2.8 Half-Life

Define the half-life of a radioactive isotope.

● The time it takes for the number of unstable nuclei in a substance to halve.

● The time it takes for the count rate from a sample to fall to half its initial level.

Why are isotopes with long half-lives particularly harmful?

● They remain radioactive for much longer periods of time.

● They must be stored in specific ways to avoid humans and the environment from being exposed to radiation.

State 2 uses of nuclear radiation in the field of medicine.

1) Examining of internal organs.
2) Controlling and destroying unwanted tissues.

Describe the nature of radioactive decay.

● Random
● Which nuclei decays and when is determined only by chance.
● It is impossible to predict which nuclei will decay and when.

Explain how to determine and verify half-life from a decay curve.

● Read off the time axis the time it takes for the number of unstable nuclei to half..
● To verify, check that the time it takes for that number to half again is the same.

Explain the basic idea behind carbon dating.

● All living things contain the same relative amount of carbon-12.
● When they die, the carbon decays at a known rate.
● By measuring the amount of carbon remaining, the age can be calculated.

Explain how a medical radioactive tracer is used.

● The tracer is inserted in the body (often through the bloodstream).
● The tracer is radioactive so decays.
● A detector outside the body can detect this radiation and locate the tracer.

What can be said about the half-life of a given radioactive isotope?

It is a constant.

Define the activity of an unstable nucleus.

Activity is the rate at which a source of unstable nuclei decays.

What is the unit of radioactive activity?

Becquerel (Bq).

What happens when a parent atom has decayed?

It produces a new atom.

Why is it important to use a larger sample size to determine the half-life?

Having a larger sample size will help reduce anomalies.

Is it better to use a graph or a table to find the half-life?

Using the graph is better as it gives a more accurate answer that is closer to the true value.

How do you calculate the mean half-life from a graph?

1st half life + 2nd half life/2

List some uses of radioactive materials.

1. Thickness control
2. Smoke alarms
3. Medical and industrial tracing
4. Cancer treatment
5. Sterilisation
6. Carbon dating

How is radioactivity used in thickness control?

If the aluminium/paper becomes too thick: \n -Less radiation is detected by the GM tube as less radiation is able to pass through \n -A message is sent to rollers to compress back to original thickness \n \n If the aluminium/paper becomes too thin: \n -More radiation is detected by the GM tube as more radiation is able to pass through \n - Message is sent to rollers to move apart back to original thickness

What radiation is used in thickness control?

Beta radiation is used as it can penetrate paper or thin aluminium \n -The source should have a long half-life so that it does not need to be replaced too frequently.

How is radioactivity used in sterilisation?

Gamma rays are most penetrating so pass through food and medical equipment. Gamma rays can also kill bacteria.

\n - The source should have a long half life so doesn't have to be replaced frequently

How is radioactivity used in smoke alarms?

Alpha radiation is used in smoke alarms as it is most ionising. It ionises the air allowing a current to flow through the alarm. If smoke is present, it will break the circuit and set off an alarm.

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Alpha sources tend to have a long half-life, meaning they don’t have to be replaced frequently in a detector.