How slowly does potassium-40 decay?

Predict, before you measure

Will a pinch of salt give more beta tracks than the background?

You are about to measure 2.0 g of low-sodium salt substitute (KCl). Pick what you think will happen.

How confident are you? Tap one.

Explain your reasoning.

Lock in my prediction

Prediction locked. You will find out at the bench.

Observe, Part 1: Background

Background measurement

Settings: Integral mode, 60 frames, 10 s each, total 600 s. Count long curved worm tracks only. Zoom in on each and confirm the deposited energy is above 700 keV before counting it.

Worm tracks counted Background rate (count divided by 600 s) Bq

If you ran a second background measurement, note it here.

Observe, Part 2: KCl sample

KCl measurement (2.0 g)

Same settings as the background. Count and verify each worm track above 700 keV.

Worm tracks counted KCl rate (count divided by 600 s) Bq

Second run result if you repeated it.

Explain

Work out the half-life, step by step

Write each answer yourself.

Step	Your answer
1. Net rate = KCl rate minus background rate
2. Corrected activity = net rate multiplied by 5
3. Mass of K-40 in your sample (KCl is 52% potassium, natural potassium is 0.012% K-40)
4. Number of K-40 atoms (divide mass by 40 g/mol, multiply by 6.022 times 10 to the 23)
5. Half-life estimate using Activity = 0.693 divided by half-life, multiplied by number of atoms
6. How does your estimate compare with the accepted value of 1.25 billion years?

Does "radioactive" mean "dangerous" in this case? What does this tell you about everyday materials?