Radiation Biology & Safety L2 PDF

MRD441: RADIATION BIOLOGY & SAFETY Effects of radiation on cells at atomic level Excitation Ionization  IONIZATION Process which occur when radiation (x-ray photon) has high energy to completely eject electrons from an atom Leaving an ion pair (positive a...

MRD441: RADIATION BIOLOGY & SAFETY Effects of radiation on cells at atomic level Excitation Ionization  IONIZATION Process which occur when radiation (x-ray photon) has high energy to completely eject electrons from an atom Leaving an ion pair (positive and negative charge ion) Amount of energy of x-ray photon is greater than the binding energy of the electron The ejected electron can cause further ionizations along the length of the path it travels – secondary ionization  EXCITATION A fraction of the x-ray photon energy of the radiation is transferred to the electrons of the absorbing material Electrons respond by jumping to another orbital level farther away from the nucleus The electrons are not ejected from the atom  LINEAR ENERGY TRANSFER (LET) The ability of ionizing radiation to cause biological damage can be determined by physical factor referred as LET LET = the rate at which an electron transfers energy to the surrounding tissue Expressed as keV per micrometer of path length (keV/µm) LET increases, biological response increases High LET o Alpha, beta, fast neutrons Low LET o X-rays RELATIVE BIOLOGICAL EFFECTIVENESS (RBE) The effectiveness of the dose of test radiation (Dtest) to the dose of the reference radiation (Dreference) RBE = Dreference/Dtest Mechananisms of damage at molecular level DIRECT ACTION INDIRECT ACTION DIRECT & INDIRECT ACTION  Relation between LET and action type Direct action is predominant with high LET radiation, e.g. alpha particles and neutrons Indirect action is predominant with low LET radiation, e.g. X and gamma rays