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diagnostic radiology (ionising) > dosimetry equipment > Flashcards

dosimetry equipment Flashcards

1
Q

what is Hp(d)

A

personal dose equivalent

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2
Q

what is short for personal dose equivalent

A

Hp(d)

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3
Q

what is Hp(0.07)

A

skin dose

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4
Q

what is Hp(3)

A

lens of eye dose

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5
Q

what is short for dose to lens of eye

A

Hp(3)

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6
Q

what is short for dose to skin

A

Hp(0.07)

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7
Q

how can Hp(d) be measured

A

dosimeter at surface of body covered with an appropriate layer of tissue equivalent material

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8
Q

what kind of quantity is personal dose equivalent

A

operational

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9
Q

essential design of a good dosimeter 6

A
  1. it can detect radiation
  2. relevant dose quantity
  3. appropriate range (energy/dose and doserate)
  4. linear response with dose
  5. flat response with energy and doserate
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10
Q

how does an ion chamber work

A

air in chamber at natural atmospheric pressure
air ionised by the radiation and charge pairs accelerated across high voltage field between two electrodes
current proportional to dose`

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11
Q

ion chamber good (4)

A

linear response across large dynamic range

can vary chamber size for specificity/spatial res

stable over time

dose accumulates

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12
Q

ion chamber bad (3)

A

electrometer used for readout limited by leakage current

requires temp/pressure corrections

requires careful calibration

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13
Q

GM tubes how they work

A

sealed chamber with low pressure gas
ion pairs created
acceleration of electrons by high voltage cause further ionisation (chain reaction) of gas molecules and large pulse measured

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14
Q

what unit does GM tube measure in

A

current per second
can be calibrated to microSV/hr within specific energy range

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15
Q

GM tubes good (4)

A

greater sensitivity

detection of all radiation types

low dose measurements

cheap

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16
Q

GM tubes uses (2)

A

spillages, monitoring

17
Q

GM tubes bad (2)

A

dead time - poor at high doserates - means that indicated rate is lower than actual

poor energy response output (pulse the same regardless of energy)

18
Q

what is dead time

A

insensitive period after each ionisation of the gas where further ionising radiation will not result in a current

19
Q

what scintillation detectors use as detector material

A

phosphor

20
Q

how do scintillation detectors work

A

when irradiated give of flash of light

amplified by PM tubes and measured

21
Q

why do scintillation detectors give off a flash of light when irradiated

A

because of their arrangement of outer shell electrons

22
Q

why are GM tubes and scintillations greater sensitivity

A

can detect individual events

23
Q

what are scintillation devices used for

A

low level radiation

24
Q

what must scintillation material match

A

radiation type

25
Q

scintillation device good

A

sensitivity - low level measurement

can determine the energy of the radiation through the size of the pulses

26
Q

scintillation device bad

A

must match radiation type

27
Q

how do semiconductor detectors work

A

radiation produces free electrons and holes ion the detector

electric field applied and electrons and holes travel to electrodes on either sides of the detector

this results in a measurable voltage pulse

28
Q

what is the number of electron hole pairs proportional to

A

the energy of the radiation

29
Q

what does irradiation of TLD cause to happen to electrons

A

they get stored in a meta stable state within the crystal structure (latent signal)

30
Q

how are stored electrons released from traps inside TLD crystal

A

heated to a few hundred degrees

31
Q

what happens when electrons are released from TLD crystal traps

A

series of light scintillations which can be measured using PM tubes

32
Q

two uses of luminescence (e.g. TLD) dosimetry

A

personal monitoring

RT patient dose monitoring

33
Q

what happens when gafchromic film exposed to radiation

A

dye changes colour

34
Q

what can be characterised using gafchromic film

A

level of exposure

beam shape

35
Q

advantage of gafchromic film over conventional x ray film development

A

no developing process - instant results

36
Q

disadvantage of gafchromic film

A

poor response below 1cGy

37
Q

what is gafchromic film useful for

A

RT and primary beam

38
Q
A

diagnostic radiology (ionising) (7 decks)

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