sentences:  Also, hydrogen is one of the few elements that has a negative scatter, which means that neutrons deflected from hydrogen are 180° out of phase relative to those deflected by the other elements.These features are important for the technique of contrast variation (see below).SANS usually uses collimation of the neutron beam to determine the scattering angle of a neutron, which results in an ever lower signal-to-noise ratio for data that
  contains information on the properties of a sample at relatively long length scales, beyond ~1 μm.