dosimetry equipment Flashcards
what is Hp(d)
personal dose equivalent
what is short for personal dose equivalent
Hp(d)
what is Hp(0.07)
skin dose
what is Hp(3)
lens of eye dose
what is short for dose to lens of eye
Hp(3)
what is short for dose to skin
Hp(0.07)
how can Hp(d) be measured
dosimeter at surface of body covered with an appropriate layer of tissue equivalent material
what kind of quantity is personal dose equivalent
operational
essential design of a good dosimeter 6
- it can detect radiation
- relevant dose quantity
- appropriate range (energy/dose and doserate)
- linear response with dose
- flat response with energy and doserate
how does an ion chamber work
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`
ion chamber good (4)
linear response across large dynamic range
can vary chamber size for specificity/spatial res
stable over time
dose accumulates
ion chamber bad (3)
electrometer used for readout limited by leakage current
requires temp/pressure corrections
requires careful calibration
GM tubes how they work
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
what unit does GM tube measure in
current per second
can be calibrated to microSV/hr within specific energy range
GM tubes good (4)
greater sensitivity
detection of all radiation types
low dose measurements
cheap
GM tubes uses (2)
spillages, monitoring
GM tubes bad (2)
dead time - poor at high doserates - means that indicated rate is lower than actual
poor energy response output (pulse the same regardless of energy)
what is dead time
insensitive period after each ionisation of the gas where further ionising radiation will not result in a current
what scintillation detectors use as detector material
phosphor
how do scintillation detectors work
when irradiated give of flash of light
amplified by PM tubes and measured
why do scintillation detectors give off a flash of light when irradiated
because of their arrangement of outer shell electrons
why are GM tubes and scintillations greater sensitivity
can detect individual events
what are scintillation devices used for
low level radiation
what must scintillation material match
radiation type
scintillation device good
sensitivity - low level measurement
can determine the energy of the radiation through the size of the pulses
scintillation device bad
must match radiation type
how do semiconductor detectors work
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
what is the number of electron hole pairs proportional to
the energy of the radiation
what does irradiation of TLD cause to happen to electrons
they get stored in a meta stable state within the crystal structure (latent signal)
how are stored electrons released from traps inside TLD crystal
heated to a few hundred degrees
what happens when electrons are released from TLD crystal traps
series of light scintillations which can be measured using PM tubes
two uses of luminescence (e.g. TLD) dosimetry
personal monitoring
RT patient dose monitoring
what happens when gafchromic film exposed to radiation
dye changes colour
what can be characterised using gafchromic film
level of exposure
beam shape
advantage of gafchromic film over conventional x ray film development
no developing process - instant results
disadvantage of gafchromic film
poor response below 1cGy
what is gafchromic film useful for
RT and primary beam
