radiation detectors and dosimetry

Question 1

what are the four radiosensitive organs

Answer 1

• Thyroid
• Eye
• Ovaries
• Testis

Question 1

why do we need to measure radiation?

Answer 1

to limit exposure to nearby organs i.e. the rectum

Question 2

how do ionisation chambers detect radiation?

Answer 2

Energy interacts with matter which liberates an ion through ionisation.
Liberated electrons x energy for one ionisation = dose.

Question 3

what are the three types of detectors and give examples

Answer 3

1. Ionisation chambers (Thimble chamber, Geiger Muller tubes)
2. Chemical devices (film, thermoluminescent dosimeters (TLD).
3. Electrical diodes go in the patient.

Question 4

what are requirements of the dectors

Answer 4

1.Accurate, reliable
2.Wide range of response
3.Be highly independent of environmental factors.
4.All devices must be calibrated to a known dose/exposure for a known radiation.

Question 5

how does a thimble chamber work? what type of detector is it?

Answer 5

ionisation chamber - Measures the amount of ionisation of a small volume of air, the liberated electrons are attracted to the electrodes. When a current is passed through one calculate dose.

Question 6

how does a geiger muller detector work?

Answer 6

Sealed chamber containing gas is exposed to radiation, ion pairs are produced. Voltage is applied between the two electrodes within the chamber, and a current flows which is measured.

Question 7

how does Thermo Luminescent Dosimeters (TLD) work and what type of detector is it?

Answer 7

chemical device dector - Mainly made of lithium fluoride works by:
1. Radiation moves the electrons into the traps which stores the energy in the TLD.
2. The TLD is then heated up which releases electrons has light photons (photoluminescence). The light released is directly proptional to the dose
After a dose reading the TDL must be annealed to remove any residual energy by heating to a higher temperature.

Question 8

what are factors that can affect dose on the TLD?

Answer 8

1.Delay in the reading
2.Exposure to heat as energy will be released so reading is reduced
3.Environmental damage i.e. putting in the washing machine.

Question 9

How does photographic film work and what type of detector is it?

Answer 9

chemical device detector - Contains filters that identify how penetrative the radiation is:
o What type the radiation is, i.e. alpha won’t penetrate.

Question 10

what are the advantages and disadvantages of photographic film

Answer 10

1. Easy to use.
2. Can be processed quickly.

Disadvantages:
1. Single-use only.
2. Energy-dependent and variably response over dose range
3. Easily damaged by water, chemicals and heat.

Question 11

what is in vivo dosimetry

Answer 11

• Implanted into the patient for dosimetry
• Instantaneous readout
• Permanent storage of data
• waterproof

Question 12

define:
1. dose area product
2. dose length product
3. dose reference levels
4. CT dose index

Answer 12

1. physical area dose (i.e. square)
2. dose across length of patient
3. accepted level of radiation for particular procedure
4. dose index for ct slice multiplied by the x amount of slices you’ve done.

Question 13

what are semi-conductor diodes

Answer 13

radiation detectors that monitor the patient during radiotherapy which provide a real time readout of radiation dose

Question 14

what is dose area product

Answer 14

measures of the total amount of radiation delivered to a person.the larger the area the more dose that is deposited influenced by distance.

Question 15

what are the features of the lINAC ionisation chamber

Answer 15

• must detect when the dose output has been reached and then cut of the output
• turns of when reaches set dose
• tend to have two, one for backup.