Physics Flashcards
Equation for maximum number of electrons for each shell.
2n^2
K=2(1)^2=2
L=8
M=18
Number of substrates for each shell.
2n-1
A: atomic mass
P+N
Z
Number of protons
N
Number of neutrons
Isotopes
Same number of protons, Z
Isotopes
Same number of neutrons,N
Isobars
Same number of atomic mass, A
Line of stability
Low z number –>p:n is 1:1
High z number –> p:n is 1:1.5
Radioactive element seeks stability, this transition is called mode of decay
Modes of decay
b- decay b+ decay Electron capture Isomeric transition/internal conversion Alpha decay
b- decay
n –> p + e- + v + energy
V =neutrino, behaves like a particle with no mass and is not critical to imaging consideration
Daughter has an extra proton
Z+1 and N-1
Atomic mass same
B- particle is of no use in imaging and may contribute to increase radiation dose
b+ decay
P –> n + e+ + v + energy
b+ particle (positively charged electron) will be attracted to and collide with a free negatively charged electron –> annihilation of both particles –> conversion of mass to energy state, E=mc2
Annihilation produce 2 photons, 511 keV, emitted 180 degree from each other. This is what is registered into an image in positron imaging, PET
Z-1
N+1
Atomic mass same
Electron capture
P + e- –> n + v + energy
Vacancy left by captured electron would be filled by an outer shell electron and lead to cascade of an electron to fill vacancy, will lead to emission of characteristic X-ray and Auger electrons
201Tl - characteristic X-rays that are imaged
Z-1
N+1
Atomic mass same
Isometric transitions and internal conversions
Excited state/metastable state exist for very short periods, less than 10^-12 sec, but may exist for hours, this lead to release of energy in form of radiation without changing p to n ratio. This is called isometric transition and results in electromagnetic emissions called gamma rays; same as X-rays but differ from origin, which is from nucleus
There is also a competing process called internal conversion
Ex. 99mTc –> 99Tc; isometric transition is 87% and rest are internal conversion
Alpha decay
Occur in unstable nuclei with high atomic masses
Alpha particle consists of 2 protons and 2 neutrons, helium nucleus; travels shirt distance given high charge and heavy mass.
No application in imaging
Z-2
N-2
A-4
B- decay of 99Mo yields 99Tc, which then decays to 99mTc by isometric transition/internal conversion
Sample of 99Mo will always have same proportion of 99mTc and 99Tc.
Since both parents are decaying, would reach equilibrium based on half lives.
This is employed by both technetium and rubidium generators
Parent-daughter equilibrium
Transient equilibrium
When parent half-life is marginally longer than daughter, amount of daughter in mixture will reach a maximum over a period of time.
Elapsed time will be multiple if half lives if daughter.
Equilibrium is reached after relatively few daughter half lives have passed
Basis of 99Mo-99mTc generators; four 99mTc half lives
Secular equilibrium
Half life of parent is markedly longer than that of daughter.
Since parent is decreasing so slowly relative to daughter, mixture appears to have half life of parent
Basis of 82Sr-82Rb generators used in PET imaging
82Sr 25 days
82Rb 1.2 min.
Many half lives of daughter must pass before equilibrium is reached
Number of decays for a given time equation
N(t) = N(0) e^-decay constant x time
N(t) is number of unstable nuclei remaining after elapsed time
Decay factor
e^-(decay constant x t)
Half life and decay constant relationship
T1/2= 0.693/decay constant
Unit of activity, A
Same equation as nuclear decay
Particles interactions with matter
Alpha or beta particles interact with matter through collisions, resulting in excitation, ionization, or bremsstrahlung.
