This was really fun! The farthest I got was level 6, after several runs. The gameplay is really fun, and there's a nice bit of strategy involved in terms of what particle to hit in order to kick off the reaction, and then if multiple particles are lit up, which ones to go for to keep it going. I also really like the various upgrades; they are really well-chosen. I always went for additional chance to spawn particles, and anything to increase speed/duration.
I agree with some of the other comments that sometimes the RNG feels a little bit unforgiving; you can set up the perfect shot right into the middle of a big group, and it can still get unlucky and shoot the particles elsewhere. But I like how easy and fast it is to start another run, so that it doesn't feel punishing even when you get bad luck. And I can imagine that if you're lucky to end up with the upgrade that gives you another swing of the fission sword, then that really opens things up (I was never able to buy that upgrade in my runs, though).
I like the art style quite a lot, and swinging the sword feels good, in no small part because of the nice animation you have going along with it. The sound effects are good, but I do feel like some music would help to build the atmosphere.
All-in-all, this was a great little game. Super fun. Nice work!