This was a story I read sometime in the 1970s, pretty sure it was a novel but not 100% positive.
The protagonist is trying to interest the government or a large corporation into funding research into a radical new type of spacecraft propulsion.
The spacecraft, once in space, would unfurl long radial arms or ribbons. Thin films of radioisotopes would be deployed on the rear-facing side of the arms. Radioactive alpha and beta particles traveling "forward" would be absorbed by the arms. Particles traveling rearward would provide thrust. Over time as the radioisotope decayed and stopped emitting particles it would be jettisoned and a fresh film would be deployed.
As I recall the concept was never realized, and most of the plot was about something totally unrelated (and was so uninteresting to me that I do not remember it).
Does this ring a bell?