interactions of charged particles Flashcards
what are charged particles?
-electrons
-heavy charged particles (i.e, heavier than 1 amu, such as an alpha particle)
what does this lecture focus on?
we will focus the interactions of electrons with matter that produce excitation and ionization
-in the lecture on x-ray production, we focused on interactions of electrons with matter that produced radiation (mostly bremsstrahlung x-rays)
what does excitation involve?
involves the displacing of an orbital electron from its ground state
-the incoming electron transfers energy to an orbital electron of an atom in the material, raising the energy of the orbital electron
what does ionization involve?
involves the removal of an orbital electron from a neutral atom
-the electron and the atom from which it was stripped form an ion pair
-the atom from which the electron was stripped becomes a positive ion
-the stripped electron quickly attaches a neutral atom to create a negative ion
-on average, 33.85 eV is required to produce an ion pair
-a 4 MeV electron will produce 4x10^6 eV/33.85 eV ion pairs or >100,000 ion pairs as it deposits its energy
what are the interactions of electron with matter?
where is energy lost during interactions of electron with matter?
-most of the energy lost is through electrostatic interactions between the electron and electrons in the matter (ionizations)
-a smaller amount is lost through brem x-rays (radiation)
what are spurs and blobs?
what is the path of the electron?
-the path of the electron is torturous because it is a very small particle and therefore changes direction when it interacts with other electrons
why is ionization important?
ionization causes chemical changes in tissue
-electrons interact with H2O and free radicals are produced
-DNA can be damaged directly by electrons (direct DNA damage) or by the free radicals (indirect DNA damage)
-damaged DNA can lead to cell death, cancer and heritable mutations in DNA of germ cells (egg and sperm)
what are interactions of heavy charged particles (HCP) with matter?
-interactions are similar to those of electrons, but HCP do not change direction when they interact with electron due to much larger mass
-pathways tend to be straight through matter
what is stopping power?
the rate at which the charged particles loses energy in a medium (MeV/cm)
-electrons in the MeV range have an average stopping power of about 2MeV/cm
what is linear energy transfer (LET)?
the rate at which the charged particle deposits energy along its track (kV/nano meter)
-LET focuses on the way the energy is deposited along the track in the medium (of more interest in radiation biology and radiation therapy)
what is the graph of linear energy transfer?
-energy deposition and DNA damage from low LET radiation (electrons) and high LET radiation (heavy charged particles)
-because photon interactions result in electrons, photons are considered low LET radiation
what is KERMA?
kinetic energy released in the medium
-the energy transferred from the photon to the electron(s) at (a)
-unit is joules/kg
what is absorbed dose?
-the energy retained in the medium
-equal to KERMA-bremss (x-rays leave the area)
-defined as the energy deposited by ionizing radiation per unit mass of material
-special unit Gray (Gy), equal to joules/kg
what is the relationship between absorbed dose and exposure?
-the ionization (coulomb/kg) resulting from the transfer of energy from electrons to a known mass of medium can be used to calculate absorbed dose (joules/kg) because we know how much energy is required for each ionization (33.97eV/ion pair, equal to 33.97 joules/coulomb)
what is the equation of absorbed dose and exposure?
