Radiation detectors Flashcards Preview

Dosimetry > Radiation detectors > Flashcards

Flashcards in Radiation detectors Deck (17)
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1
Q

What are films used for?

A

Brachy source position measurements

Some patient specific QA

2
Q

What are the advantages of gaphachromic film?

A
Characteristic response curve
Energy Independence
Dose rate linearity
Excellent spatial resolution
Gold standard of assessing radiation shape
3
Q

What are the disadvantages of graphachromic film?

A

Expensive

Planar measurement

4
Q

What are the advantages of radiographic film?

A

Excellent spatial resolution

Gold standard of assessing radiation shape

5
Q

What are the disadvantages of radiographic film?

A

Poor non-linear energy response
Messy/time consuming
Planar measurement

6
Q

How does radiographic film work?

A

Radiation produces electrons which collect in impurities creating negative charge
Positive aluminium ions collect on other impurities making a positive charge
Developing the film amplifies the signal
Grain in the positive aluminiom environment acts as an electron gate
Fixing plate removes aluminium not involved in amplification
washing removes fixer and developer

7
Q

How do radiosensitive gels work?

A

Fe2+ molecules turn into Fe3+ molecules when exposed with radiation.
Absorption spectroscopy performed to see dose.

8
Q

What is used to stop the diffusion of Fe3+ molecules in gels?

A

Polymer gels

9
Q

What is the advantage of using a gel?

A

3D resolution

10
Q

What are the disadvantages of using a gel?

A

Toxic
Messy
Need a CT/MR scanner
Polymer gels oxidise

11
Q

What do properties do scintillation crystals need?

A
Transparent
Sensitive
Fast Response
Efficiently convert KE to light
Light proportional to KE
Density and composition similar to water
12
Q

What are the disadvantages currently of using scintillation crystals?

A

Cerenkov radiation makes SNR too low without processing - need to use subtraction and spectral discrimination
Need a special photosensitive electrometer

13
Q

How does alanine dosimetry work?

A

When exposed it produces a stable ammonium radical (SAR)
The abundance of SAR can be found using electron paramagnetic resonance - the same process as MR spectroscopy
The energy difference between the low and high energy states must be known

14
Q

What is the NPL alanine service?

A

Provide traceable alanine which can be used in stacks to find the dose distribution for small fields. Precision is greater than +/-1% at 1sigma for 5-10Gy

15
Q

How do TLDs work?

A

Radiation excited electrons, which either relax to their ground state or fall into a trap.
Expose to heat to relax the electron from the trap to the ground state, which releases light
Need to do low temperature annealing before to remove low temperature peaks
anneal during readout to induce light
High temperature anneal post readout to remove residual signal

16
Q

What are TLDs used for?

A

Intra-cavity measurements

17
Q

What is the disadvantage of using TLDs?

A

Not real time readings