ИСТИНА

The majority of partial reactions (g,n), (g,2n), (g,3n) cross sections were obtained using quasi-monoenergetic annihilation photon beams at the Lawrence Livermore National Laboratory (USA) and France Centre d'Etudes Nuclearies de SAclay. There are sugnificant discrepancies between the cross sections measured at the two facilities that cannot be resolved in a systematic manner. To solve this long standing problem, a direct neutron multiplicity sorting technique of obtaining (g,xn) cross sections, whee x=1,2,3, has deen developed. The neutron sorting method relies on the constant neutron detection efficiency relative to the energy of reaction neutrons. For this, we use a flat efficiency neutron detector recently designed for neutron multiplicity sorting measurements. The method has been applied for the first time for case of 209Bi at the experimental hutch GACCO (Gamma Collaboration Hutch of Konan University) of the gamma-ray beam line of he NewSUBARU synchrotron radiation facility, where laser Compton scattered quasi-monochromatic gamma-ray beams with energies up to 45 MeV were used to irradiate the target. This is the first of series of experiments dedicated to partial photoneutron cross section measurements for the IAEA-CRP F41032 which make use of the new neutron multiplicity sorting method, We report here on th experimentsl technique, data analysis and results.