# Presentation: 364715

## A High Performance Hybrid Spectrometer for the Single Crystal Spectroscopy at the Pulsed SNS

* **Scientific case and design objectives for the proposed hybrid spectrometer for the SNS*
* **General layout of the proposed spectrometer and its place in the SNS instrument suite*
* **Comparison of different spectrometer concepts and evolution of our thinking*
* **Preliminary analysis of the instrument performance*

**  ****Igor Zaliznyak**
**  ****Laurence Passell**

- Outline

## Scientific case for a proposed instrument for the SNS

- Coherent collective excitations in single crystals:
- lattice dynamics (phonons)
- spin dynamics (magnons, critical scattering)
- Structure and dynamics of partially ordered and glassy states
- spin glasses
- charge glasses
- correlated amorphous phases
- Study of the microscopic physical properties of samples in a variety of extreme environments:
- magnetic field
- pressure
- temperature
- Characterization of spin-dependent  cross-sections by means of polarization analysis

*Neutron spectrometer for studies of the coherent low-energy states in single crystals.*

## What are the typical samples we want to study?

*CuGeO**3** sample used by M. Arai group for detailed measurement of the excitation dispersion on MAPS in 2000*

*CuGeO**3** sample used by L.-P. Regnault for the original measurement in 1993 *

## Benchmark requirements for a single crystal neutron spectrometer

-     Transmission of both primary (monochromator) and  secondary 	(analyzer) spectrometers should be close to 1 within the resolution 	acceptance range, and vary smoothly over a substantial energy 	interval, typically from 2.5 meV to 60 meV.
-     Both spectral (energy resolution) and angular (~ wavevector 		resolution) acceptances of the monochromator and analyzer should 	be flexible and easily adjustable, typical resolutions are 1% to 5%.
-     Scattering volume seen by a detector should be well defined and 	easily adjustable depending on the sample size to minimize the 	background.
-     Efficient use of the large incident neutron beam by focusing it on 	the sample is very important, and should be previewed.

## What are our main design goals?

- Use most of the monochromatic neutron intensity produced by the source, while providing the highest possible signal-to-background ratio in the detector at a good to moderate energy and wavevector resolution
- Use the pulsed time structure of the neutron beam at the SNS
- Avoid the direct view of the moderator and the source by the sample
- Make solid angle accepted by the secondary spectrometer as large as necessary
- Make range of the scattering angles accessible to the secondary spectrometer as large as possible
- Preview an easy setup of the polarized beam option
- Trade-off of the resolution for intensity should not degrade the instrument performance

## How does the instrument we propose fit SNS inelastic instruments suite?

*High resolution and low energy transfer*
- 10-100 eV Multichopper Spectrometer
-  E = 2 - 20 meV
-  Q = 0.1 - 4 Å-1

*High energy transfer*
- 10-1000 meV Fermi Chopper Spectrometer
-  E = 10 - 1000 meV
-  Q = 0.1 – 22 Å-1

*High intensity at moderate resolution and medium energy transfer + polarized beam*
- Crystal Monochromator Hybrid Spectrometer
-  E = 2.5 - 60 meV
-  Q = 0.1 – 8 Å-1

## Schematic layout of the proposed Rotating Crystal Time-of-Flight Spectrometer

## How does the instrument we propose fit SNS floor layout?

*Needs rather short, 15 to 25 m primary flight path, but large, ~7-8 m  radius floor area for the moving analyser bank and the sample table.*

- Use the space following a shorter instrument?
- Occupy a “tangential” position with no other instruments on one side?

**?**

## Evolution of our concept: from multi-chopper TOF to chopper-less hybrid spectrometer

- Optimized layout of the direct geometry hybrid spectrometer with vertically focused crystal monochromator: expected gain from vertically focusing the beam >~ 2

- Optimized layout of the direct geometry disc chopper spectrometer: curved guide loss ~6%, tapered section gain <1.4

*We choose to optimize the instrument performance at E**i** = 14.7 meV*

## Pro’s and Con’s of a Fermi Chopper

-  Advantages of a Fermi chopper
-  short burst times can be achieved with wide beams
-  works for very energetic neutrons
-  beam height is not limited
-  Disadvantages of a Fermi chopper
-  is a rotating collimator with curved slots: limits the beam horizontal divergence, does not take full advantage of the guide
-  short burst times require tight collimation, i.e. reduced transmission and angular acceptance
-  is optimized for a fixed set of incident energies, changing energy or resolution requires changing the rotor
-  finite transmission away from the nominal angular position (especially at 180°) may lead to background and non-trivial beam contamination
-  does not focus neutron beam on the sample

## Pro’s and Con’s of a Disk Chopper

-  Advantages of a disc chopper
-  rotation rate is not correlated with neutron energy, resolution can be semi-continuously varied
-  energy can be changed without impacting on transmission
-  Disadvantages of a disc chopper
-  small burst width cannot be achieved with wide beams because  tensile strength limits maximum rotation rate
-  transmission reduces when time for neutrons to traverse the disk becomes comparable with the slot open time (at lower neutron energies)
-  order contamination may occur when multiple slot discs are used
-  beam height is limited
-  does not focus neutron beam on the sample

