Gamma irradiation of coumarin in aqueous solution gives fluorescent products proportional to radiation dose. Our earlier results showed that hydroxyl radical is mainly responsible for this effect. Fast neutron irradiation gave similar results but with a dose response of 0.4 relative to that of gamma dose response. This report gives the effect of adding boric acid to coumarin and irradiating with thermal neutrons in a nuclear reactor. The relative response to the high LET alphas and7Li recoils from10B(n, α)7Li reaction, is found to be approximately 8% as compared to gamma rays. This value is compatible with theGOHvalue for these heavy charged particles as compared toGOHof gamma rays in neutral aqueous solution.