Exploring the Radioactivity of Cobalt-60: Understanding the N/P Ratio

[americast]

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...

 

[Simon Bridge]

That is only a rule of thumb.
Co60 is radioactive because it is energetically favorable for it to be so.

 

[snorkack]

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.

 

[americast]

Thanx a lot...!