P5: nuclear physics

Composed of: protons, neutrons -> overall positive charge 

protons = electrons (atomic number, Z)

Nucleon number, A (mass number) = protons + neutrons 

Nuclide notation: a group of atoms containing the same number of protons and neutrons

Isotopes:

some are radioactive 

occur naturally 

relative charge: the ratio of the charge of a particle compared to the fundamental charge (no units) 

proton: +1

neutron: 0 

electron: -1 

Nuclear fission: splitting of a large, unstable nucleus into 2 smaller nuclei 

Nuclear fusion: 2 light nuclei join to form heavier nucleus 

• requires extremely high temperature and pressure 

ionisation: an atom becomes negatively/positively charged by gaining/losing electrons 

background radiation: radiation that exists around us all the time 

natural sources: radioactive elements that have always existed 

man-made: from human activity 

sources: radon gas, rocks and buildings, food and drink, cosmic rays, medical sources, nuclear waste, nuclear accidents 

Detecting radiation: using a counter 

• detecter uses count rate measured in counts/s or counts/min (number of decays per second) 

• decreases the further away the detector is from the source (radiation becomes more spread) 

• Geiger-muller tube detects count rate 

• absorbs radiation -> transmits an electrical pulse to a counting machine 

  • higher count rate = more radiation is being absorbed 

Radioactive decay: change in unstable nucleus that can result in the emission of: 

alpha (a) particles 

beta (B-) particles 

gamma (Y) radiation 

spontaneous and random 

Alpha:

• same as a helium nucleus  

• charge of +2 

• low penetrating (stopped by paper)

• highly ionising 

Beta particles: 

• fast moving electrons 

• -1 charge 

• moderate (stopped by aluminium) 

• moderately ionising

Gamma:

• electromagnetic 

• have highest energy 

• no charge 

• high penetrating (stops at lead) 

• low ionising 

Ionising effect of radiation

• depends on: kinetic energy and electric charge

Nuclear radiation can ionise the atoms it hits by removing an electron so the atom loses a negative charge (positive charge overall)

Effects of kinetic energy and charge on ionising power:

• greater the charge → more ionising it is

• higher the kinetic energy → the more ionising it is

Deflection in electric fields:

Deflection in magnetic fields:

• alpha and beta particles are deflected in opposite directions due to opposite charges

Radioactive decay

• isotopes of an element may be radioactive due to an excess of neutrons → nucleus is too heavy

• too many protons creates a repulsive force → neutrons repel each other

  • decays into several smaller nuclei due to imbalance of protons/neutrons until stable nuclei

• nucleus too heavy → forces keeping protons and neutrons together will be weaker

• mass number of the element reduces during nuclear decay

During alpha and beta decay, nucleus changes to new element

• alpha decay: alpha particle is emitted

• beta decay: neutron → proton + electron

Decay equations:

• Alpha:

• 

Half-life: time taken for half the nuclei of that isotope in any sample to decay

Uses of radiation: the type of raditation emitted and the half-life of an isotope determine which isotope is used

• alpha radiation: fire alarms

  • alpha radiation ionises the air in the detector → creates a current

  • alpha emitter is blocked when smoke enters the detector

  • alarm is triggered when sensor no longer detects alpha particles

• gamma rays: sterilization of equipment and irradiating food to kill bacteria

  • most penetrating → can penetrate enough to irradiate all sides

  • can sterilize without removing packaging

  • kills microorganisms → food lasts longer and reduces food-borne infections

• beta radiation: measuring thickness of materials

  • can be partially absorbed by most materials

  • material moves across a radiation source: if material gets thicker → more particles are absorbed → less gets through to be detected

  • machine makes adjustments to keep thickness of the material constant

• gamma rays: diagnosis and treatment of cancer

  • radiotherapy: kills living cells

  • beams of gamma rays are directed at the cancerous tumor → can penetrate the body and can be moved around to minimise harm to healthy tissue

  • tracer: radioactive isotope that can be used to track the movement of substances around the body

Dangers of radiation: can damage living things such as human cells and tissues at high doses

• cell death

• mutations

• cancer

Safe storage

• lead-lined boxes and keep them at a distance

• minimize the amount of time you handle sources

• keep far away from the sources

• use a pair of tongs at arms length

• transported in special containers like casks

• radioactive waste is buried underground