week 6 Flashcards
what’s the difference between direct and indirect ionisation?
direct ionisation is the initial interaction of the photon
indirect ionisation is the interactions between electrons emitted from the initial interaction and other particles
what is the energy required to cause a single ionisation in air at STP
33.97eV
explain ionisation in air
indirect/direct ionisation occurs between electrons/photons and air molecules and the air gains a charge proportional to the energy absorbed from the x-rays. this charge can be collected and measured
what is KERMA and its two types?
kinetic energy released per unit mass
collision kerma
- energy lost to interactions with electrons (scattering, absorption)
radiative kerma
- electrons interact with nucleus, producing bremsstrahlung rays
what is absorbed dose?
energy transferred to the medium, energy absorbed per unit mass
what unit is used to describe absorbed dose?
J/Kg
=Gy
what is equivalent dose?
absorbed dose accounted for the weighting factors of different radiations (measure of biological damage done by different radiations)
equivalent dose (Sv) = absorbed dose x weighting factor
what is effective dose?
equivalent dose accounted for the sensitivity of different tissues to radiation
effective dose = equivalent dose x tissue weighting factor
what is linear energy transfer (LET)?
average energy deposited by electron per unit length (after photon interaction)
high LET = densely ionising
what is relative biological effectiveness (RBE)?
comparison of the dose required for the same biological damage, x-ray used as reference
RBE = D(x-ray)/D(test ray)
define radiation exposure
charge per unit mass
simply explain thermoluminescent dosimetry
energy gets trapped in a dosimeter when exposed to radiations, this dosimeter is then heated and a light is emitted with intensity relative to the amount of energy stored
what are two radiation detectors
Geiger-muller (GM) tube
ionisation chamber
how can an air cavity ionisation chamber be used to determine absorbed dose?
there will be interactions between photons and air particles which release electrons, which then interact with more particles to create a charge. this charge can be measured and converted to absorbed dose through a calibration coefficient
what are the 3 classes of radiation exposure?
occupational, medical, public
what are the 3 protection principles?
justification- sufficient benefit?
optimisation- ALARA
limitation- dose limit (1mSv/20mSv)
what is the difference between the absolute and relative risk model?
the relative model assumes the risk of cancer increases with age whereas the absolute model doesn’t account for this
