Giant Absorption Cross Section of Ultracold Neutrons in Gadolinium

The transmission of ultracold neutrons through natural Gd and isotopic enriched ${}^{157}\mathrm{Gd}$ samples has been measured. Absorption cross sections in the order of 100 Mb have been identified. Here one touches a region where fluctuations of the physical parameters influence the result and can cause a basic deviation from the exponential attenuation law and where a purely imaginary interaction potential exists within the sample.