Neutron Depth Profiling on NG-A′

For several years, neutron depth profiling (NDP) has occupied an end position on NG-5.  However, to develop an internationally competitive cold-neutron triple-axis spectrometer requires an advanced guide and more importantly a position at the end of a guide.  The cold source outage is a perfect opportunity to upgrade NG-5 to serve such an instrument. That means that NDP needs a new location.    

NG-A serves the neutron spin echo spectrometer, but the instrument only uses of the top portion of the guide.  This made it possible to use a multichannel bender to extract a beam from the lower portion of the guide.  The bender is 0.8 m long with a radius of curvature of 4.52 m. Si wafers (thickness = 0.3 mm) define 23 channels have coatings with m = 3.5 and 1.5 on the concave and convex surfaces, respectively.  The bender extracts a beam 50 mm wide by 40 mm tall at an angle of about 10^○.  The new guide, NG-A′, which transports the beam about 7½ m to the NDP instrument, has m = 2.5 and m=1.8 coatings on the sides and top/bottom respectively.  The diagram below shows the guide profile.  Note the dimensions of the x and y axes are very different distorting the guide shape.  The vertical lines delineate each segment of the guide, while the color on edge refers to the guide coating. 

NG-A bends strongly before reaching the guide hall.  This combined with the NG-A′ bender, means that relatively few short wavelength neutrons make it to the NDP instrument. This does not compromise the performance because NDP is a neutron capture technique and absorption cross-sections typically increase with \(\lambda\).  The plot below compares the calculated incident flux and capture flux as a function of neutron wavelength.  The shift to longer wavelengths at the new position is readily apparent.  With the new D₂ cold source, the expected capture flux at the end of NG-A′ is about 3.7 x 10⁹ n/cm²/s, a gain of about 50% over the current location.  This increase will enable shorter data acquisition times, reducing overall experiment time and increase the instruments ability to do in-situ, in-operando kinetic studies of light elements in dynamic systems.  Future research projects on this new beam-line will include Li-ion battery kinetic studies, evaluation of kinetic materials, astrophysics metrology studies, and the durability of infrastructure materials.