Atomic structure and production of radiotherapy Flashcards
Structure, Radioactive Decay, Production of RT
<p>What does Z in the atomic nomenclature indicate?</p>
What about N?
<p>Total protons</p> Total neutrons (N = A-Z)
<p>What does A in the atomic nomenclature indicate?</p>
<p>Total nucleons (protons + neutrons)</p>
Define isotope
Some number of Protons, different nucleons.
Ex: I-125 and I-131 are isotopes
Define isotone
Same number of Neutrons, different protons
Ex: not really used
Define isobar
Same number of nucleons, different proportion
Bar = barbell = mass
Ex: I-131 and Xe-131
What are the four fundamental forces, in declining strength?
Strong nuclear > Electromagnetic/Coulombic > Weak nuclear > gravity
What equation converts energy to mass and vice versa?
E=mc2
Which number is larger, the atomic mass or the sum of the particles, why?
Atomic mass is slightly smaller as some of that mass is converted to the binding energy of the nucleus
What is the mass (AMU) of an electron?
0.005 AMUs (1/2000th of a proton)
What is the energy equivalent (MeV) of 1 AMU? 1 electron?
931.5 MeV, 0.511 MeV
What is the expected relationship of the neutron:proton ratio?
The ratio is approx 1:1 as the nucleus becomes larger (Z=20, Calcium). For elements heavier, n:p >1
What is the significance of the n:p ratio above Bismuth (Z=83)
N:P ratio >1, up to a point where every increase yields a decreasing binding energy per nucleon. Above 83, the nuclei become unstable and decay
What impacts the electron binding energy of an atom?
The closer to the nucleus, the higher the BE. The more charge in the nucleus (Z), the higher the BE.
How is an Auger electron formed? What is its energy?
When an inner orbital electron is lost, an outer orbital electron drops into that shell. Characteristic energy is lost in the outer-inner transition. If this energy leaves the atom, it is a photon ‘characteristic x-ray’. In some cases, this excess energy is transferred to another orbital electron, giving it enough energy to eject. The ejected electron is an ‘Auger electron’.
The kinetic energy of an Auger electron is the difference in binding energy between the outer-inner shells (initial energy released) minus the binding energy of the auger electron.
Describe alpha decay
Since particles are stable in pairs, a large heavy atom emits a pair of neutrons and protons (2n, 2p, He). The decay energy is split between the daughter nucleus and alpha particle (typical energy is 2-8 MeV)
Describe beta particle decay
A nucleus is unstable and wants to convert a neutron to a proton or vice versa. In either decay, mass is not lost (isobaric). A positron/electron is produced, and an anti-matter particle.
What is the beta-minus decay?
A neutron turns into a proton. To preserve charge, an electron is released (hence the -minus decay) as well as an antineutrino (antimatter)
What is beta-positive decay?
A proton turns into a neutron. An anti-electron (positron) and neutrino are formed.
What happens with the particle produced in beta-positive decay?
A positron is released, which annihilates with a nearby electron almost immediately. With annihilation, all mass is converted to energy. Since the rest mass energy of each particle is 0.511 MeV, two 0.511 MeV photons are produced
What is electron capture?
Similar to beta-positive decay, but with lower available energy. An orbital electron is consumed by the nucleus, joining a proton to become a neutron. This releases a neutrino (just like beta-positive decay) and a photon. That photon may be emitted (gamma ray) or transferred to an orbital electron ejecting it (internal conversion electron). With the empty inner shell, a second characteristic x-ray (or auger electron) can be produced as well. If an IC electron is ejected, this can produce another characteristic x-ray.
Explain internal conversion
The nucleus has extra energy, and transfers this to an inner shell electron. The inner shell electron is ejected, leaving an unfilled shell. The outer shell electron fills this, producing characteristic x-rays (or Auger electrons)
Ex; I-125 decays by absorbing an inner shell electron –> Te-125 + 35.5 keV. That 35.5 keV is transferred to another orbital electron, lets say with a BE of 8.5 keV –> empty shell + 27 keV electron.
What is the value of 1 Ci is dps? In Bq?
1 Ci = 3.7 e10 disintegrations per second
1 Bq = 1 dps
Thus, 1 Ci = 3.7 e10 Bq
1 mCi = 37 MBq
What is the equation for exponential decay?
A(t) = Ao e^-(lambda t)
What is the equation for half life?
t1/2 = 0.693/t
What is the equation for mean half life?
tavg = 1.44(t1/2)
What is secular equilibrium?
The daughter half-life is much shorter than the parent, producing a bottleneck. After buildup, the daughter activity and half-life are the same as the parent
What is transient equilibrium?
The daughter half-life is similar to the parent, thus the daughter activity builds up as the the parent decays.
How are frequency, wavelength, and light related?
c = v lambda
What is the equation for energy of a photon?
E = hv E(keV) = 1.24/lambda
What approximate energy is needed to ionize?
10-30 keV
How do electrons produce x-rays?
As electrons bombard a target, their close approximation with matter (high Z material) causes deceleration and loss of photons (‘braking radiation’, brehmsstrahlung)
What is the significance of the angle in an x-ray tube target?
X-rays preferentially come out at a right angle; thus a slight angle will allow a small ‘focal spot’.
How are electrons produced/modulated in an electron gun?
A tungsten filament is heated, giving off electrons by thermionic emission. The speed of these electrons can be increased by increasing the tube voltage. The frequency of emission can be increased by increasing the current through the filament.
What is the function of the klystron?
Low power microwave are used to ‘bunch’ electrons in a drift tube. As they exit the tube they emit high powered microwaves. Thus, a klystron amplifies microwaves.
What is the role of a waveguide?
The waveguide carries high powered amplified microwaves to the accelerator tube, where they then accelerate electrons toward the speed of light
What is the sequence of applicances in a treatment head
Target > flattening filter > primary collimator > ion chamber > secondary collimator > MLCs
For electrons, the target/flattening filter are replaced by a scattering foil
What is a microtron?
This is a specialized linear accelerator where electrons travel in a circle. Each pass around the circle re-enters them into the linear accelerator to pick up more energy. Electrons of a desired energy (up to 1500 MeV!) can be collected by moving the collection duct to various loop diameters
What is a cyclotron?
Alternating D-shaped electrodes accelerate a particle in the setting of a static perpendicular magnetic field. Primarily used to accelerate protons
What is a synchotron?
Essentially the LHC; a long loop with magnetic segments where the segments can be alternated to accelerate a particle to GeV energies.
