Chapter 7: Radioactivity

Question 1

Atoms and radiation

Answer 1

• A radioactive substance contains unstable nuclei that become stable by emitting radiation
• There are three main types of radiation from radioactive substances – alpha, a, beta, B, and gamma radiation, y.
• Radioactive decay is a random event – you can’t predict or influence when it will happen
• Radioactive sources emit a, B and y radiation

Question 2

The discovery of the nucleus

Answer 2

• Rutherford used alpha particles to probe inside atoms. He found that some alpha particles were scattered through large angles.
• The plum pudding model could not explain why some alpha particles were scattered through large angles
• An atom has a small positively charged central nucleus where most of the atoms mass is located
• The nuclear model of the atom correctly explained why some alpha particles scattered through large angles

Question 3

Changes in the nucleus

Answer 3

• Isotopes of an element are atoms with the same number of protons but different numbers of neutrons. So they have the same atomic number but different atomic masses

See table in documents

Question 4

More about alpha, beta and gamma radiation

Answer 4

• Alpha radiation is stopped by paper and has a range of a few centimetres in air. It consists of particles, each composed of two protons and two neutrons. It has the greatest ionising power.
• Beta radiation is stopped by thin sheet of metal and has a range of about 1 meter in air. It consists of fast-moving electrons emitted from the nucleus. It is less ionising than alpha radiation and more ionising than gamma
• Gamma radiation is stopped by thick lead and has unlimited range in air. It consists of electromagnetic radiation
• alpha, beta and gamma radiation ionise substances they pass through. Ionisation in a living cell can damage or kill the cell.

• Ionisation is when the radioactive substance can make other atoms lose electrons, the atoms become charged because they lose electrons.

Question 5

Activity and half-life

Answer 5

• The half-life of a radioactive isotope is the time it takes for half of the radioactive substance to decay. The time it takes for the number of nuclei to sample to half
• The count rate of a Geiger counter caused by a radioactive source decreases as the activity of the source decreases
• The number of atoms of a radioactive isotope and the count rate both decrease by half every half life
• The count rate after n half-lives = the initial count rate / 2^n

Question 6

Nuclear radiation in medicine

Answer 6

• Radioactive isotopes are used in medicine for medical imaging, treatment of cancer, and as tracers to monitor organs
• How useful a radioactive isotope is depending on:

  o	Its half-life
  
  o	The type of radiation it gives out

• For medical imaging with a radioactive isotope and for medical tracers, the half-life should be not too short and not too long
• A gamma beam or a radioactive implant can destroy cancer cells in a tumour

Question 7

Nuclear fission

Answer 7

• Nuclear fission is the splitting of an atom’s nucleus into smaller nuclei and the release of two or three neutrons and energy
• Induced fusion occurs when a neutron is absorbed by a uranium-235 nucleus or a plutonium-239 nucleus and the nucleus splits. Spontaneous fission occurs with out a neutron being absorbed.
• A chain reaction occurs in a nuclear reactor when each fission event causes further events
• In a nuclear reactor, control rods absorb fission neutrons to ensure that, on average, only one neutron per fission goes on to produce further fission.

Question 8

Nuclear fusion

Answer 8

Nuclear fusion is the process of forcing the nuclei of two atoms close enough together so that they form a single larger nucleus

• Nuclear fusion can be brought about by making two light nuclei collide at very high speed
• Energy is released when two light nuclei are fused together. Nuclear fusion in the sun’s core release energy
• A fusion reactor needs to be at a very high temperature before nuclear fusion can take place. The nuclei to be fused are difficult to contain

Question 9

Nuclear issues

Answer 9

• Radon gas is an alpha emitting isotope that seeps into homes through the ground in some areas
• There are hundreds fission reactors safely in use in the world. None of them is of the same type as the Chernobyl reactors that exploded
• Nuclear waste contains many different radioactive isotopes that emit nuclear radiation for many years. The radiation is dangerous because it can cause cancer
• Nuclear waste is stored in safe and secure conditions for many years after unused uranium and plutonium (to be used in the future) are removed from it