Semi-Conductors and In-Vivo Dosimetry Flashcards
What are the 2 types of semi-conductor?
P and n types
p-type doped with Al or boron to have excess of holes, only 3 electrons in the valence band. When an electron fills the hole it becomes a negative ion.
n-type doped with phosphorous so 5 electrons in the valence band, with one free to wander around lattice. It is a positive ion.
What happens at a P-N junction?
N and P type semiconductors are put together.
Electrons flow across the gap until an equilibrium is reached due to the build up in charge.
What is the region with no electrons in called?
The depletion region
What happens when you apply a forward bias?
The potential hill becomes lower, making it more likely for electrons to cross the junction
How are most P-N junctions made?
A P-type silicon semiconductor with SiO2 regions has phosphorous added. The SiO2 protects most of the base semiconductor from the phosphorous but it does dope a small area. This makes a P-type semiconductor with a small region of n-type semiconductor.
How do semiconductors detect radiation?
Radiation creates electron/hole pairs
Electrons, in a P-type, and holes, in a N-type, that make it to the depletion region without recombining add to charge.
Current is proportional to dose.
The diffusion length is proportional to carrier concentration so longer for P-type (50um compared to 1um)
What is the process of radiation damage?
As radiation interacts is dislocates atoms forming traps. They shorten the diffusion length and active detection volume, reducing the sensitivity of the diode.
Damage is proportional to radiation energy so 20MeV electrons are 20x more damaging than 8MV photons.
Are P-type or N-type more affected by damage? Why?
N-type, as boron is smaller than phosphor so it is less likely to be displaced.
Are P-type or N-type more linear over dose rate?
P-type. N-types are more susceptible to traps so will over respond at high dose rates.
What is the saturation effect?
Where all the traps are full so you see a non-linear response.
Why is the PDD shallower at depth for N-types?
Because they over respond at Dmax
What causes the temperature variation in semiconductors?
There is more energy so carriers are more likely to escape traps. This plateaus at ~5kGy so use pre-irradiated diodes.
What are the advantages of using diodes?
High signal per unit volume
High resolution
Immediate dose readout
What is the stopping power ratio in the clinical range for photons and electrons?
- 8% for photons
0. 5% for electrons
What is added to photon diodes to reduce the scatter and low energies incident on it which would cause an over response?
Shielding behind the measuring volume is added
What are diodes used for?
Small fields. Large field only if validated with an ion chamber.
Measuring profiles in relative dosimetry
What is the main cause of errors for diodes?
Not being perpendicular to the beam as they have a strong angular dependence
How do EPIDS work?
Amorphous silicon photodiodes, with 1024x1024 pixels in a 40x40cm area. Each pixel is a photodiode and TFT. The TFT releases the charge which has been held in the EPID so that blocks at a time can be read out.
What is in-vivo dosimetry?
Direct measurement of the dose to the patient to identify gross errors. Uncertainty ~5%.
Who recommends IVD?
WHO
Towards Safer Radiotherapy
Manual for Cancer Standards, RT Manual
Why do IVD?
Detects major errors in patient set-up Identifies problems with understanding Identifies planning system problems Independent check of MU Defence in depth Intracavity doses OAR doses Test non-standard SSD doses
What are typical IVD equipment?
MOSFETS TLDs Diodes Film EPID
How is the dose for IVD with diodes calculated?
D = R.diode calibration.angle correction.collimator setting and SSD correction.temperature correction
What are the advantages of Diode IVD?
Simple
Instant results
SSD errors change the dose
What are the disadvantages of Diode IVD?
No IMRT Single dose point Only entry point Takes time to attach Hard to do posterior
How does EPID IVD work?
Measures the fluence transmitted through the patinet.
The images are deconvolved with the EPID kernel, which recovers the scattered fluence in the EPID and gives in air fluence map.
This is converted to RMUs using calibration image.
This is then back projected onto the planning CT using a PB alogrithm so the dose can be compared.
What are the advantages of EPID IVD?
IMRT (although not mandated) Dose anywhere in field found Not intrusive on workflow Simple to commission 3D dose distribution Uses existing equipment
What are the disadvantages of EPID IVD?
Complex Uses a PB algorithm Large field sizes so it irradiates EPID electronics Risk of collision Not for electrons Kernels need updating as EPID degrades, expensive Insensitive to SSD errors Not instant
How is QA performed on EPID?
Expose with a 100MU, 10x10 beam, send to vendor
