About 15 minutes of content buying upgrades to set up for longer and longer chain reactions. Fun while it lasts and fairly fast-paced. It's a bit unbalanced in that once you hit a tipping point, your gains explode -- I had a run that lasted until late in the second bonus tier, earned about 200K income, and completed the last 2/3 of the upgrades all at once.

Hint: Pay attention to the tip on the game page that the black-hole atoms, which do nothing but eat your neutrons, are clickable to despawn them. (Doing so does use one of your banked clicks, and triggers click effects, so make sure you buy Momentum before trying it.)