Post on 19-Jan-2016
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 H2 monitor
spectra at 5m(dashed), 10, 15, 25 and 100m, using t0=1300sec, n/cm2/sec/Å/sterad/pulse
(Å)
Pulsed
Reactor
L1 = 2 – 12 m, sample at 19m, L2 = 2- 12 m, Qmin~
0.002 Å-1
EITHER unsurpassed simultaneous Q using TWO 1m
square detectors at different L2.
OR increased count rate from two detectors at
similar L2
SANS2d at 10Hz ISIS TS-2being built !
C hopperBender
Sam ple M ovableD etectors
R em ovable G uides
SANS – recent progress
• 2006 - 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 L2 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 L2 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. Qmin ~ 10-3 Å-1 ( ~ 0.6 μ) “easy”, Qmin < 3x10-4 Å-1 ( ~ 2 μ) is harder.
• Time of flight double crystal USANS, Qmin ~ 4x10-6 Å-1 ( ~ 150 μ) – needs hot moderator !
• Spin-Echo SANS, G(z), z ~ λ2B, ~ 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