Relative Stability of 237-Np & 237-Pu Explained

[sghosh]

Hello All,

I need some explanation on the relative stability of 237-Np and 237-Pu. We know that 237-Np has 2.144x10^{6} yrs of half-life. On the other hand, 237-Pu , which has only 1 extra neutron as compared to 237-Np has a half-life of 45.2 days with EC and alpha as most prominent decay modes.

Can we explain as to how does this additional neutron makes the nuclide less stable. The number of neutrons is 145 for 237-Pu as compared to 144 for 237-Pu, and these are not magic numbers as well.

How can we explain this fact.

any answers would be useful

thanks
Suddhasattwa Ghosh

 

[mfb]

Can we explain as to how does this additional neutron makes the nuclide less stable.

Plutonium has one proton more and one neutron less. Np-237 has an even number of neutrons, which means they can occupy their energy levels in pairs. In general, this makes nuclei more stable. It has an odd number of protons, but apparently this is not so important here.
Pu-237 can decay to Np-237 while the opposite direction is not allowed by energy conservation.

One general remark about nucleus stability: sometimes general rules allow to get a good estimate, but often they do not. It is possible to calculate them numerically using quantum mechanics, but that does not give a good description "why" the half-lifes are like that.

 

[snorkack]

Plutonium has one proton more and one neutron less. Np-237 has an even number of neutrons, which means they can occupy their energy levels in pairs. In general, this makes nuclei more stable. It has an odd number of protons, but apparently this is not so important here.
Pu-237 can decay to Np-237 while the opposite direction is not allowed by energy conservation.

The odd mass is important, too.

Quite generally, due to Mattauch isobar rule, almost all isobar pairs differing by one neutron-proton difference have one unstable member, whether the other member is also unstable or completely stable. (The two exceptions are Ta-180 and Te-123, which are stable - Te-123 along with Sb-123, and Ta-180 along with Hf-180)

If the total number of nucleons is odd, then there is either an unpaired neutron or proton - either way one odd nucleon, unaffected by beta decay. In case of even number of nucleons, the pairing energy favours even-even nuclei over odd-odd.

 

[mfb]

Quite generally, due to Mattauch isobar rule, almost all isobar pairs differing by one neutron-proton difference have one unstable member, whether the other member is also unstable or completely stable. (The two exceptions are Ta-180 and Te-123, which are stable - Te-123 along with Sb-123, and Ta-180 along with Hf-180)

Ta-180 in its ground-state is unstable (half-life 8 hours). The metastable Ta-180m is not stable, just so long-living its decay has not been found (yet).
Te-123 should decay, too, but again with a very long lifetime.

 

[sghosh]

Ta-180 in its ground-state is unstable (half-life 8 hours). The metastable Ta-180m is not stable, just so long-living its decay has not been found (yet).
Te-123 should decay, too, but again with a very long lifetime.

Dear All,

thanks very much for the discussion. Extending this discussion, may I also know, why Np-241 (13.2 min), Pu-241(14 yrs.) and Am-241 (433 yrs) they have increasing half-lives.

We have for
Np-241, 148 neutrons (even) and 93 protons (odd)
Pu-241, 147 neutrons (odd) and 94 protons (even)
Am-241, 146 neutrons(even) and 95 protons (odd)

Here, if we go by what "mfb" has mentioned and I quote, "...even number of neutrons, which means they can occupy their energy levels in pairs. In general, this makes nuclei more stable". By the same reasoning, Np-241, which has 148 neutrons, should be more stable. But it is not. It works for Am-241 though, which has 146 neutrons. so, why Np-241 is less stable than Am-241.

Thanks and regards,

Suddhasattwa Ghosh

 

[mfb]

Don't forget the protons.
If you plot binding energy versus element (number of protons) for all nuclei with the same odd sum of protons and neutrons, you get a parabola. For every nucleus, either the proton or the neutron number is odd, but some have "too many" neutrons and some have "too many" protons (so higher energy levels have to get occupied). Usually, all those nuclei apart from one are unstable against beta-decay and can decay towards the nucleus with the lowest energy.
This happens here, Np can decay to Pu and Pu to Am, but this has to do an alpha decay because there is no beta decay possible.