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

Counting decays to tell the time

Science · Matter & Materials · ages 17-18
Name ______________________   Date ____________

  1. One becquerel means one decay per second.

    Circle one:   True   False

  2. How do you calculate the activity of a sample?

    • Lambda times the nuclei present
    • Lambda divided by the nuclei present
    • Nuclei present minus lambda

  3. Which assumption does carbon dating rely on?

    • Air steady while alive, sealed since death
    • All ancient wood comes from oak trees
    • Laboratories never make measurement errors

  4. Charcoal shows half the carbon-14 activity of living wood. Carbon-14 half-life is 5730 years. Give the age in years.

    Answer: ______________

  5. A sample holds 6000 nuclei with lambda 0.5 per second. Give the activity in becquerels.

    Answer: ______________

  6. Which relation converts a half-life into a decay constant?

    • Lambda equals half-life over 0.693
    • Lambda equals 0.693 over half-life
    • Lambda equals half-life times 0.693

  7. Nadia sketches activity falling in a straight line to zero. What is wrong?

    • It stays flat and then drops
    • It halves each half-life, not in a straight line
    • It rises as nuclei vanish

  8. The Shroud keeps 92 percent of living carbon-14 activity. Roughly how old is it?

    • About 5730 years
    • About 11460 years
    • About 690 years, well under one half-life
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Answer key

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

Counting decays to tell the time W1-mt_LvZw4h5wlu-s1

  1. True · The becquerel simply counts decays each second.
  2. Lambda times the nuclei present · Each nucleus adds its chance, so total decays per second are lambda times N.
  3. Air steady while alive, sealed since death · A shifted air level or later contamination both fake the leftover ratio.
  4. 5730 · Half activity means one half-life passed, which is 5730 years.
  5. 3000 · Multiply chance by headcount: 0.5 times 6000 is 3000 Bq.
  6. Lambda equals 0.693 over half-life · Lambda equals 0.693 over half-life, so long half-lives give small lambdas.
  7. It halves each half-life, not in a straight line · Activity tracks the nuclei, halving every half-life along an exponential curve.
  8. About 690 years, well under one half-life · Barely below full activity means barely any decay time, about 690 years.
Worksheet · LightMySky