Unstable Particles Is a valuable crafting material that can be used for various useful stuff, such as Escape Teleporter and Matter Converter.
Although its recipe isn't very hard, smelting radioactive ores such as Uranium Ore is probably one of the best (if not the best) methods of obtaining Unstable Particles in large quantities - Many of these ores can be extracted from blocks that are not hard to find, but more importantly is that some of them may even be extracted or centrifuged from products of Infinite mixing. For example, you can centrifuge Essentia Obscura for Plutonium Ore with huge (15%/10%/15%/16%) chance.
Alternatively, one might opt to instead look for Erchius Asteroids in order to mine vast quantities of Matter Blocks to extract. As these are up in the topmost lair of planets, you will need to use your mech or other means of flight to reach them. It might also help to check your Tricorder's GPS feature when beaming down to a planet to see whether they have this biome or not before attempting to fly up to one.
Particle Destabilization (Chemistry tree).