Revision summary

Read the nucleus and atom

Proton number Z

Number of protons; identifies the element. A neutral atom also has Z electrons.

Nucleon number A

Protons plus neutrons. Neutrons = A - Z. A and Z are counts with no unit.

Nuclide and isotope

A nuclide has specified proton and neutron numbers. Isotopes have the same Z but different neutron numbers and therefore different A.

Put A at the upper left and Z at the lower left of the element symbol. A change in electrons makes an ion; it does not change the isotope. Equal A alone does not establish the same element.

Explain decay and emissions

An unstable nucleus loses energy by emitting radiation. Decay is random: an individual decay time cannot be predicted. It is spontaneous: no external trigger is needed. A large population can still have a predictable average pattern.

• Alpha: helium nucleus, two protons and two neutrons, charge +2. Typically dense ionisation and low penetration.
• Beta-minus: fast electron produced in a nuclear change, charge -1. Typically less dense ionisation and more penetration than alpha.
• Gamma: electromagnetic radiation from the nucleus, uncharged and highly penetrating. Thick shielding reduces transmission but need not eliminate it.

All three can ionise. Energy and absorber conditions affect their penetration. Radiation leaving a source is different from radioactive material spreading to a new location.

Separate activity from detector rates

Background comes from sources such as cosmic radiation, rocks, radon and radioactive materials in food and the body, with artificial contributions possible. Counts fluctuate; removing the study source does not normally make them zero.

Activity is the number of decays per second in the source. A detector records only some of the emissions, so its count rate is smaller than the activity. The example 300 counts in 2 min minus 100 counts in 5 min gives 150 - 20 = 130 counts/min from the source.

Choose suitable detector response and rate range. Keep geometry, shielding and settings fixed, record durations and estimate background. Longer counting reduces relative random variation for a steady rate, but can blur rapid decay. It cannot repair a wrong calibration.

Use repeated halving

Half-life t_1/2 is the time for half the undecayed nuclei of one nuclide in a large sample to decay, or its activity to halve. It is measured in s, with other consistent time units possible.

After one, two and three half-lives, the remaining fractions are 1/2, 1/4 and 1/8. With half-life 6 h, a corrected count rate of 800 counts/min becomes 100 counts/min after 18 h. Daughter material remains; the whole sample does not disappear.

Use background-corrected rates for a decay curve. In the supplied model, 160 to 80 counts/min takes 4 min; 40 to 20 also takes 4 min. The total curve approaches 20 counts/min background, while the source curve approaches zero. Keep the single-nuclide, negligible-daughter and unchanged-detection conditions.

Connect uses and hazards to the radiation

• An internal gamma tracer emits towards a detector outside the body.
• A suitable beta gauge detects lower transmission through a thicker sheet under otherwise matched conditions.
• Gamma ionisation can damage tumour cells and healthy cells.
• Irradiation means radiation reaches a target. Contamination means radioactive material is present on or inside it.
• Internal alpha-emitting material can be hazardous despite low penetration. Activity alone does not give the energy absorbed by a person.
• Time, separation and suitable shielding affect external exposure; containment addresses spread of source material.