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Topic 2 - Radioactivity by

Nuclear Nomenc­lature

Ground state: Energy tends prefers arrang­ements where the lowest energy state is achieved.
Radi­oac­tiv­ity: nucleus undergos a reaction to lower its energy and become more stable
Nucl­ide: an atomic species with a definite number of protons and neutrons arranged in a definite order in the nucleus.
Radi­onu­cli­de: nuclides that are unstable and thus decay by emission of particles or electr­oma­gnetic radiation.
Isom­er: nuclides with the same number of protons and neutrons, but different energy states.

Nuclear binding energy

The energy that would be required to disass­emble the nucleus of an atom into its component parts.

Mass Deficit

The mass of a nucleus is always less than the combined masses of the nucleons (protons + neutrons). The difference in mass is termed the mass deficit
[protons + neutrons + electrons] - [mass of atom] = [mass deficit]
The average binding energy of a nucleon = the total binding energy (calcu­lated from the mass deficit) divided by the number of nucleons.

Radioa­ctivity

Radi­oactive decay: an unstable nucleus (the parent) decays into a more stable product (the daughter).
Decay energy: Nuclear energy is released as the daughter is at a lower energy level than the parent, this is in the form of particles, electr­oma­gnetic radiation (usually gamma rays) or kinetic energy
1. Partic­les: alpha (2 protons, 2 neutrons); beta- β+ (positive electron – positron) and β- (negative electron); Neutrons
2. Electr­oma­gne­tic: gamma rays; x-rays
Total radioa­ctive decay constant λ: a charac­ter­istic parameter for each radioa­ctive decay process, indepe­ndent of the age of the radioa­ctive atom and is essent­ially indepe­ndent of physical conditions

Decay rate

Decay rate/A­ctivity

For a sample of N radioa­ctive atoms
λ is the decay constant and has a charac­ter­istic value for each radion­uclide

Physical Half-life

Time required to reduce its initial disint­egr­ation rate (activity) to one half.

Nuclide Activation

Radi­oac­tiv­ati­on: Stable nuclei may be transf­ormed into unstable radioa­ctive nuclei by bombar­dment with suitable particles or photons of approp­riate energy
Nuclear reactors are the main source of radion­uclides used in medicine.

Activation with Thermal Neutrons

This uses neutron activation or neutron capture and produces neutro­n-rich unstable isotopes that decay through β− decay into more stable config­ura­tions
Two types of neutron activation processes occur commonly: (n, γ) and (n, p)
(n, γ) process results in emission of γ rays
(n, p) process results in emission of protons
This results in a mixture of stable parent nuclei and radioa­ctive daughter nuclei. The parent act as carriers of the daughter and decrease activity.
Tc-99m is the radion­uclide that is used in more than 90 percent of all nuclear medicine proced­ures
 

Activation with Proton­s/H­eavier Charged Particles

Protons produced by cycl­otr­ons are used in the production of proton­-rich unstable radion­uclides that decay through β+ decay or electron capture into more stable config­ura­tions.
When striking a target material, protons may cause nuclear reactions that produce radion­uclides in a manner similar to neutron activation in a reactor.
Because of their positive charge, protons striking the target must have high kinetic energies to penetrate the repulsive Coulomb barrier surrou­nding the positively charged nucleus.
Proton activation reactions are endo­erg­ic: energy must be supplied by the projectile for the reaction to occur.
Thre­shold energy: minimum energy that will allow the reaction to occur

Decay Schemes

Decay by Alpha Emission

The nucleus ejects an α-particle which consists of two neutrons and two protons (4He nucleus)
It is a ‘massive’ particle absorbed ~ 0.03 mm in body tissues
Common among very heavy elements.
The daughter is often in an excited state resulting in the emission of gamma rays.

Alpha Decay

Decay by β− Emission

When a radion­uclide is neutron rich (that is, it has more neutrons than the stable isotope), it decays by the emission of a beta particle (β) and an antine­utrino.
a neutron is converted into a proton so that the atomic number Z of the product increases by one.
A β particle is also an electron
A β particle originates from within the nucleus whereas an electron originates from the extran­uclear electron orbitals.
Anti­neu­tri­no: a particle which is is non-re­active, has no charge and essent­ially no mass
The beta particle and the antine­utrino are ejected from the nucleus with kinetic energy equal to the energy released in the process (ie. they carry away some of the energy released in the decay process)

β− Decay

Decay by (β-, γ) Emission

Decay by beta emission can result in a daughter nucleus in an excited state
This excited state will decay to the ground state by the emission of γ-rays as the nucleons fall to a lower energy state.

(β-, γ) Decay

 

Isomeric Transition (IT)

A daughter nucleus may be formed in a 'long-­lived' meta­stable state, hen decay by the emission of a γ-ray.

Isomeric Transition Decay

Electron Capture (EC) and (EC,γ) Decay

When a radioa­ctive nucleus has fewer neutrons than its stable isotope, it can undergo decay by electron capture
An orbital electron is 'captured' by the nucleus and combines with a proton to form a neutron.
The daughter product is often in an excited or metastable state, therefore gamma rays may also be emitted.

Electron Capture

Positron (β+) and (β+,γ) Decay

Neutro­n-d­efi­cient (proto­n-rich) radion­uclide
A proton is converted to a neutron, a positron and a neutrino, thus decreasing the atomic number of the daughter nuclide by one.
The β+ particle (positron) collides with an ordinary electron in an annihi­lation reaction in which the combined mass is converted into energy.
The energy appears in the form of two annihi­lation photons (each 0.511 MeV) travelling in opposite directions (180° apart).
The daughter may be left in an excited state resulting in additional γ-rays being emitted, i.e. (β+,γ) decay.

Positron (β+) and (β+,γ) Decay

β+ Decay and Electron Capture

Positron emission and electron capture have the same effect on the parent nuclide, they reach the same endpoint
If the decay is by electron capture, then either x-rays or Auger electrons will be emitted, not positrons.
Positron decay occurs more frequently among lighter elements
Electron capture is more frequent among heavier elements where orbital electrons are closer to the nucleus and more easily captured.
Some radion­uclides can decay by either mode and the percentage for each process is fixed for a given radion­uclide.

β+ Decay and Electron Capture

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