The New NG-5

The SPINS cold neutron triple-axis spectrometer was one of the older instruments at NCNR, and was recently decommissioned, as part of the guide upgrade project.  It will be replaced with PoLAR, a new cold triple-axis instrument with a modern beam delivery system.  Modern guides for cold-neutron triple-axis instruments are optimized for a double focusing monochromator.  This means that the guide typically provides a virtual source in the horizontal plane while supplying a “parallel” beam in the vertical dimension.  With these principles in mind, we have performed detailed simulations of the primary spectrometer of a new cold-neutron, triple-axis spectrometer on NG-5.  We have done this subject to a few constraints.  First our budget is insufficient to develop an entirely new instrument.  Thus, we will use a new monochromator drum and a double-focusing monochromator mount that we already have.  This will allow us to fully optimize the primary spectrometer to maximize the flux at the sample position of an instrument that will be located at the end of NG-5.

The new guide is straight and elliptical in the horizontal and vertical planes.  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. 

The expected flux at the sample position for PoLAR is roughly 10⁸ n/cm²-s.  This represents a substantial gain from SPINS where the calculated flux at the sample position was a little over 10⁷ n/cm²-s.   PoLAR is slated to come online no earlier than 2030.