NCNR Summer School, June 18-22, 2001 1
Time of flight (TOF) spectroscopy
How time of flight spectroscopy works
• in principle
• in practice (at the NCNR)
Trading intensity for resolution
NCNR Summer School, June 18-22, 2001 2
TOF spectroscopy, in principle
MonochromatorPulser
Sample
Detector
DPS
DSDt=0
t=tS
t=tD
v=vi
v=vf2θ
PSS
i
Dtv
=
SDf
D S
Dvt t
=-
21i,f i,f2E mv=
i,f i,f
i,f
mv kh
=
=l
hi fE E E= -
i fQ k k= -r rr
( )2 2 2i f i fQ k k 2k k cos 2= + - q
NCNR Summer School, June 18-22, 2001 3
2
DdI(2 , t ) N t
d dtsq = F DWD
W
Number of neutrons scattered at angle 2θ into solid angle ∆Ω, reaching detector within time interval tD,tD+∆t
Number of atoms illuminated
Double differential scattering cross section (w.r.t. time)
Number of neutrons per unit area in incident beam
(MEASURED)
NCNR Summer School, June 18-22, 2001 4
2 23SD
f
d d td dE d dt
s s\ µW W
2 2
f f
d d dtd dE d dt dE
s s=W W
f 3SD
1dE dtt
µ
( )2
B f
f i
d k S Q,d dE 4 k
s s= wW ph
2
SD1f 2
SD
DE mt
æ ö÷ç ÷= ç ÷ç ÷çè øf
SD
1kt
µ
( )2
4SD
dHence S Q, td dt
sw µW
Double differential scattering cross section (w.r.t. time)
Scattering function(DESIRED)
NCNR Summer School, June 18-22, 2001 5
TOF spectroscopy, in practiceThe NCNR Disk Chopper Spectrometer
(4) The sample area
(1) The neutron guide
(2) The crystalfilter
(5) The flight chamber and the detectors
(3) The choppers
NCNR Summer School, June 18-22, 2001 6
26.8m6.9m
12.0m
Guide width60mm
Guide width30mm
Cold sourceChoppers
Crystalfilter
Sampleposition
Neutron Guide Hall (G100)ConfinementBuilding (C100)
9.0m1.5m
Detectorsat 4.0m
0.25
Localshutter
*
Overall plan view of DCS (to scale)
NCNR Summer School, June 18-22, 2001 7
(1) The neutron guide
Neutrons transported by total external reflection; critical angle wavelength.
Gammas and high energy neutrons removed by “optical filter” design.
NCNR Summer School, June 18-22, 2001 8
(2) The crystal filter
Short wavelength neutrons removed by Bragg reflection in cooled pyrolytic graphite filter.
White beam monitor
Shutter
NCNR Summer School, June 18-22, 2001 9
Neutron Guide
Optical FilterSample Chamber
913 3He detectors,400x31x11mm3
MonochromaticBeam Monitor Radial Collimator
Sample
Monochromating Choppers
Frame OverlapChopper
Order Removal Choppers
Pulsing Choppers
Argon gas-filledFlight Chamber
White BeamMonitor
Crystal Filter
TransmittedBeam Monitor
BeamStop
Shutter
NCNR Summer School, June 18-22, 2001 10
(3) The choppers
A single (disk) chopper pulses the neutron beam.
A second chopper selects neutrons within a narrow range of speeds.
Counter-rotating choppers (close together), with speed , behave like single choppers with speed 2 . They can also permit a choice of pulse widths.
Additional choppers remove “contaminant” wavelengths and reduce the pulse frequency at the sample position.
NCNR Summer School, June 18-22, 2001 11
The DCS has seven choppers, 4 of which have 3 “slots”
Disk 4B
NCNR Summer School, June 18-22, 2001 12
NCNR Summer School, June 18-22, 2001 13
(4) The sample areaThe sample stage accomodates a wide variety of sample environments.A radial collimator reduces scattering fromvacuum cans, radiation shields, etc.
NCNR Summer School, June 18-22, 2001 14
(5) The flight chamber and the detectors
The flight chamber is argon-filled to reduce scattering of neutrons traveling from the sample to the detectors.There are 913 detectors in 3 banks.
NCNR Summer School, June 18-22, 2001 15
Instrument control and data acquisition system
VME choppercontrol modules
Choppers
ChopperControlComputer
InstrumentControlComputer("dcs")
Data analysiscomputer("solo")
913 detectors4 beam monitors
VME System
coincidence pulse
vacuum gauges, temperature sensors,oxygen sensors, motor controllers,pressure sensors, pressure alarms
NCNR Summer School, June 18-22, 2001 16
-3
I(E) peaks: 2.5-4.5A At long , I(E) drops~50% for every 2A
At fixed T and
.E varies
W:
as ~
λ
∆ λ
Trading intensity I(E) for resolution ∆E
2SI(E)
At fixeT / T
d ,
W:
E T∝
∆ ∝
λ3I(E
At f) ~ixed ,
W
:W
T
E ~∝
∆ ∝
λ
Q range ∝1/λ
E range increases as m increases, but I(E) ∝1/m
Quantities that can be varied:• chopper period T, and “frame overlap ratio” m=TS/T• wavelength λ• chopper slot widths W
(TS is theperiod at the sample)