My current /rad build has moderate defense (20%ish) but focuses on resistance and regen.
I played around with slotting Gamma Boost but found it wasn't worth it. Stick an endmod IO or performance shifter proc in it and call it a day.
Radiation Therapy not only provides +regen, it also provides a decent heal per foe in range. You can easily get the recharge under 30 seconds. I haven't tried using it as a proc bomb.
Beta Decay debuffs foe defense and tohit and buffs your recharge. It's autohit, so it really doesn't need more than 1 slot (I recommend an Enzyme HO), unless you want to add an Achilles Heel proc.
While the +regen from Particle Shielding is nice, the absorb shield is key. You want that perma, particularly against damage types you're weak against. You want to use it proactively, not after you've already taken damage.
There are different philosophies for slotting Ground Zero (procs vs dmg). I currently have it 6-slotted with Obliteration.