Physics Flashcards
Atomic number = # protons (lower left)
Atomic mass = # protons plus neutrons (top left)
isotoPe = same # of protons, i.e. same element but different masses, i.e. diff # neutrons
isotoNe = same # neutrons
isobAr = same Atomic mass i.e. same total sum of protons + neutrons
isoMer = is in a Metastable state, eg Tc (99m) i.e. same element but energy in the nucleus is slightly different
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Characteristic X-rays are emmission from an outer e- event, i.e. Inner shell e- is ejected, vacancy is filled by one dropping down from outer shell
X rays are one type of “photon” images in nuclear medicine (Thall gives off 70 keV X-rays)
Auger e- is when first events are same (e- ejected from inner shell, then outer e- drops down to fill its place), but instead of the released energy leaving the atom as Xray photon, it gets transferred to another e- (Auger) which is then ejected
“Particle enmission” vs photon emmission
We image photons in SPECT, not particles
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Emissions coming from events inside the nucleus
Protons and neutrons in nucleus are subject to Coulombic forces–otherwise protons would repel each other due to positive charges and wouldn’t stay packed tightly in small space of nucleus
When these nuclear forces are equal = stable state (“ground” state).
When forces are not balanced = one of two alternate states:
Excited state–fleeting bc very unstable, and quickly goes to more stable state
Metastable state–longer lived and can be used in imaging
aka “isoMers”
eg Tc99m
What determines whether nuclear forces are perfectly balanced/stable, or unstable, or Metastable, is the proton:neutron ratio
Neutrons generate the binding energy on the protons
When isomer transitions to more stable state it gives off energy = gamma ray = photons (or emits a particle)
Unstable isomers can decay via several different modes: beta-decay (Moly-Tc generator), internal conversion (Tc), isomeric transition (Tc), electron capture with gamma emmission, positron decay (PET)
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Beta decay:
A neutron is changed into a proton and electron (called beta particle);
Atomic mass is unchanged, atomic number goes up by 1–ie new element formed
Occurs in neutron rich nuclei
If this decay results in daughter isotope that’s also unstable, it then releases gamma ray photon when it next decays
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Tc undergoes Isomeric Transition (IT) and Internal Conversion (IC)
IT is when metastable parent Tc99m changes to more stable Tc99, releasing photons (gammas) when it does so
IC is when nucleus transfers energy to inner orbital ring e- (uses the same “Transition energy” that would’ve been released via IT, but instead imparts this amt of energy to e-, causing it to break free of shell and be emitted
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Electron capture
An orbital shell e- is captured by nucleus and combines with 1 proton to form 1 neutron
New element is formed with decreased atomic number by 1
Opposite of Beta decay
Occurs in proton-rich nuclei
Process often is accompanied by release of characteristic X-rays, when outer shell e- drops down to fill vacancy of the e- captured by the nucleus (happens in the new element formed from the initial event)
Thallium decays via this mode
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Positron decay
1 proton transformed into 1 neutron + 1 positron (e+)
Atomic number decreases by 1 (new element formed)
Occurs in proton rich nuclei
The positron immediately attracts electron in atmosphere–> the two undergo annihilation reaction, i.e. the matter of each is converted entirely into energy in the form of 2 photons of gamma rays oriented at 180 degrees to each other (each with 511 keV energy)
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Decay
Half life = 0.693/gamma sign
gamma sign = decay constant = the fraction of atoms that decay per a unit time
Number of atoms left at a given time point:
Nt = N0 x e (raised to exponent of - (decay constant x time (t))
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