Nuclear physics

Explain why the number of collisions needed to do this depends on the nucleon number of the moderator atoms.

A lower nucleon number means less KE is lost so more collisions are needed

Benefits of slowing down neutrons during fission

• U-235/Uranium fuel will (be more likely to) absorb the neutron

• Slow neutrons are less damaging OR cause less fatigue to the structure of the reactor/shielding/etc

• Slow neutrons (spend longer within the fissionable material and) increase the chance of causing fission

• Slowing neutrons transfers heat energy to the moderator (which can make heat easier to extract)

Explain why kinetic energy of neutrons must be reduced in a thermal nuclear reactor

• To increase the probability/chance of fission (when neutron collides with fissile material/U-235)

• To allow the neutron to be absorbed by the fuel/U-235

Decay constant meaning

The probability of a nucleus decaying per unit of time

Properties of alpha radiation

• Strong ionising power

• Slow speed

• Absorbed by paper of a few cm of air

• Affected by magnetic field

• Sources of alpha particles are dangerous if ingested

Properties of gamma radiation

• Weakly ionising

• Speed of light

• Absorbed by many cm of lead

• Radioactive tracers: A radioactive source with a short half life is injected into patient and a detector is then used to detect the emitted gamma rays

• Rotating beam of gamma rays is used to lessen the damage done to surrounding tissue.

• Should be stored in a lead box

Sources of background radiation

• The air

• The ground and buildings

• Cosmic radiation

• Living things

• Man-made radiation

It can be difficult to get a reliable age from radioactive dating as:

• The object may have been contaminated by other radioactive sources

• There may be a high background count that obscures the object’s count

• There may be uncertainty in the amount of carbon-14 that existed thousands of years ago

• The sample size or count rate may be small

Suggest which type of radiation is likely to be most appropriate for the sterilisation of medical instruments

Gamma as it is the most penetrating

Explain why the public need not worry that irradiated instruments become radioactive once sterilised

Ionising radiation does not affect the nucleus which means the atoms in the instruments do not become radioactive

Explain why the metastable form of the isotope technetium is a radioactive source suitable for use in medical diagnosis

• Technetium has a long enough half life for the diagnosis to be performed but short enough it does not stay in the body for a long time

• The metastable form is a pure gamma emitter so weakly ionising so position where gamma rays are emitted can be determined because it is most penetrating

State what happens to a neutron that is incident on the moderator

The neutron undergoes elastic collisions with less speed

A slow moving neutron is in collision with a nucleus of an atom of the fuel which causes fission. Describe what happens in the process

U-235 absorbs slow moving neutron causing the nucleus to split into daughter nuclei releasing several fast moving neutrons

Suggest one use for Strontium-90 which produces beta radiation

Controlling aluminium foil thickness as beta radiation would only be partially absorbed by aluminium foil but is totally absorbed past a certain thickness so changes in absorption can be used to monitor thickness.

State what is meant by the critical mass of nuclear fuel

The mass of nuclear fuel needed for chain reaction to continue at a stead rate where one fission follows another

Describe the function of both the coolant and the control rods in a thermal nuclear reactor

The coolant transfers energy away from the reactor

The control rods absorb neutrons to limit the rate of nuclear fission

Material used in control rods

Boron

3 measures taken within nuclear power stations to limit the risk to employees

• Emergency shut down

• The core of the reactor where fission occurs has a thick concrete casing which absorbs most of the ionising radiation

• Air in the nuclear power plant is filtered to remove radioactive isotopes

Explain what would happen to the nuclear chain reaction within a reactor if the moderator was removed

The neutrons would not be slow enough to cause further fission so the fission reaction would be slowed down

Describe how a moderator slows down neutrons in a nuclear reactor

The neutrons produced by fission undergo elastic collisions with atoms in the moderator. The neutrons lose some kinetic energy to the moderator atoms during the collision so slow down.

State one factor that affects how efficient a material would be as a moderator

The size of the material’s particle, a moderator with particles of a similar mass to a neutrons mass is most efficient

Explain in terms of binding energy, why energy can be released when two nuclei undergo nuclear fusion

In order to release energy the total binding energy of the 2 nuclei < binding energy of nuclei formed.

If binding energy increases reaction releases energy

The mass of the products < mass of the reactants for reaction to release energy.

For too many neutrons what decay mode will occur?

Beta minus emission

For too many protons what decay mode will occur

Beta-plus emission or electron capture

For too many nucleons what decay mode will occur?

Alpha emission

For too much energy what decay mode will occur?

Gamma emission

State what is meant by binding energy

The energy required to separate the nucleus into its constituent nucleons