THE PRECISION NEUTRON INTERFEROMETRIC MEASUREMENTS OF THE NEUTRON COHERENT SCATTERING LENGTH IN SI AND 208PB. David L. Jacobson, Alexander Ioffe, Muhammad Arif and Samuel A. Werner. Building 235, Room A159, NIST, Gaithersburg, MD, 20899 (301-975-6207, email:

The precise determination of the coherent neutron scattering length, bcoh, in 208Pb, is important in determining the neutron-electron interaction amplitude, ane. The current value for bcoh, in 208Pb is, bcoh=9.494(29)fm. However, the accuracy of this measurement is still too low for a conclusive determination of the sign of ane.

A new technique has been developed at the NIST Cold Neutron Research Facility s (CNRF) Neutron Interferometer Facility, to allow precise measurement of bcoh in any material. This technique has been calibrated against Si, for which bcoh=4.149(1), is known very well. The technique is based on the use of the neutron interferometer which can directly measure bcoh. In the neutron interferometer, a cold neutron beam (here the wavelength may range from 2 to 4 Å) is coherently split by a perfect, monolithic, Si crystal, and then recombined after passing through a flat, parallel sided, homogeneous, sample. The sample is precisely oriented in the non-dispersive geometry in order to eliminate dependence on the energy of the neutron. The measured phase shift is directly proportional to bcoh. This technique only requires precise determination of the density and the thickness of the sample and is useful for the measurement of bcoh in many different, neutron transparent materials (solids, liquids or gasses).