Radioactivity at A Level

A Level Radioactivity INDEX

Radioactivity at 16+ level differs from that at 14-16 level primarily in the application of mathematical interpretation that is required. It is useful to revise the subject at the lower level before beginning this section of study. An extensive glossary of terms is accessible from this section  this should

From radioactivity studied at level (14-16) the following should be fully appreciated:

Symbols
Why radioactivity occurs
Background radiation
Half Life
Uses
Safety

The exponential decay of a radioactive isotope requires the use of natural logarithms. A section on the mathematical skills required for this section is included so that you can revise the basic rules for manipulation of equations, exponents and logarithms. Being able to 'speak mathematics' fluently is very important at 16+ level

Mathematical skills

New material at 16+ level:

The equations relating to radioactive decay are exponential in character. This causes problems with calculations relating to half-life. See Mathematical skills before continuing.

Radioactive decay and half life calculations at 16+

When considering radioactive decay as represented in equations you now have to also consider particle physics and the balancing of lepton number and baryon number (from your AS syllabus!).

Balancing Equations

The Feynman diagrams will also have to be known for beta decay.

Practical knowledge is imperative at this level (see safety).

A Geiger-counter meter only records the particles that interact with it. Students often think that the true activity of a sample is measured by the instrument. The count rate is always lower than the true activity.

Illustrative question

In order to answer questions that relate the number of atoms in a sample with its mass students must understand what a mole is.

The mole and the Avogadro number

Try this question: The half-life of Pb 210 is 20 years. Find the initial number of disintegrations per second for 1.0g of this isotope of lead.

Try this question: A speck of uranium dust contains 10¹⁴ atoms. It has a mass of 3.95 x 10^-8g. The half life of Uranium 238 is 4.5 x 10⁹ years. It is inhaled and lodges in the lung. What is the initial rate of disintegration of the dust spec?

Carbon 14 dating is used to date artefacts that contain once living material.

Try this question: If the equilibrium concentration of carbon 14 in living plants gives 16 disintegrations per minute per gramme of carbon, estimate the age of a piece of timber if 2.0g of carbon prepared from it gives 15 disintegrations per minute.
(The half-life of carbon 14 is 5.57 x 10³ years).

Medical Physics Applications

Medical use of radioisotopes forms an important part of Medical Physics content syllabuses. This makes an understanding of the physiological effects off radioactive materials important, but even if you are not studying that option this is 'good for your soul'

Effects of radiation exposure

When a patient is injected with a radioactive tracer the bodily functions expel the isotope from the body by natural processes such as excretion, respiration and perspiration. Therefore the effective half-life of the isotope is less that the physical isotope but also depends upon the rate at which the body removes it: the biological half-life. The combination of these is called the effective half-life.

Calculations related to effective half-life

Choice of tracer isotope