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For background: I have some sci-fi technology based on sci-fi physics which depends on some fictional forces that interact differently with up quarks vs. down quarks. So, to get certain types of effects, you want to use materials with lots more down quarks than up quarks, and to get other types of effects, you want materials with an excess of up quarks over down quarks. In practice, that means that sometimes you want materials that are neutron rich (to get an excess of down quarks), and sometimes you want materials that are proton rich (to get an excess of up quarks), because Delta- and Delta++ aren't suitable for building stable matter out of.

Now, getting high neutron excesses in small spaces isn't too hard--elements get more neutron rich as they get heavier, so a block of U-238 is just about ideal**

Getting proton excess, however, is a little trickier. The ideal material should be hydrogen--it's nothing but protons. Except, hydrogen gas isn't very dense, and neither is liquid hydrogen, so the total number of protons you can pack into a given space in the form of liquid hydrogen is considerably lower than the total number of unpaired neutrons you can pack into the same space in the form of uranium.

So, what is the best way to get a really large excess of protons packed really densely? Materials that are solid, and furthermore materials with high tensile strength, are preferred, but if necessary this can be simulated with high-proton liquids or powders embedded in a matrix of less-protonaceous-but-stronger materials.

**Actually, you also want materials that are electrically non-conductive, so actually complex uranium ceramics are most ideal, but that's relatively less important.

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  • $\begingroup$ Are you okay with electrons to balance out the positive charge of all of your protons? If not, you'll have problems. $\endgroup$ – Joseph Sible-Reinstate Monica Dec 21 '19 at 4:53
  • $\begingroup$ Would metallic hydrogen do? That has a reasonable density... $\endgroup$ – Starfish Prime Dec 21 '19 at 10:58
  • $\begingroup$ The best way is to use neutron stars, they can pacl protons ridiculously densely. Not terribly portable though. $\endgroup$ – John Dec 21 '19 at 16:30
  • $\begingroup$ @JosephSible-ReinstateMonica Yes; the electrons are irrelevant. $\endgroup$ – Logan R. Kearsley Dec 21 '19 at 20:44
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I seriously doubt that you can beat compressed hydrogen to get excess protons. If you can't live with compressed hydrogen, but require a solid it will be difficult and the material will be radioactive.

There are isotopes with more protons than neutrons. This talk has several slides about proton-rich nuclei. You will note that the proton-rich isotopes heaver than hydrogen are all unstable with one exception, and that's helium-3 (two protons and one neutron). The problem is that it's rare and it's a gas and gives you the same excess proton density as hydrogen, but at much greater expense.

So I think your choices are hydrogen or some heaver radioactive isotopes.

If you decide that radioactivity is OK, then you need to look at the table of isotopes to find the right balance between radioactivity and number density (number of atoms/cc) which yields the number of excess protons/cc. (You may be have real difficulty finding a proton-rich isotope that is solid in pure form due to the heat of radioactivity melting any macroscopic samples.)

Note that you're probably going to find a light element being better since as the number of protons in a nucleus increases, the number of neutrons needed for stability increases faster (heaver nuclei tend to be more and more neutron rich), so proton-rich isotopes will tend to be less stable as atomic weight increases.

The one place to look where there may be some hope is where the number of either protons or neutrons is 2, 8, 20, 28, 50, 82, or 126 -- the "magic numbers" of the shell model, since those nuclei tend to be unusually stable. I'd look for proton-rich nuclei with a magic number of protons or a magic number of neutrons in a search for only somewhat radioactive proton-rich nuclei.

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