P7 - Radioactivty

• What did Dalton think the atom was and what did his model not include

  He thought that the atom was a solid sphere that could not be divided into smaller parts

  His model did not include protons, electrons or neutrons
• What was the plum pudding model

  Sphere of positive charge with negative electrons embedded in it
• What were the results and conclusions from the alpha scattering experiment

  Some of the particles went straight through, some were deflected and some bounced straight back

  Scientists suggested that the positive charge and the mass of an atom must be concentrated at the centre of the atom
• What did the nuclear model show

  That electrons orbit the nucleus but not at fixed distance and the mass of the atom was concentrated in the charged nucleus
• What did Bohr's model show

  That electrons did actually orbit the nucleus at a fixed distance and there were called energy levels/shells.
• what is the structure of the nucleus

  Has a radius about 10,000 smaller then the radius of the atom

  Contains protons and neutrons

  Is where most of the mass in concentrated
• What is the structure of an electron

  Orbits the nucleus at fixed distances called energy levels/shells

  They can gain energy by absorbing electromagnetic radiation. This causes them to move to a higher energy level.
• What is radioactive decay

  When nuclear radiation is emitted by unstable atomic nuclei so that they become more stable

  This is a random process.

  This radiation can knock electrons out of atoms in ionisation.
• Describe alpha particles


  Nucleus loses 2 protons and 2 neutrons

  Highest ionising power

  Travels a few cm in air

  Is stopped by a sheet of paper
• Describe Beta particles

  A neutron changes into a proton and a electron

  High ionising power

  Travels about 1m in air

  Stopped by a few mm of aluminium
• Describe Gamma radiation


  Some energy is transfered away from the nucleus

  Low ionising power

  Virtually unlimited range in air

  Stopped by several cm of thick lead or a few m of concrete
• What is activity and count rate measured in and how is it measured.

  Measured in Bq and is detected by a Geiger-muller tube
• How do we protect against irradiation and contamination

  Maintain a distance from the source

  Limiting time near the source

  Shielding from radiation
• What is the difference between irradiation and contamination

  Irradiation is when an object is exposed to nuclear radiation. Can cause harm through radiation. Is prevented by shielding or moving away from the source.

  Contamination is when atoms of radioactive material are on or in an object
• How do we use nuclear radiation in medicine for the exploration of internal organs

  Gamma-emitting tracers are injected or swallowed by the patient. Gamma cameras can then create an image showing where the tracer has gone.

  The half life of the tracer must be short enough so most of the nuclei decays shortly after the image has been taken.
• How do we use nuclear medicine in the control or destruction of unwanted tissue

  Narrow beams of gamma radiation aimed at the tumour cells to destroy them. Gamma is used because it can penetrate the body from the outside.

  Beta of Gamma-emitting implants can be surgically placed in or next to tumours. Half-lives must be long enough to be effective but short enough to not to continue to irradiate the patient.
• Explain ionising radiation

  Living cells can be damaged or killed by ionising radiation

  The risk depends on the type of radiation and the half life of the source.

  Alpha radiation inside the body is very dangerous because it effects the surrounding tissue. Outside the body it only effects the skin and the eyes because it can't penetrate any further.

  Beta and gamma radiation aren't as dangerous outside and inside the body because they can't penetrate into the tissue.
• What is background radiation and where does it come from

  Radiation that is around us all the time.

  Comes from natural sources like rocks and cosmic rays, nuclear weapons and nuclear accidents.
• What is nuclear fission

  Nuclear fission is when a large unstable nucleus absorbs an extra neutron and splits into 2 smaller nuclei of roughly equal size.

  During nuclear fission:

  Gamma radiation is emitted and energy is released

  2 or 3 neutrons are emitted that can go on to cause a chain reaction.

  The chain reaction in a power station reactor is controlled by absorbing neutrons.
• What is nuclear fusion

  Nuclear fusion is when two light nuclei join to make a heavy one

  Some of the mass is converted to energy and transferred as radiation.