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W/Biophysics > 15.16. Measurement of radiation, dosimetry > Flashcards

15.16. Measurement of radiation, dosimetry Flashcards

1
Q

What is the type of damage done by the physical phase of direct radiation?

A

Ionization

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2
Q

What is the type of damage done by the chemical phase of direct radiation?

A

free radical reaction

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3
Q

What are the 2 types of damages done by the biological phase of indirect radiation?

A

alteration in the tissues, somatic damage

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4
Q

Time to cause damage for the physical phase

A

10^(-17) - 10^(-12) seconds

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5
Q

Time to cause damage for the chemical phase

A

10^(-10) - 10^(-1) seconds

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6
Q

Time to cause damage for the biological phase

A
  • Tissue alteration : hours

- Somatic damage : days- years

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7
Q

What is a stochastic effect?

A

random effect, severity is independant of the dose. Probability of damage increases with the dose.

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8
Q

What is the deterministic radiation effect?

A

Occurs when dose is above the “threshold dose”. Severity increases with increasing dose.

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9
Q

What is the absorbed dose?

A

Energy absorbed from the radiation by the mass. Can be used for all types of radiation

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10
Q

What is the unit of the absorbed dose?

A

J/kg : Gray

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11
Q

How is the absorbed dose measured?

A

Indirectly measured : ionization chamber, semiconductor detector, thermoluminescent detector

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12
Q

What is the exposure? (+unit)

A

The positive charge produced (ionized) in the air of mass m. Coulomb/kg

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13
Q

What radiations can exposure be used for?

A

for gamma and X-rays, in the AIR

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14
Q

How is exposure measured?

A

In an ionization chamber

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15
Q

What is the equivalent dose?

A

stochastic health effect of low level ionizing radiation on human body : Ht = sum of (Wr * Dtr)

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16
Q

What is the unit of the equivalent dose?

A

Sievert : J/kg

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17
Q

What is a radiation weighting factor?

A

how many times greater the effectivity of the given radiation is compared to x-ray or gamma radiation

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18
Q

What is the effective dose?

A

risk of stochastic effect of a whole body : takes into account the different tissues

19
Q

What is a tissue weighting factor?

A

relative contribution of an organ or tissue to the total damage in case of stochastic effects

20
Q

What is the dose rate?

A

the amount of radioactive dose received by a person over a period of time : sv/h

21
Q

What is a scintillation counter?

A

an instrument used to detect and measure ionizing radiation (usually X-ray or gamma photon) by using the excitation effect of incident radiation on a scintillating crystal

22
Q

What is the role of the scintillator?

A
  • incident photon produces high E electrons by photoeffect, compton or pair production
  • electrons excite atoms of crystal (valence band to conduction band)
  • crystal emits photon in the blue light range
23
Q

What is the “dopant” used to activate the crystal?

A

thalium : shortens the energy gap between valence band and conduction band

24
Q

What is the current of the anode proportional to?

A

to the energy of the incident photon

25
Q

How do detectors based on gas ionization work?

A
  • high E particles interacts with matter, ionizes it and creates ion / electron pairs.
  • electrons lead to a potential drop across RC, which is detected
  • Size of pulse is dpdt of the E of the incident radiation
26
Q

What is an ionization chamber?

A
  • Low voltage difference but high enough to have no recombination
  • NO secondary ionization
  • Pulses are weak but make it easy to differenciate bw strong and weak E particles
27
Q

Which region of the I/U curve does the ionization chamber belong to?

A

ionization region

28
Q

What are the proportional counters?

A
  • Energy is high enough that there are secondary ionizations for each electron: avalanche
  • Good for detecting even low E signals
29
Q

In which region of the I/U curve do Geiger muller tubes work?

A

In the plateau region

30
Q

What does a thermoluminescent dosimeter do?

A

detect low quantity of radiation by measuring the intensity of visible light emitted by a crystal inside the detector, when the crystal is heated (200°)

31
Q

What is the crystal used for thermoluminescent dosimeters?

A

calcium sulfate crystal

32
Q

What is the material used for the trap in the thermoluminescent dosimeter?

A

dysprosium

33
Q

What is a semiconductor detector?

A

uses a semiconductor (Si/Ge) to measure the effect of incident charged particles or photons (radiation)

34
Q

What is the depletion region of the semi-conductor detector?

A

Region where electrons and holes recombines = no charge

35
Q

What happens when radiation arrives on semiconductor detector?

A
  • creates e- / hole pair (excites an electron)
  • Number of pairs is proportional to the E of radiation
  • electrons transferred to conduction band from valence band
  • electrodes detect electrons and electron holes
36
Q

Why is a semiconductor detector advantageous?

A

3eV needed to create e/h pair but 35eV needed for e/ion pair, so this detector required less energy for a pulse : can detect different energies easier

37
Q

ALAR principle (3)

A
  • application of radiation must be useful
  • risk of applying should be lower than risk of not applying
  • radiation source must not cause deterministic effects on humans
38
Q

Dose limit for working personel (whole body)

A

20mSv/year

39
Q

Background radiation dose

A

2,4 mSv/year

40
Q

Radiation dose conventional X-ray

A

0,2-1 mSv

41
Q

Radiation dose CT scan

A

2-8 mSv

42
Q

Radiation dose radiotherapy

A

45-60 Gy

43
Q

How much energy is needed to produce 1 pair of ions in air?

A

34eV

44
Q

What is the relationship bw dose and exposure?

A

dose = J/kg, exposure = C/kg - since volt = J/C, 1C = 34J

W/Biophysics (24 decks)

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