LET: a cold neutron chopper
spectrometer on ISIS TS2
Project team:
Scientific Advisory Committee:
Rob Bewley
Steve Bennington
Jeremy Balchin
James Treadgold
Andrew Boothroyd
Roger Eccleston
Des McMorrow
Alan Tennant
Javier Bermejo
Toby Perring
LET overview
1
10
● High incident flux of cold neutrons
4% resolution
LET
MAPS
MERLIN
15
2
● Wide dynamic range
Sample flux (n/cm /s) x104
20
(0.5 < Ei < 80 meV)
● Enormous position-sensitive detector:
Horizontal –40 to +140 deg
Vertical
±30 deg
10
5
0
1
10
Incident energy (meV)
● Highly flexible
● Polarization analysis option
LET detector tank
LET features
Double Funnel Guide
(Diverging)
Double Funnel Guide
(Converging)
Chopper 1
Beam Jaws
Sample point
Main
Blockhouse
Chopper 5
Chopper 4
Chopper 3
Front
Blockhouse
Beamstop
Glass Guide
( Insert )
Main Vessel
Vac Pumps
Neutron Attenuator
Glass Guide
( Shutter )
Chopper 2
Interchangeable Guide
Section
Detector Packs
(Roof entry port shown)
Distance (m)
28.5
Vacuum Vessel
25
23.5
15.6
11.75
7.8
0
LET features
Double Funnel Guide
(Diverging)
Double Funnel Guide
(Converging)
Chopper 1
Beam Jaws
Sample point
Main
Blockhouse
Chopper 5
Chopper 4
Chopper 3
Front
Blockhouse
Beamstop
Glass Guide
( Insert )
Main Vessel
Vac Pumps
Neutron Attenuator
Glass Guide
( Shutter )
Chopper 2
Interchangeable Guide
Section
Detector Packs
(Roof entry port shown)
Distance (m)
28.5
Vacuum Vessel
25
23.5
15.6
15.6
11.75
11.75
7.8
0
7.8
0
Choppers
chopper configuration allows multiple measurements in one time
Distance from moderator
25
1.52
3.2
4.89
6.57
8. 26
9.94
11.6
13.3
1.52
3.2
4.89
6.57
8. 26
9.94
11.6
13.3
35.5
7.98
3.43
1.89
1. 2
0.828
0.605
0.462
35.5
7.98
3.43
1.89
1. 2
0.828
0.605
0.462
20
15
10
5
0
0
2
4
6
Time (us)
8
0
10
4
x 10
2
4
6
Time (us)
8
10
4
x 10
LET features
Double Funnel Guide
(Diverging)
Double Funnel Guide
(Converging)
Chopper 1
Beam Jaws
Sample point
Main
Blockhouse
Chopper 5
Chopper 4
Chopper 3
Front
Blockhouse
Beamstop
Glass Guide
( Insert )
Main Vessel
Vac Pumps
Neutron Attenuator
Glass Guide
( Shutter )
Chopper 2
Interchangeable Guide
Section
Detector Packs
(Roof entry port shown)
Distance (m)
28.5
Vacuum Vessel
25
23.5
15.6
15.6
11.75
11.75
7.8
0
7.8
0
Chopper 5
Novel double-funnel guide and chopper system
m=2
20 mm
m=3.8
m=3.8
1.1m
Advantages of new design:
Up to 4x more flux in high
resolution mode
Beam size is independent of
resolution
40 mm
fast
disks
sample
Chopper 5
Novel double-funnel guide and chopper system
m=2
20 mm
m=3.8
m=3.8
Advantages of new design:
Up to 4x more flux in high
resolution mode
Beam size is independent of
resolution
fast
disks
sample
Intensity at sample (arb. units)
1.1m
40 mm
4000
3000
2000
1000
0
-4
-3
-2
-1
0
1
2
Beam Divergence (degrees)
3
4
LET features
Double Funnel Guide
(Diverging)
Double Funnel Guide
(Converging)
Chopper 1
Beam Jaws
Sample point
Main
Blockhouse
Chopper 5
Chopper 4
Chopper 3
Front
Blockhouse
Beamstop
Glass Guide
( Insert )
Main Vessel
Vac Pumps
Neutron Attenuator
Glass Guide
( Shutter )
Chopper 2
Interchangeable Guide
Section
Detector Packs
(Roof entry port shown)
Distance (m)
28.5
Vacuum Vessel
25
23.5
15.6
15.6
11.75
11.75
7.8
0
7.8
0
Detector Tank
• New tank design
— easier to construct
• No windows
— pumped to 10–6mbar
• Non magnetic
— can use magnets and
polarized neutrons
LET features
Double Funnel Guide
(Diverging)
Double Funnel Guide
(Converging)
Chopper 1
Beam Jaws
Sample point
Main
Blockhouse
Chopper 5
Chopper 4
Chopper 3
Front
Blockhouse
Beamstop
Glass Guide
( Insert )
Main Vessel
Vac Pumps
Neutron Attenuator
Glass Guide
( Shutter )
Chopper 2
Interchangeable Guide
Section
Detector Packs
(Roof entry port shown)
Distance (m)
28.5
Vacuum Vessel
25
23.5
15.6
15.6
11.75
11.75
7.8
0
7.8
0
Detector Tubes
• 4m long PSD tubes, virtually gapless
• Detectors in vacuum, electronics in air
Tests of 4m tubes on MERLIN
give good results
Position resolution less than 25mm
4m
Detector Tubes
• 4m long PSD tubes, virtually gapless
• Detectors in vacuum, electronics in air
LET features
Double Funnel Guide
(Diverging)
Double Funnel Guide
(Converging)
Chopper 1
Beam Jaws
Sample point
Main
Blockhouse
Chopper 5
Chopper 4
Chopper 3
Front
Blockhouse
Beamstop
Glass Guide
( Insert )
Main Vessel
Vac Pumps
Neutron Attenuator
Glass Guide
( Shutter )
Chopper 2
Interchangeable Guide
Section
Detector Packs
(Roof entry port shown)
Distance (m)
28.5
Vacuum Vessel
25
23.5
15.6
15.6
11.75
11.75
7.8
0
7.8
0
Polarisation option
Polarising
SM
π flipper
Guide field
Polarisation on LET
XYZ Helmholtz
colis
He-3 analyser cell
• PASTIS type XYZ polarization analysis
• Non-magnetic environment around He-3 cell
• Larger flange to insert XYZ coils
• Movable section of guide to insert Polarising SM
• ISIS (Chris Frost) just awarded £1.3 million to design He-3 cells and coils
LET Summary
• Versatile
• Wide dynamic range
• Polarization analysis option
• First neutrons in summer 2008