Yes a character/creature can intercept the fireball bead in order to prematurely detonate it.
I would reconsider the AC score though.
If you set a fixed AC like 20 then you'll going to be getting players to have an Archer or even a summoned monster with ranged attacks intercepting all fireballs with a ready action. Turning the fireball against its own caster.
As a GM I treat it a bit differently.
It is an opposed check among the Spellcaster's Spellcraft and an attack roll.
Sometimes I might allow an easier check especially if the player uses something like a shield or his own body. But setting a fixed AC would motivate the players to start shooting arrows at it.
One more point: Fireball can travel 400ft+40ft/lvl instantly
That is rather fast, actually faster than an Arrow. The pee sized bead is like a sling stone flying! What would be the chance a readied action would intercept it?
If you would put an AC to the bead you should consider also its speed.
Cheers
