Solid State Detectors

What is most useful characteristic of solid-state dosimeters? Why?

Small size = high spatial resolution

What does a trap consist of? What does it play significant role in?

A trap consists of either a chemical impurity or an imperfection in the spacing of the atoms. Traps play a significant role in photoconduction, luminescence, and operations of various devices

How are charge carriers released from traps?

by adding energy, such as with irradiation, light, or heat.

How does an electron gains energy to cross the band gap?

Various processes can supply the energy needed such as ionizing radiation, which ultimately leads to TL or OSL.

What are traps characterized by?

Activation energy, trap lifetime

What is trap lifetime?

The time between activation of the trap and emission of light

With higher Ea comes an increase in τ and for very high τ, phosphorescence can be accelerated by what?

heat, UV or visible light.

What is the general concept for TLDs and OSLDs?

The general concept is that the thermoluminescent dosimeters (TLDs) or optically stimulated dosimeters (OSLDs) receive radiation, store that energy using solid-state effects, and then release the energy in the form of light at a later time. The amount of light emitted is proportional to the amount of absorbed energy.

For TLDs and OSLDs, how is the amount of light and the amount of absorbed energy related?

They are proportional

Is a TLD a passive or active dosimeter?

Passive

What is meant by passive?

The dosimeter must be read out after irradiation

How are TLDs readout?

The readout process involves thermal activation, which is recorded as a measured current from a photomultiplier tube (PMT). While the TLD is being heated up, the heating rate is held constant, resulting in a plot of current vs time, referred to as a thermogram. This is more commonly referred to as a a glow curve in medical physics.

What is a glow curve?

The resulting plot of current vs. time when heating a TLD at a constant rate

What two parts make up a TLD system?

signal storage and signal readout

What is signal storage?

The irradiation of the phosphor results in trapping of charge carriers (electrons and holes) in storage traps and recombination centers.

What is signal readout?

The luminescence process is activated by heat. This releases charge carriers trapped in storage traps and initiates subsequent recombination. Recombination energy is released in the form of UV or visible light, which is read out.

What are the three different ways a TLD can be heated?

Oh I ally heated metallic pad, nitrogen gas, focused laser light

What is this?

Whole body TLD

What does each peak in a glow curve correspond to?

Each peak corresponds to a different trap’s energy depth. Each material will have its own characteristic glow curve

One thing you will notice about all of those TLD materials is that they contain relatively high-Z materials. What does this mean?

They are highly sensitive by are not very tissue equivalent

A LiF:Mg,Ti glow curve is dominated by "peak 5" that occurs at a temperature slightly above 200 degrees celcius at a heating rate of 2 degrees per second. The shoulder on the low temperature side of peak 5 is due to an additional glow peak referred to as "peak 4". What does the area of these two peaks normally serves as?

the measured signal that is proportional to the dose. Some people will separate the peaks out for analysis, while others will integrate over all the peaks. The integration is the more common method of reading out the TLD

Why do TLD peaks fade post irradiation?

The TLD peaks have different half lives

Why is an irradiated TLD susceptible to signal fading?

This is due to crystal lattice vibrations causing the random release of trapped charge carriers, resulting in charge recombination at luminescence and recombination centers.

What is the standard pre-irradiation annealing process for TLDs?

heat for 1 hr in a 400 ◦C oven and then for 24 hrs in an oven at 80 ◦C.

TLDs are linear up to a point. When do the start experiencing supralinearity?

5 Gy

What is a beneficial characteristic of LiF TLDs?

The effective atomic number is 8.2, so it’s almost water equivalent

What is a disadvantage of LiF TLDs?

There is an over response of approx. 1.4 to 40 keV photons. They are also not easy to use reliably and reproducibly.

What is the difference between OSLDs and TLDs? How are they alike?

OSLD has the same band-structure characteristics as TLDs, but the stimulating agent is light instead of heat.

What colored laser light is the most convenient for stimulating OSLD.

Green

What is a notable feature of an OSLD?

One notable feature of OSLDs is that the readout only depletes ≈1% of the stored signal, in contrast to TLD where it is mostly erased.

What configuration for a diode when placed in a circuit is used in medical physics?

Short circuit / no bias - P-N junction is in equilibrium with a potential difference formed across the junction of ≈0.7 V.

What are the advantages of a diode dosimeter?

1. increased sensitivity per unit volume

2. increased spatial resolution makes diodes good for high dose gradients

3. very good mechanical stability and strength

4. no external bias needed.

What are the shortcomings of diode dosimeters?

1. As they accumulate dose, there is a loss in sensitivity.

2. They are sensitive to temperature changes and their response varies by about 0.2% per centigrade.

3. The atomic number of Si is 14, which makes these detectors less water equivalent

4. They are directional dependent.

What are the advantages of a diamond dosimeter?

1. the effective atomic number is 6

2. low leakage

3. exhibit very little change in response with irradiation over time

4. less prone to damage

5. can be sterilized for in-vivo use due to their high melting point.

What are the disadvantages of a diamond dosimeter?

1. They require quite a bit of warmup dose to achieve an equilibrium of recombination within the detector.

2. They also exhibit a dose rate dependence.

MOSFETs are controlled how?

Voltage controlled. A certain voltage threshold, VT , on the gate is required for current flow. After being irradiated, the voltage threshold shifts to a higher voltage.

What are the downsides of MOSFETs?

1. limited dose range (Commercial MOSFET detectors used in radiotherapy are only good for around 20 measurements of 2 Gy before they become depleted and have to be discarded)

2. lack of water equivalence

3. extreme energy dependence

What are MOSFETS commonly used for?

in-vivo dosimetry

Advantages of MOSFEs?

1. relatively cheap

2. robust