Neutron-proton scattering as a primary standard in the 100-1000 MeV region

Neutron-proton scattering is used as standard cross section in neutron-induced nuclear data measurements. Recently, there has been an intense debate on the np scattering cross section above 100 MeV, indicating that this cross section might not be as well known as was previously thought. The world database on np scattering differential cross section data from 100 to 1000 MeV incident neutron energies has been reviewed. In addition, the status of the np total cross section and the pp -> dπ⁺ total cross section is discussed, as these have frequently been used normalize np scattering data. It appears that the shapes of the largest np data sets tend to fall into two groups, with different steepness at backward angles. Also, it seems as the two major techniques for normalizing data yield incompatible results. Both these effects have consequences not only for the cross section itself, but also for determinations of the absolute strength of the strong interaction in the nuclear sector, the pion-nucleon coupling constant, g_πNN². This fundamental constant is often derived from np scattering data, and the conflicting data situation has led to an intense debate about its exact value. (author)