Characterization of photons produced in solid films of organic molecules by the impact of 252Cf-fission fragments

Experiments on photon production in solid films of organic molecules by the impact of 252Cf-fission fragments are presented. The samples were mainly prepared with coronene or POPOP sprayed onto an aluminized polyester foil using the nebulizer spray technique. It is demonstrated by scanning electron micrographs that this technique is a suitable method to prepare homogeneous solid films of organic compounds by producing layers of microcrystals. Time profiles and photon spectra have been determined by bombarding these samples with 252Cf-fission fragments as well as, for comparison, by exciting with light. The results give evidence that the photons induced by fission fragment bombardment originate from molecular fluorescence in the solid sample. Approximately 500 photons per fission fragment have been produced within a 100 ns time window in about 2 μm thick POPOP samples. The photon yield has been observed to increase linearly with the thickness of the sample up to about 16 μm. Only very few photons per fission fragment has been obtained with Rhodamine 6G samples and the same low photon yield has been observed under the bombardment with 2 keV electrons. These results probably indicate that the excitation of molecular species by energetic δ-electrons is the necessary step in the process of photon production by fast heavy ions. Photons which have been additionally produced when extracting positive ions come also from molecular fluorescence in the samples. But this fluorescence is excited by electrons which originate from impacts of fission fragments on components of the acceleration system and which are accelerated back to the sample. (orig.)