I think he is referring to himself as "Marblehead", as someone whose head is made of marble or full of marbles, i.e. a dummy-head. In hindsight it seemed obvio

I think he is referring to himself as "Marblehead", as someone whose head is made of marble or full of marbles, i.e. a dummy-head. In hindsight it seemed obvious that the radiation from the thorium would act as low level illumination for the photodiode and so ruin its dark current (the electricity that flows when there is no illumination).