How to represent isotopes?

[rowlesmr]

Kind of raised in #390

I don't think there is currently a way to denote isotopes using the current _atom_type.symbol definition*, and core seems very X-ray biased, so I don't think it would be an easy fix (but I think worthy to do).

*
    The identity of the atom specie(s) representing this atom type.
    Normally this code is the element symbol followed by the charge
    if there is one. The symbol may be composed of any character except
    an underline or a blank, with the proviso that digits designate an
    oxidation state and must be followed by a + or - character.

[jamesrhester]

Agree that this is important. It should be separate from the atom type symbol, e.g. _atom_type.isotope. The scattering length table could be indexed by both element and isotope number, or perhaps atomic number and isotope number. As the scattering length table is not yet used we have some flexibility in how we rework it.

[rowlesmr]

Idea. We already have _atom_type.atomic_number. Let's create _atom_type.mass_number.

Scattering lengths are then keyed on both atomic and mass number. Mass number has a default value of zero, which indicates to use the "natural" element abundance value for the scattering length, rather than a specific isotope.

.
Although, I can't see how to do a nested loop, a la, _atom_type.atomic_number-style to make a value depends on two keys with a loop_ _enumeration_default.index _enumeration_default.value construct.

[jamesrhester]

Although, I can't see how to do a nested loop, a la, atom_type.atomic_number-style to make a value depends on two keys with a loop _enumeration_default.index _enumeration_default.value construct.

Yes indeed, that is a problem. The hacky solution is to define a data name that is a list of [_atomic_number, _atomic_mass] with which to index the enumeration list. The more elegant solution would be to replace _enumeration.def_index_id with a loopable data name, e.g. _enumeration_index.id where the data names to use to index the default list could be provided.

As much as I hate touching DDLm, the latter is really the right way to do it. I'll raise an issue for separate discussion.

[rowlesmr]

From #397 (comment), I think it is better to loop on _atom_type.element_symbol and _atom_type.mass_number. This allows for an explicit use of D for deuterium, which is in the element listing.