[lukeluke]

In a forum that i read often, a nuclear chemist is saying that nobody knows why silicon or oxygen does not
absorb neutrons. This was related to the fact that cross sections are completely empirical in their nature.

I am absolutely not a chemist and i tried to learn more about cross section from wikipedia and
around the net, but the fact that there is no justification for a particular cross section to be in that way
is something that makes me curios...

Thank you.

Ps.
The forum i am reading is not about nuclear chemistry, it is a forum in italian language about 'cold fusion' theories like the
one of Widom and Larsen. It happens that a lot of problems are related to the cross sections that are so small
that some type of particle interactions are just impossible to happen.

[Enthalpy]

Natural oxygen is mainly ¹⁶O which is very stable with even numbers of protons and neutrons (permits spin pairing), well-balanced numbers between protons and neutrons (equal numbers are fine for light elements)... So adding an absorbed neutron wouldn't make oxygen more stable, hence the small section.

So stable in fact, that when a star has transformed all hydrogen into helium in its core, the next step is to transform helium into ¹²C and ¹⁶O.

We could add arguments about so-called magic numbers, but they don't work very well. For instance ⁵⁶Ni is radio-active.

You can save your time about cold fusion, it's a crook.

[manonemission]

This is a very interesting thread.  Are there any theories as to how neutrons and protons bond within the nucleus?    It almost sounds as if different atoms could be attributed different levels of neutron-affinity the way they are attributed electronegativity.

[Borek]

http://en.wikipedia.org/wiki/Quantum_chromodynamics

[Enthalpy]

...almost sounds as if different atoms could be attributed different levels of neutron-affinity

Neutron affinity wouldn't bring much, because:
- You never have equilibrium situations in a lab. Only brief dynamic situations which end at the first oportunity, not at the best one.
- Under conditions where neutrons could hop from one nucleus to an other, so would protons as well, and alphas... Hence neutron-to-nucleus affinity wouldn't suffice. You'd use the heat of formation of every possible nucleus. That's part of the theory of primordial nucleosynthesis, explaining how much H, D, He, Li, Be result from the big bang - but even the big bang didn't result in an equilibrium.
- Consequently, nuclear people define figures for the dynamic evolution, not for equilibrium situations. They call it "neutron cross-section" (in barns or 10^-28 m²) and use tables for thermal neutrons, fission neutrons, and sometimes fusion (14MeV) neutrons - or they have curves of cross-section versus energy.

-----

Theories about how neutrons and protons are bond... Basically QCD among quarks, yes, but even n-n, p-p, n-p bonds aren't perfectly clear, and complete nuclei even less so...

The latest I've seen suggests that light nuclei are solids while heavy ones are "liquid", with nucleons delocalized over the whole nucleus. As well, it seems that n-p bonds occur more often than others.

Elementary questions like "why no di-neutron" are still open. Very foggy field.

[manonemission]

The latest I've seen suggests that light nuclei are solids while heavy ones are "liquid", with nucleons delocalized over the whole nucleus. As well, it seems that n-p bonds occur more often than others.

Phase continuum is a logical consequences of any field whose force decreases with distance; or is that an overgeneralization?

I wonder if n-p bonds are more common because the 'amazing disappearing electron' is still lurking somewhere within the neutron.  I've sometimes wondered if neutrons aren't miniaturized hydrogen atoms.

[Enthalpy]

I've sometimes wondered if neutrons aren't miniaturized hydrogen atoms.

This is abandoned for about one century. Present-day theory is quarks and gluons, aka QCD.
And, yes, electrons can appear and disappear. Other quantities are constant, but not the number of electrons.

[disputes123]

- Under conditions where neutrons could hop from one nucleus to an other, so would protons as well, and alphas... Hence neutron-to-nucleus affinity fedloan irs.gov easybib wouldn't suffice. You'd use the heat of formation of every possible nucleus. That's part of the theory of primordial nucleosynthesis, explaining how much H, D, He, Li, Be result from the big bang - but even the big bang didn't result in an equilibrium.


agreee!!