Radioactive Decay & Detector Lab
Count a decaying source in timed readings, measure the background with the source removed, and plot ln(C − B) against t to find the decay constant.
Learning goals
- Analyse random radioactive decay, activity, decay constant and half-life.
- Apply conservation laws to nuclear equations and beta decay, including antineutrino evidence.
Protactinium-234m (β emitter) 3.00 cm from a GM tube, with no absorber. Start counting to take readings.
- Activity of the source, A
- 3000 Bq
- Counts in the last reading, N ± √N
- —
- Count rate, C
- — s⁻¹
- Background, B (measured)
- — s⁻¹
- Net count rate, C − B
- — s⁻¹
Try this
0 of 4 doneTake the source out and measure the background count rate. (not done yet)
Background comes from rocks, buildings, food and cosmic rays. Subtract it from every reading before using the decay law.
Record at least six net count rates as the protactinium-234m decays. (not done yet)
ln(C − B) against t is a straight line with gradient −λ. For protactinium-234m, λ = ln 2 / 69.5 s = 0.0100 s⁻¹.
With cobalt-60, take readings at one distance and at twice that distance. (not done yet)
The count rate drops to about a quarter, but the activity is unchanged: the tube now catches a smaller fraction of the radiation.
Take three 10 s and three 30 s readings of one source. Which scatter more? (not done yet)
A count of N fluctuates by about √N, so the fractional scatter 1/√N is smaller for longer counts.
Your readings
| # | t / s | N | C − B / s−1 | ln((C − B) / s−1) | Remove |
|---|---|---|---|---|---|
| No readings yet. Set up a measurement, then record it. | |||||