Gamma rays can scatter when they interact with some material; you're probably reading gamma rays which have travelled above the shielding and scattered towards you off the ceiling. The intensity of scattered radiation is significantly reduced however, and it's had to travel that much further to get to you so the inverse square law has some impact.
Quixeh t1_iuh3ahz wrote
Reply to comment by ILikeDaWubbs in Is there such a thing as a gamma radiation mirror? by AlarmingAffect0
Gamma rays can scatter when they interact with some material; you're probably reading gamma rays which have travelled above the shielding and scattered towards you off the ceiling. The intensity of scattered radiation is significantly reduced however, and it's had to travel that much further to get to you so the inverse square law has some impact.