The radioisotope 40K decays either by electron capture (~10% of the time), releasing about 1.31 MeV, or by beta decay (90% of the time), releasing about 1.51 MeV. Thus the mean heat released is about 1.33 MeV per decay.(adsbygoogle = window.adsbygoogle || []).push({});

There are 1.5x10^25 40K atoms in 1 kilogram. Therefore the energy which could be released from complete decay of 1 kg is [tex]\phi[/tex]=3.2x10^12 Joules.

The half-life of 40K is T=1.277x10^9 years, or 4.027x10^16 sec.

Therefore, we can calculate the heat produced (W/kg) from the decay of 1 kg of 40K atoms as:

[tex]=\phi\exp(-tln(2)/T)\frac{ln(2)}{T}=\phi\frac{ln(2)}{T}[/tex]

I calculate 5.53x10^-5 W/kg, but numerous sources which I would otherwise consider authoritative give values which are closer to 3x10^-5 W/kg. I don't understand why our values are so different. I have calculated the heat released from many other radioisotopes (e.g. 235U, 232Th, etc.) without problems.

Am I correct, or where have I gone wrong?

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# Heat of 40K decay

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