ESS - SANS Instrumentation pulsed source SANS, using a wide range of wavelengths, uses more of the available flux than a continuous source. For same time average flux, count rates can be ~ 5 to 10 times better AND a wider simultaneous Q range with improved Q resolution (over most of Q) BUT for Long Pulse, need long beam lines to keep good λ resolution. BUT not too long as λ band will eventually be less than a 10% velocity selector! 2msec long pulse, coupled H 2 monitor spectra at 5m(dashed), 10, 15, 25 and 100m, using t 0 =1300  sec, n/cm 2 /sec/Å/sterad/pulse Pulsed Reactor

L 1 = 2 – 12 m, sample at 19m, L 2 = m, Q min ~ Å -1 EITHER unsurpassed simultaneous Q using TWO 1m square detectors at different L 2. OR increased count rate from two detectors at similar L 2 SANS2d at 10Hz ISIS TS-2 being built ! SANS – recent progress

Revise these calculations from 2004 (and earlier ones from R.Heenan & ESS working group). More realistic instrument (e.g. gravity) More practical (e.g. chopper locations) Comparison with the way D22 is actually used. Perhaps limit L 2 to 20m ? (smaller Q by tightening collimation, or using other methods) SANS – recent progress

ESS - SANS Instrumentation Count rate ~ (Q resolution) -4, needs careful comparisons ! At long L 2 for smaller Q count rate falls off very rapidly – and can become unusable, despite improved Q resolution. - SANS users understand this trade-off. 1m square detector on D22 at ILL is usually run “off axis” – wider simultaneous Q ranges are a scientific necessity. ESS running at 1/2 of 16.6Hz may be very attractive for much science (if prompt pulse & backgrounds are controlled).

Generic SANS Instrumentation Trend to using larger detectors – which if are arrays of linear psd gas tubes can be more flexible in size. Still a need for high count rate, 5mm x 5mm pixel, 2d detectors. High m guides or benders do not generally help SANS as they simply increase divergence (and bring more neutrons closer to the sample, increasing background).

Other SANS techniques ? Several ideas are being actively developed around the world. Pulsed sources can help or hinder … Focussing neutrons at detector can (just) out perform pinhole collimation at smaller Q. e.g. mirrors or polarised neutrons and magnetic sextupole lenses. Q min ~ Å -1 ( ~ 0.6 μ) “easy”, Q min < 3x10 -4 Å -1 ( ~ 2 μ) is harder. Time of flight double crystal USANS, Q min ~ 4x10 -6 Å -1 ( ~ 150 μ) – needs hot moderator ! Spin-Echo SANS, G(z), z ~ λ 2 B, ~ 1nm to ~ 150 μ ? All are exciting but depend crucially on development of high quality hardware, which is on-going. Hard at present to design “reference” instruments for ESS ? Trial mirrors JNOP Time of flight offSPEC/ SESANS prototype at TU DELFT