MOSSBAUER EXPERIMENTS WITH HIGH ENERGY GAMMA RAYS: THE 158 keV TRANSITION IN lg9Hg (*)

Rbum6.- On a 6tudi4 la resonance Mossbauer de 158 keV de 199Hg dans une gkomCtrie de transmission en mesurant le courant integral du detecteur, aussi qu'en experiences de retrodiffusion. A partir des deplacements isomeriques observCs entre HgFz et les sources de 199Au dans les matrices de Pt, V, Nb et Rh, on estime une valeur de A < r2> = + 1 x 10-3 fm2 pour le changement du rayon quadratique moyen nuclkaire. Abstract.- The 158 keV Mossbauer resonance in 199Hg has been studied both in a transmission geometry by measuring the integral detector current and in backscattering experiments. From the isomer shifts observed between HgFz and sources of 199Au in Pt, V, Nb and Rh matrices one estimates A < r2> x + 1 x 10-3 fmz for the change mean square nuclear charge radius. 1, Introduction.- Mossbauer experiments with gammarays having energies above approximately 100 keV become increasingly difficult because of the small recoilfree fractions then encountered even in heavy elements. The 158 keV transition in lg9H