## Pro’s and Con’s of a Crystal Monochromator

-  Advantages of a crystal monochromator
-  beam may be compressed on the sample using focusing Bragg optics, beam height is not limited
-  sample and its environment are exposed only to a reflected monochromatic beam: reduced background
-  works for wide beams
-  resolution is easily adjusted by putting the collimators
-  (Heusler) may provide a polarized beam for energies up to ~60 meV
-  Disadvantages of a crystal monochromator
-  lower than one and energy-dependent reflectivity of the crystal, is not well-suited for providing high-energy incident neutrons
-  energy and wavevector resolutions are coupled and depend on the instrument position
-  sample table and secondary spectrometer need to be moved when beam energy is changed

## What’s Good About Rotating Crystal?

-  Advantages of a rotating crystal monochromator
-  short burst times can be achieved with wide beams without impacting on either transmission or collimation
-  burst time decrease with decreasing neutron wavelength
-  rotation rates are well below the tensile strength limit
-  takes full advantage of the horizontal divergence introduced by the supermirror-coated guide
-  no impact on vertical focusing
-  possibility of time focusing (or defocusing) by changing the sense of rotation
-  Disadvantages of a rotating crystal monochromator
-  introduces some Doppler broadening (but it decreases as the neutron wavelength increases)
-  ?

*Defines the time structure of the neutron pulse at the sample: no chopper is necessary*

## Technical case for the rotating crystal monochromator

*A PG(002) crystal array 23 cm wide is needed to reflect a 4 cm wide beam  of 60 meV neutrons*

-  We assume a composite crystal monochromator, mounted on a 24 cm wide segmented vertically focusing device (GMI type), made of a high tensile strength Al alloy
-  At a rotation rate of 240 Hz the peripheral velocity of the frame is 181 m/sec, well below  the maximum peripheral velocity (500 m/s) for high tensile strength Al alloys
-  At a rotation rate of 240 Hz the crystal velocity at the periphery of the spot illuminated by a 4 cm wide guide is about 5.1% of the velocity of a reflected neutron. For Ei > 14.7 meV this corresponds to a Doppler broadening of the incident energy distribution of less than 3.6%.
-  Narrowing the beam reduces the Doppler broadening, a phased disc chopper upstream could be used for Doppler focusing

## Performance of the rotating crystal monochromator

- Crystal rotation rate (Hz)	Burst FWHM (s)		t/t	E/E
- Ei=5.0 meV (0 = 2.4°)
- 	240		32		0.0065		1.3%
- 	180		42		0.0105		2.1%
- 	120		63		0.0158		3.2%
- 	60		126		0.0315		6.3%
- Ei=14.7 meV (0 = 1.4°)
- 	240		26		0.0111		2.2%
- 	180		34		0.0146		3.0%
- 	120		52		0.0223		4.4%
- 	60		103		0.0441		8.8%
- Ei=60.0 meV (0 = 0.69°)
- 	240		22		0.0190		3.8%
- 	180		29		0.0251		5.2%
- 	120		43		0.0372		7.4%
- 	60		87		0.0753		15.6%

-  PG(002) rotating crystal viewing the m=3 supermirror guide
-  Crystal mosaic 1.2°, downstream angular acceptance 1 = 0.66°
-  Secondary flight path 4.0 m

## Secondary flight path and analyzer performance

- 	LSD 	t/t		E/E
- Ef=5.0 meV
- 	4 m	0.0098		2.0%
- 	5 m	0.0078		1.6%
- Ef=14.7 meV
- 	4 m	0.0168		3.3%
- 	5 m	0.0134		2.6%
- Ef=60.0 meV
- 	4 m	0.0339		6.8%
- 	5 m	0.0271		5.4%

- Analyzer resolution for the length of the secondary flight path LSD =4 m and 5 m and for the t=40 s burst width at the sample (FWHM)

*Uncertainty of the flight time in the analyzer  gives largest contribution to the energy resolution.*

- kf/ki=0.5

- kf/ki=1.0

- 60° - 90° coverage of the scattering angle by the detector array gives simultaneous access to large enough interval in Q for 0.5<ki/kf<1

*Moving the analyzer is cost-effective!*

## Choice of the moderator 

*“**Benchmark” the instrument performance at E**i** = 15 meV*

- E*i** * = 10 meV

- E*i** * = 20 meV

- coupled H2

- decoupled H2

- coupled H2

- decoupled H2

- t

- t

- Intensity

- Intensity

- Figure of merit is the integral flux within 30-50 s time window, defined by the length of the secondary flight-path

- Coupled supercritical H2  moderator

## Performance of the guide.

- Impact of the guide curvature
- (relative to the similar straight guide)

- Impact of the guide coating
- (relative to 3c supermirror guide)

-  L = 20 m curved 3c supermirror guide at a reasonable offset of 8 cm ~ 2 times the width of the guide provides ~75% transmission at 60 meV
-  Shorter (L = 15 m?) but narrower (3 cm?) guide may still be O.K.

## Punchline: technical features of a proposed direct geometry hybrid instrument for the SNS

- Spectrometer is optimized for
- energy resolution dE/E~0.03-0.05
- wavevector resolution dQ/Q~0.01-0.03
- High flexibility
- continuous variation of the incident energy from 2.5 to 60 meV with no higher-order contamination
- full range of energy transfers available at each Q
- easily and broadly variable energy and Q resolution
- possibility of time focusing/defocusing
- easily adaptable for polarization analysis
- Very low background
- sample is 1.8 m away from the direct beam
- scattering volume is well defined by collimators
- High sensitivity to small scattering cross-sections (signal to BG ratio)
- Can be operated at small scattering angles (near the forward direction)
- User friendly: allows for easy on-line data monitoring and analysis and experiment planning

*Efficient vertical focusing provides reasonable data collection rates even for very small samples ! *