CHRNS Instrumentation Development Activities

• Major Modernization of Medium Resolution SANS

  The current 8 m long SANS instrument on neutron guide NG-1 will be replaced by a completely new 10 m long instrument, with installation tentatively planned for the early 2005. The new instrument will have a continuously variable sample-to-detector distance from 1 m to 5 m. Choices of source aperture and beamstop size will be automated. A seven converging beam option will be available in the long instrument configuration, producing a Q_min = 0.0019 Å with 12 Å neutrons. A computer model of the instrument is shown below.
  

  Contacts:	Charlie Glinka, ext. 6242, cglinka@nist.gov
  	John Barker, ext. 6732, john.barker@nist.gov

• Corrective Refractive Optics Sharpens Focus of SANS Instrument

  Lens systems consisting of linear arrays of up to 30 biconcave magnesium fluoride single crystals, are now used routinely on two 30-m SANS instruments at the NCNR to achieve a near doubling of the size of microstructural features that can be resolved. The lens system would be even more effective at longer wavelengths were it not for the spreading of the focal spot in the vertical direction due to gravity. A recent development has been to add large-apex-angle, single-crystal prisms in front of the lenses to correct for the gravitational aberration in the system. With the added prisms, the lenses are now effective at wavelengths up to 20 Å. The combination of lenses and prisms now enables SANS measurements to be made at Q-values as low as 0.005 nm^-1, a full factor of 3 improvement over the previous conventional pinhole collimation. The new optics are enabling microstructural studies that link nanoscale with micron scale features in, for example, polymer-clay nanocomposites, gels, and fluxoid lattices in superconductors.

  Contacts:	Steve Kline, ext. 6243 steven.kline@nist.gov
  	Boualem Hammouda, ext. 3961 boualem.hammoud@nist.gov

• DCS gains more than a factor of 2!
  

  One of the most attractive features of the Disk Chopper Spectrometer (DCS) is the design of the pulsing and monochromating chopper disks, each of which has three slots with different angular widths, enabling the user to choose among three distinct intensity/resolution conditions without having to change the wavelength or the speed of the choppers. To optimize intensity at the sample, to the extent that this is feasible for all three intensity/resolution conditions, the guide that passes through the choppers is internally fitted with sets of reflecting glass plates, creating vertical channels that restrict the lateral motion of neutrons as they pass down the guide. Originally four sets of plates were employed creating five channels, but detailed simulations convinced us that by removing the inner two sets of plates we would realize significant intensity gains in all modes of operation. Accordingly the choppers and guides were disassembled in July 2002, the inner glass plates were removed, and the primary spectrometer was reassembled. The measured gains are indeed substantial particularly for the intermediate resolution (see figure), the energy resolution is unaffected, and there is no increase in the background.

  Contacts:	John Copley john.copley@nist.gov
  	Jeremy Cook jeremy.cook@nist.gov

• Focusing Neutron Guide increases neutron flux at SPINS!
  

  A focusing neutron guide with a super mirror coating has been recently added to the CHRNS supported Spin-Polarized Triple-Axis Spectrometer (SPINS). Previously, SPINS has not had any device to focus the monochromatic neutron beam in the horizontal plane. This has resulted in a beam width of ~6 cm at the sample position. However, typical samples have a width of only 2-3 cm. Thus about half of the available neutrons have actually missed the sample for most experiments. To alleviate this mismatch, a focusing neutron guide has been obtained for placement between the monochromator and the sample. This guide, which has a super mirror coating with a critical angle three times that of natural nickel, can focus the entire incoming neutron beam into a small area with little loss. Thus the whole neutron beam can now be utilized, enhancing the signal by as much as a factor of 2. The construction of the supporting structure made of polyethylene to block the neutrons that come through the sides has recently been completed, and the guide can now be used by visiting researchers.

  Contacts:	Sungil Park, ext 8369 jmspark@nist.gov
  	Seung-Hun Lee, ext 4257 shl@nist.gov

• DAVE debuts
  

  CHRNS personnel are developing a new software tool for the reduction, visualization, and analysis of inelastic neutron data. DAVE, short for the Data Analysis and Visualization Environment, is an integrated suite of interactive software tools with a visual interface for treating and analyzing time-of-flight, backscattering, and triple-axis data sets. With a few clicks of the mouse, users can reduce their data from one of the inelastic spectrometers, make a cut through the scattering function, and fit it with the lineshape of their choice from a library of model functions. The integration of these functions greatly simplifies moving between CHRNS instruments for users. Moreover the software is designed so that it is straightforward to add other instruments or more advanced analysis functions. DAVE is a standalone executable application freely available for PC, LINUX, and MAC platforms.

  Contacts:	Richard Azuah richard.azuah@nist.gov
  	Rob Dimeo robert.dimeo@nist.gov
  	Alan Munter alan.munter@nist.gov

• NSE gets Shorty!
  

  The Neutron Spin Echo (NSE) Spectrometer , part of CHRNS, has recently been upgraded with a short time (shorty) option, that extends the dynamic range of the NSE spectrometer by an order of magnitude. (See Figure) These short times, which range from about 5 ps to 1 ns, give the spectrometer a dynamic range of 4000 in time for any particular configuration of the spectrometer. The upgrade has been accomplished by the addition of small precession coils on either side of the sample region that allow small field integrals yet with applied fields that are large enough to avoid the depolarizing effects of laboratory ambient fields.

  Contact:   Dan Neumann dan@nist.gov

• Sub-millisecond Time-Resolved SANS Measurements

  The capability for making time-resolved SANS measurements with sub-millisecond time resolution is being developed for the CHRNS 30-m SANS instrument. This capability, based on a concept proposed by R. Gähler (ILL) called TISANE (Time-Involved Small-Angle Neutron Experiments), would enable kinetic phenomena, such as the onset of shear thickening in concentrated colloidal solutions, which occur on millisecond time scales to be probed with SANS for the first time.

  Implementation of the TISANE concept requires a rotating disk chopper between the velocity selector and the pre-sample flight path of the CHRNS 30-m SANS instrument to pulse the incident beam. The neutrons in each pulse would arrive at the sample at different times due to the 10-30% wavelength spread in the beam from the velocity selector. By applying a periodic stimulus to the sample with a frequency (10 to 10³ Hz) that is simply related to the chopper frequency and the pre- and post-sample distances, the condition is set up whereby neutrons that arrive at the detector at a particular time, even though having the same broad band of wavelengths that emanate from the velocity selector, have been scattered from the sample at the same time, relative to the period of the stimulus. Therefore by simply recording not only the position of a detected neutron but also its time of arrival, “snapshots” of the response of the sample at several times withi n the period of the stimulus are gradually accumulated. The time-distance diagram below illustrates this method.

  Time-distance diagram illustrating a method for performing submillisecond time-resolved SANS measurements. The slopes of the lines are proportional to neutron wavelength. For L1 = L2, for example, the frequency of the sample stimulus, 1/T_s, must be twice the chopper frequency, 1/T₁, in order for neutrons arriving at the detector to have scattered from the sample at the same time, modulo T_s.

   
  The counter-rotating disks for the chopper system have been manufactured and construction of the chopper housing and associated electronics is well underway with delivery of the complete system expected in March 2005. The chopper system will be installed in the SANS instrument soon thereafter when offline testing is completed.

  Contact:		Charle Glinka, cglinka@nist.gov

Last modified 18-January-2007