Concepts Flashcards
Nuclide vs Nucleons
Electrons determine the chemical properties of an atom.
“Nuclide” refers to the composition of the nucleus (number of protons and neutrons).
“Nucleons” include protons and neutrons.
A is Atomic Mass Number
Z is Atomic Number (Total protons)
The four “isos”:
A. Isotope: same number of protons, different neutrons (Same chemical behaviour, different mass & nuclear decay properties)
* Ex: 125I and 131I
B. Isotone: same number of neutrons, different protons.
C. Isobar: same number of nucleons, different nuclide. (more protons and less neutrons, or vice versa)
* Beta decay & electron capture always result in an isobar.
* Ex: 131I decays to 131Xe,
D. Isomer: same nuclide, different energy state. (excited vs. non-excited)
* Isomers release their energy through gamma decay
* Ex: 99mTc decays to 99Tc
The Four Fundamental Forces
In order of descending strength these are:
1. Strong Nuclear Force:
(Holds the nucleus together, counters the repulsive effect of protons’ positive charge)
- Electromagnetic (Coulombic) Force (Opposites attract)
- Weak Nuclear Force:
(Works inside particles (b/w quarks) and is responsible for radioactive
decay. - Gravity: (Not important on the atomic scale)
Decay energy sharing
In α decay: almost all of the energy goes into the α particle.
In γ decay: all of the energy goes into the photon (or an internal conversion electron, in internal conversion).
In β decay: energy is shared between a beta particle and a neutrino or an antineutrino.
Alpha (α) Decay
(2 neutrons and 2 protons)
“The nucleus is bloated, proton heavy and wants to lose weight!”
- Occurs in very heavy (Z > 52) nuclei
- Decay energy is split between the daughter nucleus and the alpha particle, but almost all of it goes to the alpha particle.
Typical alpha energies range from 2 to 8 MeV.
Alpha particles are monoenergetic.
Beta-Minus (β-) or Negatron Emission
“The nucleus is proton-poor and wants more protons!”
neutron turns into a proton, the nucleus spits out an electron and an antineutrino
Atomic number goes up by 1; mass stays the same.
Beta-Plus (β+) or Positron Emission
“The nucleus is proton-rich and wants more neutrons!”
Protons turns into a neutron, It spits out an anti-electron (positron) and a neutrino
Atomic number goes down by 1; mass stays the same.
Positrons are antimatter with mass! When the positron meets a regular electron, it annihilates!
This releases the electron and positron’s mass as energy.
The rest mass of an electron is 0.511 MeV; so two identical 0.511 MeV
photons are emitted in opposite directions. This is useful for imaging (PET scans).
Since the positron carries 1.02 MeV of annihilation energy, it costs 1.02 MeV
to make the positron.
Therefore, the kinetic energy is 1.02 MeV less than if the particle had
decayed by electron capture.
Electron Capture (EC)
“The nucleus is proton-rich and wants more neutrons!”
- When nucleus is proton rich but does not have an excess of 1.02 MeV, the
nucleus eats one of its electrons.
During electron capture it emits a neutrino, but most of the decay energy
remains in the daughter nucleus.
This energy is immediately emitted as a gamma ray or as internal conversion electrons.
- Proton-rich nuclei with insufficient energy to produce a positron can only
decay by electron capture.
More energetic nuclei can decay by either beta-plus or EC.
- Because there is a vacancy in one of the inner electron shells, one of the outer shell electrons will fall into this vacancy and produce characteristic X-rays or Auger electrons.
Electron capture with internal conversion results in two electron vacancies!
Gamma Emission
“The nucleus is excited and wants to settle down!”
Excess energy is released as a photon.
* Gamma emission is always isomeric
- Most gamma emission occurs during or after alpha or beta decay:
When the nucleus is excited it gets rid of energy through gamma emission or
internal conversion.
* For example, 60Co decays to an excited 60Ni which immediately releases the excess energy as γ ! rays.
The β- rays are negligible (they do not escape the machine head) so we
call 60Co a gamma emitter with 2 photon energies of 1.17 and 1.33 MeV
(average 1.25 MeV).
- Metastable nuclear isomers such as 99mTc can exist in an excited state and
emit gammas more gradually
Internal Conversion
“The nucleus is excited and it kicks out an electron!”
Instead of releasing excess energy as a photon, the nucleus transfers that
energy to an inner shell electron.
This electron is not a beta particle. It does not emerge from the nucleus, it is
a pre-existing electron.
- For example, 125I decays by electron capture with energy of 35.5 keV.
(see above)
Most of the time, it releases internal conversion electrons instead of
gamma rays.
If 35.5 keV is used to eject an electron with a binding energy of 8.5 keV, the
electron is emitted at 27 keV.
Since different electrons have different binding energies, IC results in a
spectrum of energies.
- This produces a vacancy in an inner electron shell. Outer shell electrons will fill the vacancy, producingcharacteristic X-rays or Auger electrons.
The useful radiation from an 125I brachytherapy seed is actually characteristic X-rays
