Experimental verification of the 1/v law for the absorption cross section of ultracold neutrons in natural gadolinium

The results of a new experiment on the transmission of ultracold neutrons through a natural gadolinium film are reported. The results indicate that the transmission of the sample is unchanged when the sample moves parallel to its surface. The neutron velocity in the sample coordinate system varies in a range of 6–35 m/s. It follows from the constancy of the sample transmission that the imaginary part of the scattering length is constant; i.e., the law 1/v is valid for the capture cross section of the free nucleus with an accuracy of about 0.5%.