Ionizing Radiation Flashcards
Photoelectric effect
- Emission of electron when light is shone onto material
- the light - photon encounters an electron of the material, which absorbs all of the photon’s energy and is ejected from the atom
- pair of ions is produced
- Distribution of photon energy: hv = l + Ek, (I = required minimal photon energy, Ek =remaining energy as kintetic energy)
- probability of photoelectric effect depends on energy and matter - represented by:
- linear attenuation coefficient: t ~p×Z⁴/ (hv)³
Ionisation - short explanaition, effectiveness dependence
- Ionization occurs when an electron is being broken off from the atomic shell, and this way a pair of opposite charged particles appears
effectiveness of ionizing process depends on following quantities:
- Mass, energy and charge of incident ionizing particle
- atomic characteristics of the matter.
Ionisation - direct and indirect Ionisation
- direct IR: includes charged particles (α,β, e+, e-, p) which ionize atoms by direct collision
- indirect IR: photons with high energy - gamma rays, roentgen rays, and electrically neutral particles - neutrons.
Do not possess electric charge and interact with atoms by their electromagnetic field. As a result electrons are being ejected from atomic shells.
Ionisation - basic quantities
- Linear ionization N1 - the number of ion pairs produced by the given type of ionizing particles during their passage through the unit path in matter
- Linear energy transmission (LET) - the locally transmitted energy by particles per unit path.
- Range ( R ) - the average depth in a certain substance at which the intensity of radiation decrease to half.
Compton effect
Compton scattering
- occurs when a photon interacts with free electron or with weakly bonded electron from the atomic outer shells.
- The photon transfers a part of energy to the electron (the broken off electron assimilates this energy) and continues propagation with altered direction and energy. - partial transfer of energy!
The probability for appearance of CS is evaluated by linear coefficient of Compton Scattering :
σ ~p×Z/ hv
Pair production
- high energy photon (energy at least hv = 1,02 MeV + Ek(e-) + Ek(e+)) interacts with electric field of an atom.
- produces pair of electron-positron (antiparticle of electron; same mass but positive charge).
- total transfer of photon energy
- The positron produced interacts quickly with electron from the first atom encountered and provokes the reverse process - particles annihilation.
- The probability is estimated by linear attenuation coefficient for pair production:
X ~ Z² ln(hv)
Attenuation of photonic ionizing radiation
Half value layer
- total attenuation (Abschwächung) of photonic radiation is quantitatively determined by decrease of radiant flux (Strahlungsfluss) of the ionizing photons
ψ = ψ₀ e^(-µd) where (µ = t +σ + X)
Half-value-layer (HVL)
- used for determination of absorbing ability of a given substance.
- defined as a distance of penetration for which the photon flux decrease to 50% in terms of the initial Value
Roentgen radiation - short facts
- photons with no electric charge but high energy
- short wavelength, from 3pm to 10nm
- natural sources mostly cosmic rays or radioactive decay of some elements (f.ex. thulium) by process of K-catch:
K-catch:
nucleus absorbs inner atomic electron from K- shell, vancancy is filled by electron from high energy shell emitting x-rays) )
Roentgen radiation: production
X-Ray tube
- in vacuum - no air collision
- thermo-cathode is heated up by separated electric circuit
- electrons within the cathode acquire energy sufficient to be ejected from the metal lattice;
- they fall under the action of high voltage and are accelerated towards rotating anode,
- electron beam hits the tungsten anode and produce emission of X rays.
Medical imaging by roentgen radiation: explanation of contrast in picture
○ The photons penetrate into the tissues and interact with X-ray tube different structures at different ways (mainly by photo-effect and Compton effect ).
- causes attenuation of photon beam, just part of radiation passes through the body and reaches the registration device.
- The contrast is obtained due to the Zonal distribution of the attenuation.
- The intensity of absorption depends on density of the biological matter and its effective atomic number Z
- high-Z substances and high density absorb much more = dark colour, than tissues with low Z
- use of contrast substance when examining tissues to see details, f.ex. Iodine solution in blood vessels
Modification of x ray beam properties by anode current and voltage
- change in current (Ia) changes intensity of X rays (expressed by radiant flux $)
= change in amplitude of x-ray emission spectrum at all energies, shape of the curve will remain the same
-higher current =graph of spectrum increases in heigh
$ = k.I.Z.U^2
Kinetic energy of electrons: E = e (charge) * U (voltage applied)
- voltage modification affects the image contrast
- increase in x ray tube voltage = more powerful x rays, increased penetration, left shift of braking radiation spectrum, decrease of minimal wavelength
Roentgenography, Roentgenoscopy
- production of image on film plate by transformation of photon energy into chemical energy
- compton and photoelectric effect cause attenuation and contrast
Roentgenoscopy:
- same but image is obtained on fluorescent screen
- intensity of rays proportional to brightness of picture
- higher radiation dose on patient than Roentgenography
Digital Subtraction Angiography
- goal: to exclude bones and soft tissue from picture to investigate blood vessels
- first do roentgenography to get bone picture
- then introduce iodine solution as contrast medium to make vessels visible
- at the end, computer filters parts of image so that only vessels are visible
Breaking roentgen radiation
- electrons are decelerated or “braked”by magnetic field when they are fired at and hit the metal target atom
- During this abrupt deceleration, the beam electrons emit bremsstrahlung / braking radiation
—a continuous spectrum of electromagnetic radiation with a peak intensity in the X-ray region
