How time of flight spectroscopy works • in principle • in … · NCNR Summer School, June...

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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

Transcript of How time of flight spectroscopy works • in principle • in … · NCNR Summer School, June...

Page 1: How time of flight spectroscopy works • in principle • in … · NCNR Summer School, June 18-22, 2001 1 Time of flight (TOF) spectroscopy How time of flight spectroscopy works

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

Page 2: How time of flight spectroscopy works • in principle • in … · NCNR Summer School, June 18-22, 2001 1 Time of flight (TOF) spectroscopy How time of flight spectroscopy works

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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

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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)

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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)

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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

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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)

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(1) The neutron guide

Neutrons transported by total external reflection; critical angle wavelength.

Gammas and high energy neutrons removed by “optical filter” design.

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(2) The crystal filter

Short wavelength neutrons removed by Bragg reflection in cooled pyrolytic graphite filter.

White beam monitor

Shutter

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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

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(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.

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The DCS has seven choppers, 4 of which have 3 “slots”

Disk 4B

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(4) The sample areaThe sample stage accomodates a wide variety of sample environments.A radial collimator reduces scattering fromvacuum cans, radiation shields, etc.

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(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.

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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

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-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)