MEASUREMENTS OF THE 235U(n,f) STANDARD CROSS SECTION AT THE NATIONAL BUREAU OF STANDARDS
National Bureau of Standards, Gaithersburg, MD 20899, U.S.A.
*Los Alamos National Laboratory, Los Alamos, NM, U.S.A.
** Atomic Energy Research Establishment, Harwell, United Kingdom
***Instituto de Pesquisas Energeticas e Nucleares, Sao Paulo, Brasil
The Neutron Interactions and Dosimetry Group at the National Bureau of Standards (NBS) has had a long-term program for the measurement of standard neutron cross sections. The group has maintained a significant effort on the measurement of one of the most important of these cross sections - the neutron-induced fission cross section of 235U. Since the ENDF/B-VI evaluation has been recently released, it is appropriate to review the measurements of the 235U(n,f) cross section which have been made at the NBS using accelerator-based neutron sources.
In the 0.1 to 20 MeV region where this cross section is a standard, six separate measurements of the differential cross section, using a variety of techniques have been made. Both the NBS 150-MeV Electron Linac and the 3-MV Positive Ion Accelerator have been used as neutron sources. Two of the measurements are relative to the H(n,p) cross section while the remainder are absolute. These measurements will be reviewed and compared to ENDF/B-VI. The current status of this program and possible future improvements will be discussed.
KEYWORDS: cross section, fission, neutron, review, standard, 235U