Nuclear Physics

Discovery of the nucleus• Rutherford carried out experiments to see what

happened when alpha particles (2 neutrons and 2 protons) were fired at metal foil.

• His Discoveries:• Most alpha particles passed straight through the foil• A few were deflected through a small angle• About 1 in 1000 were deflected straight back

through 180 degrees.

What this told us about the nucleus

• The inside of atoms must contain small positively charged nuclei as a few of the alpha particles were repelled and deflected.

• The nucleus is very small as only a few alpha particles were deflected.

• The nucleus is concentrated in a tiny volume as only a tiny percentage are deflected back toward the observer.

Isotopes

• Isotopes of a certain element have different numbers of neutrons in their nucleus, but the same number of protons.

• The mass number (A) will be different but the atomic/proton number (Z) will be the same.

Beyond fundamental particles

• There are two distinct groups of particles.• HADRONS/BARYONS: Particles affected by the

strong nuclear force (protons/neutrons)• LEPTONS: Particles unaffected by the strong

nuclear force (electrons)

Inside hadrons

• Inside hadrons are quarks – each hadron is made up of three quarks.

• There are six ‘flavours’ of quarks – up, down, strange, top, bottom, charm.

• Each quark has its opposite antiquark.• The properties of hadrons are the sum of the

properties of its constituent quarks.

Quark properties Charge Baryon no. Strangeness

Up +2/3 +1/3 0

Down -1/3 +1/3 0

Strange -1/3 +1/3 -1

• In any interaction between hadrons charge, baryon number and strangeness are conserved.

• A PROTON is made up of two up and one down quark• A NEUTRON is made up of one up and two down quarks• A PI+MESON is made up of one up quark and one down

antiquark• A PHI MESON is made up of one strange quark and one

strange antiquark

Nuclear density• NUCLEONS (neutrons and protons) are very dense.

They have similar mass so it follows that they have similar density. Their density is approximately 1.5*1017kgm-3.

• Nuclear density is much bigger than atomic density. This suggests:

1. Most of an atoms mass is in its nucleus2. The nucleus is small compared to the atom3. An atom must contain a lot of empty space.

The strong nuclear force• The strong nuclear force is repulsive for very small

separations of nucleons.• After 0.5fm the strong nuclear force becomes

attractive, and reaches a maximum attractive value before falling rapidly towards zero after 3fm.

• The repulsive electrostatic force between protons extends over a much larger area but is weaker than the strong nuclear force.

• The strong nuclear force must be an attractive force greater than the electrostatic force of repulsion between two protons – approximately 90N.

The strong nuclear force• It can only hold the nucleus together if its diameter

is less than 10fm.• The strong nuclear force must be repulsive at very

small separations otherwise it would crush the nucleus.

• If a nucleus has more protons than 83, the strong nuclear force cannot hold it together and it becomes unstable and emits radiation, this is because the electrostatic force of repulsion between the protons becomes too great.

The strong nuclear force

Alpha (α) Radiation

• A 42He nucleus.

• Most strongly ionising• Shortest range in air – 10cm• Can be absorbed by a sheet of paper

Beta (β) Radiation

• Β+ is a positron and is emitted when a proton decays into a neutron

• Β- is an electron and is emitted when a neutron decays into a proton

• Fairly ionising• Range of a few metres in air• Absorbed by a thin sheet of aluminium

Gamma (γ) Radiation

• Photons of high energy electromagnetic radiation

• Emitted after α and β radiation to release excess energy from the nucleus

• Also emitted when a positron collides with an electron

• Can never be fully absorbed• Reduced to safe levels by 10cm of lead.

Radioactive Decay• Nuclear decay is spontaneous and random• We cannot predict when an individual nucleus will

decay, however we can work out the probability that a nucleus will decay in a given time period.

• This is the decay constant – λ• It is worked out by measuring how many nuclei

have decayed out of a known number in a sample in a certain time.

Radioactivity

• The ‘activity’ of a radioactive sample is measured in Bequerels (Bq).

• 1Bq is one decay per second• Also known as the count rate

Random• Nuclear decay is spontaneous and random because:• The decay of one nucleus does not affect any other• The decay of one nucleus is not affected by any

external factors (pressure/temp etc.)• Each nucleus in a sample has the same chance of

decaying per unit time• This makes it impossible to predict when any

particular nucleus will decay

Half-life (t1/2)

• The half life of a sample is the time taken for the number of nuclei/count rate to decrease by half.

• Nuclear decay is exponential• The mathematical formula for nuclear

exponential decay is:

Half-life (t1/2)

• Half life can be worked out using the previous equation.

• =0.5

Half-life

Mass-energy• Mass is essentially the same as energy• If a chemical undergoes an exothermic reaction (i.e.

loses energy), the product will have slightly less mass than the reactant.

• The same theory applies with nuclei• If you split a nuclei into its constituent nucleons, you

have to do work to overcome the strong nuclear force (i.e. put energy in), so the individual nucleons will be heavier than the original nucleus.

• Einstein related mass and energy in his famous equation:

•

Binding Energy

• The mass deficit of a nucleus is defined as the difference between the total mass of the individual separate nucleons and the mass of the nucleus itself.

• The binding energy of a nucleus is the energy needed to break the nucleus into its individual nucleons. However binding energy is often measured per nucleon.

Binding Energy

Nuclear Fission• Nuclear fission is when an unstable nuclei (proton number

above 83) splits into two smaller nuclei, releasing energy.• This usually occurs when a neutron collides with the

unstable nucleus.• When an unstable nuclei splits into two smaller, stable

nuclei, more neutrons are released which can go on to create further fission.

• In nuclear power stations Uranium235 is normally used.• Energy is released in nuclear fission because the binding

energy of the products are higher. Also, the mass of the products is less than the mass of the original nucleus.

Nuclear Fission

• The energy released from fission is used to heat water and turn it into steam which drives a turbine to produce electricity.

• MODERATORS inside the reaction chamber slow down the neutrons released from fission reactions, so that they are more likely to collide with other nuclei. The moderators PROMOTE fission. The slower moving neutrons are called thermal neutrons.

Nuclear Fission

• CONTROL RODS which can be raised or lowered into/out of the reaction chamber absorb neutrons produced from fission reactions, and hence slow down the reaction. They are INHIBITORS of fission. They are often made of Boron and are used to regulate the rate of the reaction or stop it completely if need be.

Nuclear Waste

• When the fission reaction stops, the spent fuel rods contain the fission fragments which are highly radioactive

• Depending on the fragments that the fuel rods contain, some will decay rapidly and remain hot, so they must be kept cool for months afterwards

• Some decay very slowly and remain a hazard for thousands of years

Nuclear Waste

• The fuel rods may be stored on the surface where they can be monitored

• Or they can be stored deep underground which raises concerns about leakage into water supplies.

• Any method used must ensure that the radioactive fuel rods are not exposed to the environment as this could put human health at risk

• This is one of the main problems with nuclear power, and adds greatly to the cost of generating electricity this way

Nuclear Fission