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