PHYSICS NUCLEAR RADIATION

Y2

Nuclide notation

The top number is the nucleon number A, the number of protons and neutrons, and the bottom number is the proton number Z

Nucleon

A proton or neutron

Isotope

Atoms with the same number of protons but different numbers of neutrons, so different nucleon numbers

E =

c²Δm

Mass deficit

The difference between the sum total of the masses of the individual nucleons that make a nucleus and its measured mass.

Mass deficit =

(Number of protons x mass of a proton) + (number of neutrons x mass of a neutron) - mass of the nucleus

Mass deficit is always positive because...

The sum of the individual nucleon masses is greater than the measured mass of the nucleus. This mass is converted to energy and released when the nucleus forms.

Binding energy

The energy equivalent of the mass deficit.

The energy required to separate a nucleus into its individual nucleons.

or the energy released when a nucleus is formed from its constituent nucleons.

Measured in J or eV

1 atomic mass unit =

1/12th the mass of a carbon-12 atom. Roughly the mass of one proton or neutron

To convert from amu to kg, you...

Multiply by 1.66 x 10^27

To convert from kg to amu, you...

Divide by 1.66 x 10^27

The mass of an atom in amu is roughly equal to...

Its nucleon number, with actual mass being slightly lower due to the mass deficit

Total binding energy increases with...

Nucleon number as bigger nuclei have more nucleons to be separated.

This means it cannot be used to compare stability of nuclei.

Binding energy per nucleon

The average energy required per nucleon to separate a nucleus into its individual nucleons.

Used to compare stability of nuclei.

Has the same units as binding energy (J or eV)

Binding energy per nucleon =

Total binding energy/number of nucleons

A larger binding energy per nucleon means the nucleus is...

More tightly bound and more stable as more energy is required to separate its nucleons on average

Both fission and fusion can release energy because...

Any nuclear process that produces more stable nuclei results in a higher binding energy per nucleon. The products are more tightly bound and have a lower total mass, so the decrease in mass is released as energy

Graph of average binding energy per nucleon against nucleon number

The peak is at Iron-56, the most stable nucleus as it has the highest binding energy per nucleon.

Nuclei left of it will undergo nuclear fusion as larger nuclei will have a higher binding energy per nucleon.

Nuclei right of it will undergo nuclear fission as smaller nuclei will have a higher binding energy per nucleon

Iron cannot release energy because...

It has the maximum binding energy per nucleon so fission or fusion starting from it would move down the curve, decreasing binding energy per nucleon.

Energy would need to be absorbed, not released.

Nuclear fusion

When small-mass nuclei approach sufficiently closely to join and form a more massive nucleus.

The products have a greater binding energy per nucleon, so energy is released.

When fusion occurs, energy is released because...

The total mass of the products is less than the total mass of the reactants, and the mass is converted to energy.

The main barrier to fusion is...

Repulsion. All nuclei are positively charged, so repel each other due to the electrostatic force.

Even though fusion would release energy and increase stability, the nuclei cannot get close enough

Conditions needed for fusion

Extremely high temperatures and densities

Extremely high temperatures are needed for fusion because...

This ensures nuclei have sufficient kinetic energy to overcome the repulsive electrostatic force.

Extremely high densities are needed for fusion because...

The nuclei must be close together to maintain a sufficient collision rate.

Fusion is hard to achieve on Earth because...

The extreme conditions would cause reaction containment issues.

Plasma fusion conditions

Plasma, made of free electrons and positive ions, is used for fusion.

Moving charged particles experience magnetic forces.

Strong magnetic fields help keep the plasma away from the walls.

Less energy escapes through wall contact, helping maintain fusion conditions.

Nuclear fission

When a large nucleus splits into 2 smaller nuclei.

The products have a greater binding energy per nucleon than the reactants so energy is released

Background radiation

The ionising radiation which is always present in the environment, originating from natural and artificial sources, not from the radioactive source being investigated

To improve the reliability of count measurement...

Measure the background count over a longer time interval (at least 60s).

This gives a larger total count, reducing percentage uncertainty

Natural sources of background radiation

Radon gas and gamma rays from rocks and soil, cosmic radiation, food and drink.

Artificial sources of background radiation

Medical (x-rays, CT scans, radiotherapy), nuclear fallout and weapons testing, nuclear industry and accidents

Ionising radiation

Transfers energy to an atom, removing an electron and forming a positive ion.

GM tube

Detects ionising radiation as the gas in the tube is ionised, creating free electrons.

The tube has an electric field, which accelerates the electrons, creating a current pulse

Radioactive atom

An atom with an unstable nucleus that can emit particles or electromagnetic radiation (photons) to become more stable.

Ionising power

How many ion pairs (an ion and electron) are produced by radiation per cm of travel

Alpha particle

2 protons and 2 neutrons or a Helium-4 nucleus

Alpha radiation ionising power

Very high

Alpha radiation range in air

2-10cm

Alpha radiation can be shielded with...

Paper as it has a very low penetration power

Alpha radiation has the shortest range because...

It loses energy quickly due to frequent ionisation

When alpha decay occurs, proton number...

Decreases by 2

When alpha decay occurs, nucleon number...

Decreases by 4

Beta particle

A high-speed electron or positron

Beta radiation ionising power

Medium

Beta radiation range in air

~1m

Beta radiation can be shielded with...

Aluminium foil (~3mm), as it has a medium penetration power

Beta radiation has a shorter range in aluminium than air because...

Aluminium is denser than air, so radiation has more frequent interactions with atoms. This means there is more ionisation per cm, energy is lost more quickly and there is a shorter range

When beta decay occurs, nucleon number...

Stays the same

Beta negative decay

Decay of a neutron into a proton, with emission of an electron and an anti-electron neutrino

When beta negative decay occurs, proton number...

Increases by 1

Beta positive decay

Decay of a proton into a neutron, with emission of a positron and an electron neutrino

When beta positive decay occurs, proton number...

Decreases by 1

Gamma radiation

An electromagnetic wave or high-energy photons

Gamma radiation ionising power

Weak

Gamma radiation range in air

Theoretically infinite

Gamma rays can be shielded by…

Thick lead or concrete, as it has a very high penetration power

Radioactive decay is...

Random and spontaneous.

This means decay is not influenced by external forces and that we cannot predict when an individual nucleus will decay but each nucleus has a fixed probability of decay per unit time.

For large numbers of nuclei, decay is statistically predictable, leading to an exponential decay in activity

Decay constant

The probability of a nucleus decaying per unit time

Activity

Number of radioactive decays per second. Measured in Bq

A =

λN, where λ is the decay constant and N is the remaining nuclei

A =

-dN/dt, where dN is the no. of nuclei that have decayed

-λN =

dN/dt

N =

N₀e^-λt

N = no. of nuclei, N₀ = original no. of nuclei

Half-life

The time taken for half the radioactive nuclei in a sample to decay or for the activity to half

After n half-lives, the number of remaining particles is...

N₀/2^n

After n half-lives, the activity is...

A₀/2^n