Radioactivity (Lecture 3) Flashcards
Define “isotope” (2)
Isotopes are nuclei with the same number of protons but a different number of neutrons. They have the same chemical properties but different nuclear properties.
Describe the two main forces acting in the nucleus of atoms.
Attractive strong nuclear force between nucleons - only acts over small distances
Repulsive electrostatic force between protons - acts over a larger distances
[Heavier nuclei need excess neutrons to overcome the long-range repulsion between protons.]
What ratios of N/Z give stability for small and large atomic mass numbers?
Small A N/Z = 1.0 gives stability
Large A N/Z = 1.5 gives stability
Describe what is meant by “ground state” (2).
A stable arrangement of nucleons.
This is the lowest energy state of a system.
Describe what is meant by “excited state”
Any quantum state of the system that has a higher energy than the ground state (unstable) and may transform into some other state.
Describe what is meant by “metastable state” (2)
A stable state of a dynamic system other than the system’s ground state (least energy).
This leads to an unstable arrangement with a relatively long lifetime before transforming.
What is radioactive decay?
The transformation of an unstable nucleus to a stable state with the emission of particles, photons, or both.
Describe alpha decay, including which particles are emitted and what happens to the atomic and proton numbers .
Alpha decay is when heavy nuclei eject an α-particle (a helium nucleus).
The proton number drops by 2, the atomic number drops by 4.
Describe β- decay, including which particles are emitted (4), and what happens to the proton and atomic numbers?
β- decay can be considered as a neutron splitting into a proton and an electron.
A high energy electron (β- particle) and electron-antineutrino are emitted.
This conserves the lepton number when the number of electrons is increased.
The proton stays in the nucleus, which increases the proton number of the atom by one. The atomic number does not change.
Why do electrons from β- decay have a spectrum of energies?
The β- particles (high energy electrons) from β- decay have a spectrum of energies because the released energy is shared between the β- particle and the electron antineutrino.
What is the mean energy of the particles produced by β- decay?
1/3[Emax].
What are isomeric transitions?
Isomeric transitions are the decay of a metastable state to a stable state with the emission of a gamma ray, originating in the nucleus.
Why are isomeric transitions important in Nuclear Medicine?
They only emit gamma rays and have no particulate emissions.
Describe electron capture.
A process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. This forms a neutron and an electron neutrino, which is ejected from the atom’s nucleus.
Is electron-capture or positron decay (β+ decay) more likely for nuclei with larger Z values?
Why? (2)
Positron (β+) decay is more likely for nuclei with larger Z-values than electron capture.
This is because:
β+ decay can occur spontaneously when energetically allowed
For electron capture the weak force requires that the electron must come into close contact with protons in the nucleus, and the probability of this happening is low.
