Why is Cobalt-60 Radioactive with an n/p Ratio of 1.22?

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SUMMARY

Cobalt-60 is radioactive despite having a neutron-to-proton (n/p) ratio of 1.22, which is below the commonly referenced threshold of 1.56 for radioactivity. This phenomenon is explained by the Mattauch rule, which states that among two isotopes with the same mass and differing by one proton, at least one must be radioactive. Cobalt-60, being an odd-odd isotope, does not fall into the category of the five stable isotopes that defy this rule, thus confirming its radioactive nature.

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americast
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I have heard that if n/p ratio of a substance becomes more than 1.56, it becomes radioactive. But for Cobalt-60, n/p= 1.22. Yet it is radioactive. Why is that so?

Thanx in advance...
 
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That is only a rule of thumb.
Co60 is radioactive because it is energetically favorable for it to be so.
 
There is a simple rule named Mattauch rule with just 2 exemptions:

Out of two isotopes with same mass and whose proton number differs by one, at least one must be radioactive.

The 2 exemptions are:
Antimony-123 and tellurium-123 are both stable
Tantalum-180 and hafnium-180 are both stable (Also tungsten-180 decays by alpha decay, not electron capture as it should by Mattauch rule).

A consequence of Mattauch rule is:
By Mattauch rule, the only stable isotopes with odd number of protons and neutrons are the 4 light isotopes D, Li-6, B-10 and N-14. All others should be radioactive.
Tantalum 180 is violating Mattauch rule and so is the 5th stable odd-odd isotope. There are still just these 5. Cobalt 60 is odd-odd, and is not one of these 5 stable isotopes, so it is radioactive.
 
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Thanx a lot...!
 

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