Coming soon to a SANS instrument near you: sub-millisecond time-resolved SANS measurements

Charlie Glinka, NCNR

Time-dependent microscopic phenomena that occur on micro-to-millisecond time scales are not readily amenable to conventional neutron methods; the phenomena are too slow for neutron spectroscopy and too fast for neutron "snapshots". A time-of-flight (TOF) technique proposed by Roland Gähler (ILL) for time-resolved SANS measurements with sub-millisecond time resolution offers a solution to this dilemma under certain conditions. The technique requires a high-speed disk chopper, preceding the instrument's pinhole collimation, to pulse the beam at a frequency that is linearly related to the frequency (10 to 1000 Hz) of an applied field at the sample. In TOF terminology, the instrument operates in the far frame-overlap regime such that neutrons from several successive pulses arrive at the detector at a particular time corresponding to a definite (sub-millisecond) time interval within the period of the applied field at the sample.

Gähler's method has been under development for implementation on the NG-3 30-m SANS instrument in 2005. The current status of this development and plans for installation will be described along with estimates of the attainable time-resolution (~ 50 μsec). Possible applications for the technique in macromolecular systems and structured fluids will be discussed.

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