dosimetry Flashcards
what is dosimetry?
the measurement of a dose caused by radiation deposited in a medium
aka measurement of radiation from x-ray exposure
what is the importance of dosimetry
- monitoring of exposures
- identification of variation
- legislation
- protection of patients and staff
what are the quantities and units of exposure
- exposure (X)
- Kerma (K)
- Absorbed dose (D)
- Equivalent dose ( Ht)
- effective dose (E)
what is required to measure radiation dose
a dose-meter
what does a dose-meter contain
some sort of material whose response to radiation is measurable
produces an electrical signal when irradiated
what are the two types of dose-meters
- absolute
- practical
- thermoluminescent dosemeter (TLD)
- film
absolute dose meter
- very specialised equipment
- kept as national physical laboratory
- used under carefully controlled conditions
practical dose meter
- smaller and more compact
- used in hospitals
- calibrated regularly against absolute dose meter
- sensitive to external factors
what is intensity
measure of number of photons in the beam
what is the ionising effect on air
exposure (X)
what is an air ionisation chamber
radiation detector used for determining intensity of a beam of radiation or for counting individual charged particles
what are some examples of applications of air ionisation chambers
- as detectors in automatic exposure devices (AEDs)
- measure exposure (X) accurately
- measure x-ray tube output and perform QA tests on x-ray equipment
-a dose area product (DAP) meters
what is air kerma
the number of individual x-ray photons per unit of area
- doesnt say anything about the extent of the x-ray beam
what is the energy to create one ion pair in the air
34eV
- very sensitive process
what can a calibrated air ionisation chamber provide an accurate measurement of
- exposure (X)
- dose in air (Dair)
- air kerma (Kair)
what is the calibration factor
ratio of absolute dose meter measurement of exposure to practical chambers reading delivered under standard conditions
what type of chamber is practical dosimeter
- thimble ionisation chamber
thimble ionisation chamber
every measurement made by practical ionisation chamber must be multiplied by calibration factor
what happens to thimble ionisation chamber when irradiated
electrons liberated in wall and attracted to electrode
central electrode will lose some of its positive charge
electrometer measures charge collected
why ionisation in air?
- readily available
- composition close to being universally constant
what is dose a measure of
average energy absorbed from radiation beam into medium
what does air ionisation chamber measure
- exposure (X)
- measure absorbed dose in air
- air kerma
absorbed dose
biological effects in any tissue are proportional to amount of energy absorbed per unit mass of tissue
equivalent dose
we need to consider how absorbed dose is influenced by biological effects of different radiations
effective dose
different organs of body are assigned tissue weighting factors (Wt) to reflect different sensitivity of each organ to radiation damage
what is the dose area product meter
parallel plate ionisation chamber connected to electrometer where the radiation beam never exceeds area of chamber
what does the DAP measure
entire area of x ray beam exposure
measures radiation dose to air multiplied by area of x-ray field
radiographic film
an emulsion containing radiation sensitive silver halide crystals and a transparent, flexible blue tinted base make up x-ray films for general radiography
TLDs
thermoluminescent dosimeter
what are TLDs used for
used for measuring x-rays and beta radiation exposures
what are features of TLDs
- versatile material
- different forms; powder, chips, discs or rods
- small size - a few ml
- reusable
- reliable if calibrated
