Radioactive Banana! Peeling Away the Mystery
All bananas contain potassium (element K). All potassium contains 93+ % Potassium 39, stable potassium, and a little bit of the isotope Potassium 41, perhaps 6+ %. But, all potassium also contains a tiny fraction of the radioactive isotope of potassium, Potassium 40 (K40).
Potassium 40 undergoes three forms of decay, beta -, rarely beta+, and electron capture. The last step emits a gamma ray with an energy of 1461 keV. It is this gamma ray that I detected.
My calculations for the typical radioactivity of a banana:
The number of Potassium (K) atoms per gram of potassium:
(Avogadro’s Number / Atomic Weight of K40) = 6.022 x10^23 / 39 = 1.544 × 10^22 K Atoms/gram
The amount of Potassium in a Banana (approx):
grams of Potassium in a banana = 0.442 grams
Natural abundance of K40 per normal Potassium (A): 0.000117
Half life of Potassium: 3.9357×10^16 seconds (T 1/2).
((Avogadro’s Number) / (Atomic weight)) x (0.442 g) x (A) x (ln 2) / (T1/2)
(((6.022*10^23 / 39)*0.442) x 0.000117) x ln2 / (3.9357×10^16)
=14.0633 decays per second per banana
= 14.0633 Bq Banana^-1
Practical Gamma Spectroscopy Links
For $350 USD – $500 USD
http://beeresearch.com.au/ – Quality and inexpensive MCA
http://www.physics.usyd.edu.au/~marek/pra/index.html – PRA software for viewing peaks.
You can often find a good scintillation probe on eBay for a few hundred dollars (USD), but you have to shop for it.
http://spectrumtechniques.com/ucs30_system.htm — Entry lab-grade setup.
Some potassium and banana sites!!!