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The photofission yields of Bi induced by the quasi-monochromatic photon beam from in-flight annihilation of positrons of the Frascati linac were measured. The experiment was performed at sixteen different positron energies, from 120 MeV up to 280 MeV, by collecting the annihilation photons at angles 0.5° < θ < 1°. Fission fragments were detected with glass sandwiches. Photofission cross-section values were deduced by means of an appropriate unfolding method.

As a m a t t e r of fac t t h e absolute values of t he average to t a l k inet ic ene rgy 2 ~ obt a ined in different labora tor ies (1,a.5) disagree somewhere more t h a n the expe r imen ta l unce r t a in t i e s would allow, as Fig. I shogs . Some ag reemen t the re is in the behav iour o f / ~ vs. En: Never the less , some au tho r s f ind t h a t t he average to ta l k inet ic ene rgy of t he ~3sU f r a g m e n t s decreases as E . increases