ESS - SANS Instrumentation

daughterduckUrban and Civil

Nov 15, 2013 (3 years and 9 months ago)

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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
m
獥cⰠn⽣/
2
/sec/Å/sterad/pulse


(Å)
Pulsed

Reactor

L
1

= 2


12 m, sample at 19m, L
2

= 2
-

12 m,
Q
min
~ 0.002 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 !

Chopper
Bender
Sample
Movable
Detectors
Removable Guides
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 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

~ 10
-
3

Å
-
1

( ~ 0.6
μ
)

“easy”,

Q
min

< 3x10
-
4

Å
-
1

( ~ 2
μ
)

i
s 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