The New NG-6

The previous version of NG-6 provided neutrons to three monochromatic side positions (including \(\alpha\)-\(\gamma\) which will move to NG-7) and the Cold Neutron Imaging Instrument (CNII).  The current guide is straight and thus CNII suffers from core \(\gamma\)-rays and fast neutrons.  The many guide cuts also introduce non-uniformity into the neutron distribution, which is undesirable for an imaging station.  Finally, the instrument is not as long as one might wish.  The new NG-6 is designed to address these issues.

To eliminate unwanted radiation from the reactor, the new NG-6 will be curved slightly to eliminate line of sight to the core.  It is well known that curved guides introduce unwanted beam inhomogeneities.  To reduce this problem the guide will use a technique called phase-space tailoring which uses a curved section with a higher m coating on the outer radius followed by a straight section that clips off the unwanted phase space populated by the most highly divergent neutrons.  This provides a perfectly uniform spatial distribution for neutrons above a certain wavelength (in this case about 3 Å), which is determined by the chosen m-coatings.  Due to this as well as imperfect illumination and non-perfect coatings, there is a very minor deviation from uniformity of perhaps 2%.  The capture flux at the end of the guide will be about 6 x 10⁹ n/cm²-s.

The diagram below shows the guide profile.  Note the dimensions of the x and y axes are very different distorting the guide shape allowing one to clearly see the curve.  Note that the end of the guide is out of the line-of-sight by more than 30 cm.  The vertical lines in the figure delineate each segment of the guide, while the color on the edge refers to the guide coating.  The side view shows that the bottom section ends upstream of the new imaging station allowing a monochromator to be installed to serve a side position (NG-6a) which will not interfere with the imaging station.

The guide upgrade outage presents a unique opportunity to enhance the capabilities CNII. The most important improvement will be the curved, m=2 supermirror guides from the cold source to the end station. The curved guide will eliminate backgrounds and ease radiation protection measures due to the line of sight fast neutrons and gamma rays. The m=2 supermirror guides will result in a factor of about 3.4 increase in thermal equivalent flux over the present straight ⁵⁸Ni guides. The use of phase space tailoring ensures that for wavelengths above 3 Å, the phase space is uniformly filled producing a uniform field of view.

The relocation of the \(\alpha\) - \(\gamma\) instrument and the 9 Å beam line permits lengthening the hutch by about 5 m, so that the maximum flight path is about 15 m. As well, there is space to have a permanently installed double monochromator assembly and velocity selector so that transitioning between polychromatic and monochromatic measurements will not require changing the elevation of the optical axis. These new features will facilitate fine tuning dark field tomography measurements over a broad range of autocorrelations, which is a new method being developed under a NIST Innovations in Measurement Science Project. The upgraded CNII will offer the first working prototype of this method, which will provide SANS-like measurements in a volume element that is about 50 \(\mu\)m.

The additional flight path length provides an opportunity to design a Wolter optics microscope that includes a condenser lens as well as an objective lens. Ray tracing indicates that this neutron version of Hooke’s microscope will produce neutron images with a spatial resolution of about 3 µm with an acquisition time of about 0.1 s. This will bring neutron imaging closer to the performance of coarser synchrotron imaging instruments. The Wolter optic should be available when the upgraded CNII begins operation.