refine look of nucleus in the Rutherford Atom screen

[pixelzoom]

@ariel-phet asked to create this issue in 3/10/16 dev meeting.

Requests are:

(1) Refine the look of the nucleus in the Rutherford Atom screen, so that it looks more spherical and doesn't contain holes. Either use the approach in build-an-atom, or have @andrewadare come up with something else. If something else, consider apply it to build-an-atom (create an issue).

(2) Programmatically generate the icon for the Rutherford Atom screen, so that the look is the same. Jettison the related .png and .ai files.

[pixelzoom]

@schmitzware This is not something that you need to do. It will likely be done by @jessegreenberg or @andrewadare.

[jessegreenberg]

@ariel-phet is this something that you would like @jessegreenberg and/or @andrewadare to work on now? As mentioned in https://github.com/phetsims/rutherford-scattering/issues/59#issue-139980432 we could either generalize the approach used by build-an-atom and isotopes-and-atomic-mass (hopefully already in shred) or @andrewadare could work on a new representation, which could replace the one in shred.

[ariel-phet]

@jessegreenberg it seems like it is worth investigating generalizing the approach used in build-an-atom. It just needs to "look" reasonable, but I would advice consulting with @andrewadare to makes sure that the reasonable look does not break any major rules of nuclear physics!

[jessegreenberg]

Sounds good @ariel-phet, thanks. I will consult with @andrewadare before proceeding.

[jessegreenberg]

@andrewadare and I discussed the representation and some possible alternatives. One possibility that @andrewadare suggested was to use the Woods-Saxon formula to model the density distribution of nucleons as a function of the radius. ( http://slideplayer.com/slide/8620364/ )

The result might look like this Glauber Monte Carlo model, but with a single nucleus. (http://entropybound.blogspot.com/2008/05/phobos-glauber-monte-carlo.html)

Not sure if this would have implications for the 'unstable' animations in isotopes-and-atomic-mass and build-an-atom.

@ariel-phet for now I will use the algorithm in shred as it is, since it isn't fundamentally rule breaking. I am not sure if the representation shown in the Glauber Monte Carlo model would be desirable or important for the learning goals of sims that use shred.

[jessegreenberg]

Looking pretty good:

But I had to make some changes in shred and I would like to ask a couple of questions about ParticleAtom.js before making the commit.

[jessegreenberg]

If we go through with this new representation, we will need to also replace icons for

• [x] the Rutherford Nucleus Scene radio button
• [x] the Rutherford Nucleus Screen icon

[ariel-phet]

The screenshot looks quite good to me!

[jessegreenberg]

Home screen icon and radio button icon both match the new nucleus representation.

[jessegreenberg]

I think that completes this issue, ready for review.

[jessegreenberg]

This was verified, closing.