OCR A alevel Physics -> Radioactivity, Particle physics and Nuclear physics

What were 3 observations of Rutherford particle scattering experiment?

1. Most alpha particles passed straight through the gold foil

2. Some alpha particles were deflected through large angles

3. Very few alpha particles were deflected through angles > 90

What was the conclusion found from the observation that most particles passed straight through the gold foil in alpha particle scattering experiment?

Most of the atom is empty space, with the mass concentrated in the nucleus

What was the conclusion found from the observation that some alpha particles were deflected through large angles in the alpha particle scattering experiment?

The centre of the atom (nucleus) must have a highly +ve charge

What was the conclusion found from the observation that very few particles were deflected through angles > 90 in the alpha particle scattering experiment?

Nucleus must be very small and dense

Size of an atom

10^-10 m

What is the diameter of the nucleus in comparison to the diameter of an atom?

1/10,000 th

What is the name of the isotope hydrogen-1?

Protium

What is the name of the isotope hydrogen-2?

Deuterium

What is the name of the isotope hydrogen-3?

Tritium

Who discovered electrons?

JJ Thompson

What are the four fundamental forces?

• electrostatic

• gravitational

• strong nuclear

• weak nuclear

Describe the nature of the strong nuclear force

• Repulsive at very short distances < 0.5 fm

• Strongly attractive 0.5 - 3 fm

• Rapidly falls to zero > 3 fm

• Acts on quarks and hadrons

Give three examples of quarks

up, down, strange

Give two examples of leptons

electron, electron antineutrino

What is a hadron?

particle made of quarks

What is a meson?

hadron made of a quark and antiquark (held together by strong nuclear force)

What is a baryon?

hadron made of any three normal matter quarks

What is an anti-baryon?

hadron made of any three antimatter quarks

Define beta minus decay

A neutron in an unstable nucleus decays into a proton, an electron and an electron anti-neutrino

What happens to the quarks in beta-minus decay?

down quark decays into up quark

Define beta plus decay

A proton in an unstable nucleus decays into a neutron, a positron and an electron neutrino

What happens to the quarks in beta plus decay?

up quark decays into a down quark

What does alpha decay consist of?

2 protons and 2 neutrons (helium nucleus)

What can alpha decay be stopped by?

A thin sheet of paper

What type of decay is most ionising?

Alpha

What is the range of alpha decay in air?

2cm

What does beta minus decay consist of?

A fast moving electron

What can beta minus decay be stopped by?

3cm of aluminium

What is the range of beta minus decay in air

1m

What does gamma decay consist of?

high energy photons

What can gamma decay be stopped by?

few cms of lead

What is the range of gamma decay in air?

very long

When is radiation emitted?

When a nucleus decays

What is the nature of radioactive decay?

• random → cannot predict when a nucleus will decay / which will decay next

• spontaneous → not affected by presence of other nuclei or external factors

What is count rate?

The number of detected decay events per second recorded by a detector

What is activity?

The actual number of decays per second for a radioactive sample

What is half life?

The average time taken for half the number of nuclei in the sample to decay

What is annihilation?

• particle and antiparticle combine

• releasing their mass as the energy of a pair of photons

• travel with equal speeds in different directions

• mass → energy

What is pair production?

• particle antiparticle pair produced if enough energy is supplied

• energy from gamma photon / KE of particle collision

• energy → mass

What is the mass defect?

The difference between the mass of a nucleus and the mass of its completely separated constituent nucleons

What is binding energy?

The minimum energy required to completely separate a nucleus into its constituent protons and neutrons

Which type of nuclei have the highest binding energy?

Large nuclei

What is binding energy per nucleon a measure of?

how tightly bound nucleons in a nucleus are

What is fusion?

• light nuclei combine to make heavier nuclei

• binding energy per nucleon increases

• energy is released

What is fission?

• heavy nuclei split to make light nuclei

• binding energy per nucleon decreases

• energy is absorbed

Purpose of control rods

• adjusting the rate of fission

• lowered rods = more neutrons absorbed so rate of fission decreases

• raised rods = fewer neutrons absorbed so rate of fission increases

Purpose of the moderator

• slow down fast moving neutrons

• so they can react efficiently with fuel

• neutrons collide with moderator molecules

• so they lose momentum and slow down

Purpose of fuel rods

• regulate rate of fission

• capture excess neutrons within the reactor core

• withdrawn rods = more fission so increased power output

How is nuclear waste disposed of?

• placed in cooling pond for a few years

• plutonium / uranium isotopes collected for reusing

• waste mixed with molten glass and made solid

• encased in steel containers and stored deep underground

How is electricity generated through nuclear fission?

• energy released in fission is trapped as thermal energy

• this is used to boil water producing steam

• which turns a turbine

• powering a generator

Explain the fission process in a nuclear fission reactor

• neutron is absorbed into a nucleus

• forming a highly unstable isotope

• nucleus splits into two daughter nuclei and two or three neutrons which explode out of the fission reaction

• these neutrons collide with other uranium nuclei

• forming a chain reaction