually
how often is local/secondary standard calibrated at lab?
every 2 years
how do you measure stem effect
-dummy stem
-irradiate thin rectangle- once with stem in field and other time without (rotate chamber or field)
issues with small volume ion chambers
In addition to stem effect (and leakage) being larger proportion of signal, high Z electrodes (or diode material) resulting in over-sensitivity to low energy photons (and note that this will affect output factor measurements as well – i.e., this effect won’t cancel in the ratio due to variation in scatter with field size), other issues with small volume chambers may include:
Sensitivity to irradiation history, anomalous recombination behavior (due to smaller volume, potentially lower recommended voltage bias), large polarity effect
pros of diodes
-high sensitivity
-small volumes (0.01 mm3 to 0.1 mm3) - good for small fields and penumbra
-
cons of diodes
-high Z gives them energy dependence (solid state dosimeters including MOSFETS)
-orientation dependence (especailly if shielded to compensate for energy dependence)
-sensitive to radiation damage
-temperature depedence
-dose rate dependence
con of diode for in vivo dosimetry compared to TLD, OSLD, MOSFET
diodes are not integrating detectors and require simultaneous read out during irradiation (like an ion chamber).
why are some diodes shielded?
Diodes intended for use in photon fields commonly also have a shield of a high atomic number material (usually tungsten) integrated into the encapsulation to selectively absorb low-energy photons to which silicon diodes would otherwise over-respond. This assumes that the shielding and encapsulation does not perturb the rest of the spectrum. Unshielded diodes should be used for measuring beam profiles since shielding may perturb response in an unpredictable way. Diodes with shields/buildup are known as energy-compensated diode detectors. Diodes for measurements in electron beams are typically unshielded.
o Encapsulation and shielding contributes to their orientation dependence.
why are diodes useful for measuring electron PDDs directly?
o Useful for measuring electron PDDs directly since restricted mass collisional stopping power ratio of silicon to water is ~constant as a function of energy for clinically relevant energies (unlike water to air ratio for ion chambers, which decreases due to density effect). However, using diode measurement as PDD directly would ignore changes in Pion, Ppol and Pfl with depth
pro of diamond vs other diodes
stopping power ratio of diamond to water is closer to being constant as a function of energy than is the stopping power ratio of silicon to water (they are ~tissue equivalent). Both of these ratios are more constant versus energy than is the ratio of air to water hence why diodes and diamond detectors can be used to measure electron PDDs directly
-2-4X more sensitive than diode detectors
-less orientation depedence than diodes
-less radiation hardness than diodes
-negligible temperature dependence
-small volume (0.004 mm3)
cons of diamond
o A higher density of material in the diamond means that recombination is more significant (since there is a relatively high concentration of charge carriers present per unit volume). An issue with diamond detectors is their dose-rate dependence due to the recombination rate being proportional to the square root of the dose rate.
how does TLD work?
Incident radiation excites electron into trap state, where it stays until read out, when it is heated, resulting in the release of visible light as the electron drops back down to its ground state.
pros and cons of TLD
o Thermoluminescent dosimeters have nearly tissue-equivalent atomic composition (LiF; Z~8), but can exhibit nonlinear integrated dose response, and energy-dependent response.
Careful calibration required; calibration should be conducted using the same beam energy as the intended use.
- TLDs are integrating dosimeters.
-Each TLD requires individual absolute dose cross-calibration at each use
-signal fading occurs over time post-irradiation.
-can only be read out once, but are re-usable
achievable accuracy with TLD
5%
how do OSLDs work?
-integrating dosimeter
-Similar mechanism as TLDs except readout process involves light instead of heat.
why do we store OSLDs and TLDs in dark?
For both TLDs and OSLDs, which rely on electrons in trap states, these trapped charges may escape at room temperature by thermal stimulation. Short term fading can occur due to electrons escaping from shallow traps. OSLDs (and TLDs) should be stored in the dark.
pros and cons of OSLD vs TLD
-OSLD readout is better controlled with respect to duration, intensity and wavelength (since light is easier to control than heat) and is faster.
- OSLD response independent of irradiation beam angle, independent of temperature of irradiation
As with other solid state detector, OSLD sensitivity increases for lower energies (e.g., below Co-60). Typical material is Al2O3.
Unlike TLDs, OSLDs ~permanently store dose information and can be read out multiple times. The amount of signal depletion due to readout (i.e., the amount of traps emptied) depends on the stimulation intensity and duration. Typical commercial readers use ~1 s of illumination which saves time and preserves the signal for future re-evalutation, if needed.
o More sensitive than TLDs with lower detection limit ~10 μSv (TLD lower limit ~ 0.1 mSv)
OSLD dose range at NSHA
0.005 to 1500 cGy. Need to establish calibration curve over range of doses of interest.
-characterize and document linearity
RPL glas dosimetry
-radiophololuminescent
-used for personnel dosimetry
-• When exposure to radiation, stable luminenscence centers are created. These luminescence centers emit light upon excitation (e.g., with UV radiation).
• Signal not erased during readout so can be readout multiple times
direct reading personal dosimeters
-allow for instantaneous display of accumulated dose
-self-reading pocket dosimeter (capacitor)
-electronic personal dosimeter (mini GM)
range of dose for film and TLD personal dosimeter
0.1 mSv to 10 Sv
range of dose for OSL and RPL personal dosimeter
0.01 mSv to 10 Sv
range of dose for pocket dosimeters
0.05 mSv - 0.2 Sv
range of dose for electronic personal dosimeter
0.1 uSv to 10 Sv
when is calibration for film required?
-for each batch of film
-for each energy used
advantage of radiochromic film
-nearly tissue equivalent
-doesn’t require processing like radiographic film
-excellent spatial resolution
