Half-Life and Activity · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Halving again and again

Science · Matter & Materials · ages 15-16
Name ______________________   Date ____________

  1. What is the half-life of an isotope?

    • The time for all atoms to decay
    • The time for one atom to pop
    • The time for the activity to halve

  2. What does activity measure?

    • The number of decays each second
    • The size of each atom
    • The heat of the sample

  3. What fraction is left after two half-lives?

    • One half
    • One quarter
    • One eighth

  4. A half-life is 5 minutes. A count rate falls from 800 to 100 Bq. How many minutes passed?

    Answer: ______________

  5. A sample reads 640 Bq. What is its activity after two half-lives?

    • 320 Bq
    • 160 Bq
    • 80 Bq

  6. Where do you read the half-life on an activity graph?

    • Where the activity drops to half its start, read across to time
    • Where the curve first starts to fall
    • Where the curve hits zero

  7. A bone sample holds one eighth of its starting carbon 14. How many half-lives passed since death?

    • Two
    • Eight
    • Three

  8. You can predict exactly when a single nucleus will decay.

    Circle one:   True   False

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Halving again and again W1-mt_S5snTqUjyi-s1

  1. The time for the activity to halve · Half-life is the time for half the activity to go.
  2. The number of decays each second · Activity counts pops per second in becquerels.
  3. One quarter · Halve twice: one half, then one quarter.
  4. 15 · 800 to 100 is three halvings, times 5 minutes.
  5. 160 Bq · Halve twice: 640 to 320 to 160.
  6. Where the activity drops to half its start, read across to time · Find the half value on the axis, then read the time.
  7. Three · One eighth means three halvings.
  8. False · Single pops are random; only the crowd is steady.
Worksheet · LightMySky