Half-life and Radioactive Decay Graphs Flashcards

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What is meant by the half-life of a radioactive isotope?

The time it takes for the activity, or the number of radioactive nuclei, to fall to half its starting value.

What is activity and what is its unit?

Activity is how many decays happen each second. Its unit is the becquerel (Bq), where 1 Bq = 1 decay per second.

What is count rate in a decay experiment?

The number of counts detected each second by the detector. It is proportional to the source’s activity.

On a decay graph (activity or number of undecayed nuclei) against time, how do you read the half-life?

Choose a starting value, find when it has halved, and read the time difference between those points.

Why do radioactive decay graphs curve downwards rather than form a straight line?

Because the amount halves in equal time steps. The drop is by a fraction, not by the same amount each time.

What does it mean that radioactive decay is random?

You cannot predict when one nucleus will decay, but with many nuclei the overall drop follows a steady half-life.

Does the half-life depend on how much of the sample you start with?

No. Half-life is constant for an isotope and does not depend on sample size.

Does temperature or pressure change the half-life of an isotope?

No. Half-life does not change with temperature or pressure.

How do you work out how much remains after n half-lives?

Multiply the starting amount by one half for each half-life.

An isotope has a half-life of 2 hours. What fraction remains after 6 hours?

1/8, because 6 hours is three half-lives.

A source’s count rate falls from 200 counts/s to 50 counts/s in 10 minutes. What is its half-life?

5 minutes. It halves twice in 10 minutes, so one half-life is 5 minutes.

What should you do about background radiation when using count rate data to draw a decay graph?

Measure the background count first, then subtract it from all readings before plotting.